Publications

Tracing the ancestral roots of Polish Jews before the introduction of metrical data in 1808 represents a unique and complex challenge for genealogists and historians alike. Indeed, limited official records, shifting geopolitical boundaries, and the absence of standardized documentation practices characterize that early era. Sometimes, however, genealogical sources and records unique to Jews, based on religious daily life and traditions, have subsisted. When available, they open unforeseen avenues into identifiable family histories for which no other record, or personal memories, are available. In other cases, less well-known archival records unexpectedly emerge to elucidate a perplexing genealogical problem. The present article deals with two such instances with a similar starting point, namely, the apparent impossibility of merging two family clusters with the same surname in a given town. The first case deals with two separate KUMEC clusters in the small Polish town of Konskie. Research of this specific case, using limited official records, leads to the discovery of a single-family line dating back to the early 1600s, by means of complementary metrical and rabbinical data. The second case deals with two distinct KRELL clusters in the city of Warsaw, which, after 25 years of extensive but unsuccessful research, finally leads to merging into a cohesive KRELL ancestral line dating back to the early 1700s, by means of a less exploited source of archival records. The present study puts forward guiding principles for searches back to pre-1808 Jewish family history. As such, it should be useful to researchers encountering similar roadblocks in the quest for their Jewish ancestors.

Beaded fiber composites represent ingenious design strategies capable of resolving the conflicts between strength and toughness in most engineering materials. Although intermittent beading holds great potential for improving mechanical properties, the underlying mechanisms responsible for strengthening and toughening of beaded fiber composites are largely unknown. In this study, we explore near-tip fields of an interface crack between the fiber and the polymeric bead, which is associated with fiber-bead debonding in the course of fiber pullout. The post-yield strain softening followed by strain hardening of polymer matrix, and friction between the bead and matrix are accounted for in the numerical analyses. It is found that pullout of fibers leads to development of multiple shear bands near the tip of the interface crack between the fiber and bead, and coalescence of shear bands gives rise to a zone of high plastic strain. Compared with the low level of friction between the bead and matrix, the bead-matrix interface with high friction coefficient can generate a larger zone of high plastic strain, increasing the propensity of interfacial debonding between the fiber and bead. Furthermore, we have revealed the role of thin coating of carbon nanotubes (CNT). Adding a CNT coating on the fiber enables the emergence of a small zone of high plastic strain near the interface crack tip and reduces the shear stress levels, thereby delaying bead debonding. Additionally, the introduction of CNT coating facilitates stress transfer from the bead to fiber, leading to high pullout force. The findings of this study provide important mechanistic insight into the design principles of beaded fiber composites.

Contemporary designs of engineering structures strive to minimize the use of material in order to reduce cost and weight. However, the approach taken by focusing on materials selection and on the design of the exterior shape of structures has reached its limits. By contrast, nature implements bottom-up designs based on a multiple-level hierarchy, spanning from nanoscale to macroscale, which evolved over millions of years in an environmentally sustainable manner given limited resources. Natural structures often appear as laminates in wood, bone, plants, exoskeletons, etc., and employ elaborate micro-structural mechanisms to generate simultaneous strength and toughness. One such mechanism, observed in the scorpion cuticle and in the sponge spicule, is the grading (gradual change) of properties like layers thickness, stiffness, strength and toughness. We show that grading is a biological design tradeoff, which optimizes the use of material to enhance survival traits such as endurance against impending detrimental cracks. We found that such design, when applied in a more vulnerable direction of the laminate, has the potential to restrain propagation of hazardous cracks by deflecting or bifurcating them. This is achieved by shifting material from non-critical regions to more critical regions, making the design sustainable in the sense of efficient use of building resources. We investigate how such a mechanism functions in nature and how it can be implemented in synthetic structures, by means of a generic analytical model for crack deflection in a general laminate. Such a mechanical model may help optimize the design of bioinspired structures for specific applications and, eventually, reduce material waste.

The electrospinning of thermoplastic polymers is widely used in applications such as filters and coatings, but has only recently been applied to thermosetting polymers because of their chemical structure and reactivity. Epoxy is a thermosetting polymer which, when combined with a curing agent, chemically reacts to form a crosslinked matrix. In the present study, we demonstrate that to electrospin epoxy and obtain continuous micro and nanofibers, one must precisely control the curing reaction. Epoxy was mixed with triamine curing agent and, to enable electrospinnability, was dissolved in a mixture of tetrahydrofuran and dimethylformamide solvents. We identified a narrow working window wherein a proper solution for electrospinning is close to the gel point, right before the transition from liquid to solid gel state. The solution was characterized by means of (i) Fourier-transform infrared spectroscopy to monitor the extent of reaction, (ii) steady shear viscosity to detect the divergence near the gel point, and (iii) oscillatory loss and storage shear moduli to identify the liquid-to-gel transition. Based on these measurements, it was possible to monitor the chemical transformations that the epoxy solution underwent with time, such as chemical interconnections and gelation, and thus define the working window for electrospinning.

The multiscale structure of biomaterials enables their exceptional mechanical robustness, yet the impact of each constituent at their relevant length scale remains elusive. We used SAXD analysis to expose the intact chitin-fiber architecture within the exoskeleton on a scorpion's claw, revealing varying orientations, including Bouligand and unidirectional regions different from other arthropod species. We uncovered the contribution of individual components constituent behavior to its mechanical properties from the micro- to the nanoscale. At the microscale, in-situ micromechanical experiments were used to determine site-specific stiffness, strength, and failure of the biocomposite due to fiber orientation, while metal-crosslinking of proteins is characterized via fluorescence maps. At the constituent level, combined with FEA simulations, we uncovered the behavior of fiber-matrix deformation with fiber diameter

Drawn epoxy fibers possess increasingly high mechanical properties when their diameter is decreased down to a limit of about 10 μm. Smaller diameters with even higher properties can potentially be obtained by electrospinning. Yet, electrospinning of standalone epoxy fibers has not been possible so far, because of the reactivity of epoxy and the fragility of such fibers. These difficulties are overcome in the present study by dissolving the epoxy in methyl ethyl ketone solvent, and achieving suitable electrospinning conditions by controlling the degree of epoxy crosslinking in the solution. The fibers are captured on a net screen, with the positive electrode placed behind it. The resulting electrospun fibers exhibit about 80% higher strength and stiffness compared to bulk epoxy, and striking 900% higher elongation and 1200% higher toughness. Also observed is a size-dependence of the strength on the fiber diameter, which shows a sharp strength rise to more than 300 MPa (compared to about 70 MPa in bulk epoxy) in fibers of diameters down to 3 μm. This rise in properties is likely due to anisotropic molecular rearrangement resulting from the strong stretching forces induced by electrospinning. Highly performant epoxy nanofibers could find applications in diverse fields, including reinforcement in composites, sensors, coatings and electronic devices.

The geometric modification of fibre-matrix interfaces is a promising approach for simultaneous improvement of strength and toughness of composites. In this study, we investigate the effect of discrete epoxy droplets deposited on glass fibres embedded in an epoxy matrix, using single fibre experiments. Pullout tests reveal increases in both the pullout force and work in beaded fibre samples compared to beadless ones. This is somewhat unexpected as the bead and the matrix are made of the same material (cured epoxy) and possess the same mechanical properties. Interestingly, upon pullout, only the fibres are extracted from the matrix whereas the beads remain within the matrix, indicating that failure occurs at the fibre-bead interface. The simultaneous improvement in pullout force and work seen during the pullout of beaded fibres is explained by a conceptual anchoring mechanism based on friction lock, which agrees well with the experimental results. Smaller beads yield higher increases in pullout force and work, leading to the possibility of denser packing of multiple beaded fibres in a practical composite.

The reinforcement of composites by carbon-nanotubes (CNTs) is typically limited by agglomeration and non-uniform dispersion. Thus, achieving a nanocomposite with high density of reinforcement material is a tough challenge. In this work, rather than mixing CNTs in a matrix, we first construct a dense CNT-network scaffold, and then impregnate it by the matrix to obtain a composite. To that end, we explore an evaporation-driven self-assembly approach to form 3D CNT scaffolds on quartz fibers, which combines high CNT density and nanoscale pore size with a straightforward, efficient process. The scaffold is thicker than the fiber by more than an order of magnitude, with a typical pore size of 70 nm and porosity of 60%. The strength of a scaffold-reinforced composite is evaluated by a fragmentation test. μCT 3D-reconstruction of the fragmented scaffold reveals that the matrix-impregnated scaffold creates a multiscale structure that under load behaves much like a fibrous composite. The fragmentation results are analyzed by a mechanical model, demonstrating a scaffold-composite strength of ∼200 MPa. The improved strength and relatively high CNT volume fraction (∼20%), along with the capability of tuning the scaffold thickness and density, make the proposed structure a promising prospect for composite reinforcement, as well as for diverse nanoscale applications.

Biological structures such as bone, nacre and exoskeletons are organized hierarchically, with the degree of isotropy correlating with the length-scale. In these structures, the basic components are nanofibers or nanoplatelets, which are strong and stiff but anisotropic, whereas at the macrolevel, isotropy is preferred because the direction and magnitude of loads is unpredictable. The structural features and mechanisms, which drive the transition from anisotropy to isotropy across length scales, raise fundamental questions and are therefore the subject of the current study. Focusing on the tibia (fixed finger) of the scorpion pincer, bending tests of cuticle samples confirm the macroscale isotropy of the strength, stiffness, and toughness. Imaging analysis of the cuticle reveals an intricate multilayer laminated structure, with varying chitin-protein fiber orientations, arranged in eight hierarchical levels. We show that the cuticle flexural stiffness is increased by the existence of a thick intermediate layer, not seen before in the claws of crustaceans. Using laminate analysis to model the cuticle structure, we were able to correlate the nanostructure to the macro-mechanical properties, uncovering shear enhancing mechanisms at different length scales. These mechanisms, together with the hierarchical structure, are essential for achieving macro-scale isotropy. Interlaminar failure analysis of the cuticle leads to an estimation of the protein matrix shear strength, previously not measured. A similar structural approach can be adopted to the design of future synthetic composites with balanced strength, stiffness, toughness, and isotropy.

Strain sensors are used in varied applications, including personal healthcare monitoring, human-machine interaction, and artificial skin. Here, we report fabrication of a highly sensitive capacitive strain sensor comprising a self-healing polydiacetylene-polyacrylic acid-Cr3+ hydrogel. The dielectric hydrogel medium was prepared through a simple synthesis scheme from readily available ingredients. The elasticity and pronounced sensitivity of the composite hydrogel are attributed to the distinct components of the system. The Cr(H2O)(6)(3+) complexes function as cross linkers, maintaining stability of the hydrogel framework through electrostatic binding to carboxylate moieties within both the polyacrylic acid and polydiacetylene, additionally facilitating incorporation of high concentration of water molecules essential for maintaining hydrogel elasticity. In parallel, polydiacetylene, employed here for the first time as a vehicle for strain sensing, endows the system with high intrinsic capacitance sensitivity to mechanical stimuli, further exhibiting a major contribution towards greater flexibility and resilience under high strains. The sensor exhibits high stretchability of 500%, and high sensitivity exhibiting a gauge factor (GF) of up to 160. Applications of the polydiacetylene-polyacrylic acid-Cr3+ capacitive hydrogel sensor for physiological strain monitoring are presented.

Electrospinning is one of the most investigated methods used to produce polymeric fiber scaffolds that mimic the morphology of native extracellular matrix. These structures have been extensively studied in the context of scaffolds for tissue regeneration. However, the compactness of materials obtained by traditional electrospinning, collected as two-dimensional non-woven scaffolds, can limit cell infiltration and tissue ingrowth. In addition, for applications in smooth muscle tissue engineering, highly elastic scaffolds capable of withstanding cyclic mechanical strains without suffering significant permanent deformations are preferred. In order to address these challenges, we report the fabrication of microscale 3D helically coiled scaffolds (referred as 3D-HCS) by wet-electrospinning method, a modification of the traditional electrospinning process in which a coagulation bath (non-solvent system for the electrospun material) is used as the collector. The present study, for the first time, successfully demonstrates the feasibility of using this method to produce various architectures of 3D helically coiled scaffolds (HCS) from segmented copolyester of poly (butylene succinate-co-dilinoleic succinate) (PBS-DLS), a thermoplastic elastomer. We examined the role of process parameters and propose a mechanism for the HCS formation. Fabricated 3D-HCS showed high specific surface area, high porosity, and good elasticity. Further, the marked increase in cell proliferation on 3D-HCS confirmed the suitability of these materials as scaffolds for soft tissue engineering.

Nanodiamonds (NDs) were synthesized under atmospheric conditions by heating a precursor powder mixture consisting of naphthalene and a microwave (MW) absorbing material inside an ordinary MW oven for 10 min. Pyrolysis of naphthalene led to the formation of onion-like carbon particles which then converted to NDs after prolonged MW irradiation. Different carbon-based materials like graphite, carbon black, graphene, and carbon nanotubes were used as microwave radiation absorbers that assisted in the dissociation of naphthalene and formation of NDs. ND particles were formed in both isolated as well as aggregated forms. Size of the particles ranged from 2 to 700 nm. Scanning electron microscopy, transmission electron microscopy, electron energy loss spectroscopy, Raman spectroscopy and thermogravimetric analysis were used to characterize the NDs present in the MW-synthesized product. The proposed MW-based ND synthesis technique is simple, fast, inexpensive, energy efficient and could be suitable for industrial scale production.

Epoxy fibers with different diameters were prepared by hot drawing and their mechanical properties were measured under tension. The stiffness, strength, ultimate strain, and toughness revealed substantial scale-dependent effects as they all significantly increased with a decrease in size. Compared to bulk epoxy, an intrinsically brittle material, thin epoxy fibers displayed a highly ductile behavior under tension. A drop in stress observed immediately beyond the yield point was followed by the development of a stable necking region propagating through the entire fiber length, then by strain-hardening up to final rupture. Necked fiber segments tested in tension were found to have even higher strength and modulus compared to the initial as-prepared fibers. Possible reasons for the highly ductile mechanical behavior and the size effects of epoxy fibers are discussed. Size effects for the strength of epoxy can be elucidated in principle either by means of a classical fracture mechanics argument (strength ~ 1/d
^1/2), or via a stochastic model argument (strength ~ 1/d
^1/β, where β is a function of the material and is generally larger than 2). In both models the presence and size of critical defects play a key role. However, defects cannot explain the colossal ductility (plastic deformation) seen in our experiments, nor can the presence of defects justify a size effect in an elastic property, namely Youngs modulus. Only scarce evidence exists in the literature for similar (milder) size effects in epoxy fibers but without any structural justification. We find here that highly cross-linked necked epoxy fibers exhibit partial macromolecular anisotropy which likely explains the observed high mechanical characteristics.

In view of their facile fabrication and recycling, functional materials that are built from small molecules ("molecular plastics") may represent a cost-efficient and sustainable alternative to conventional covalent materials. We show how molecular plastics can be made robust and how their (nano)structure can be tuned via modular construction. For this purpose, we employed binary composites of organic nanocrystals based on a perylene diimide derivative, with graphene oxide (GO), bentonite nanoclay (NC), or hydroxyethyl cellulose (HEC), that both reinforce and enable tailoring the properties of the membranes. The hybrids are prepared via a simple aqueous deposition method, exhibit enhanced mechanical robustness, and can be recycled. We utilized these properties to create separation membranes with tunable porosity that are easy to fabricate and recycle. Hybrids 1/HEC and 1/NC are capable of ultrafiltration, and 1/NC removes heavy metals from water with high efficiency. Hybrid 1/GO shows mechanical properties akin to covalent materials with just 2-10% (by weight) of GO. This hybrid was used as a membrane for immobilizing β-galactosidase that demonstrated long and stable biocatalytic activity. Our findings demonstrate the utility of modular molecular nanoplastics as robust and sustainable materials that enable efficient tuning of structure and function and are based on self-assembly of readily available inexpensive components.

Epoxy nanocomposites combining high toughness with advantageous functional properties are needed in many fields. However, fabricating high-performance homogeneous epoxy nanocomposites with traditional methods remains a great challenge. Nacre with outstanding fracture toughness presents an ideal blueprint for the development of future epoxy nanocomposites. Now, high-performance epoxy-graphene layered nanocomposites were demonstrated with ultrahigh toughness and temperature-sensing properties. These nanocomposites are composed of ca. 99wt% organic epoxy, which is in contrast to the composition of natural nacre (ca. 96wt% inorganic aragonite). These nanocomposites are named an inverse artificial nacre. The fracture toughness reaches about 4.2 times higher than that of pure epoxy. The electrical resistance is temperature-sensitive and stable under various humidity conditions. This strategy opens an avenue for fabricating high-performance epoxy nanocomposites with functional properties.

We present a theoretical analysis of the elastic stresses in a composite reinforced with beaded fibers by extending the classic Cox shear lag theory. The motivation for reinforcing a composite with beaded fibers is to improve both strength and toughness, two often conflicting properties. It is found that owing to their geometry beads intermittently placed on a fiber enhance fiber anchoring in the matrix, and can potentially dissipate energy by deforming the matrix during failure. The composite stiffness is shown to improve compared to a composite with beadless fibers, particularly when the beads are large and stiffer than the surrounding matrix. The stress profiles in the fiber, bead, matrix and along their respective interfaces incur periodic perturbations induced by the beads, modeled by Hill equation. For given elastic constants and bead geometry, these profiles reveal the weakest link loci in the structure, and consequently determine the composite strength and failure mode. A finite element analysis is presented that confirms our results. The bead-fiber and bead-matrix interfaces may be tuned by choice of materials and coatings to achieve desired mechanical properties.

The flexural rigidity of cylindrical specimens, composed of epoxy reinforced by short, magnetized glass fibers, was enhanced using weak magnetic fields (

The effect of basalt fibre sizing on the mechanical and interphase properties of fibre-reinforced composites was studied. Two different chemical preparations of the fibre surface (PBT-compliant and PP-compliant) were used. The polymer matrix was prepared from polypropylene/poly(butylene terephthalate) (PP/PBT) immiscible polymer blend and the effect of different compatibilizers on the composite properties was evaluated. SEM hints at improved fibre adhesion to the polymer matrix when a PP-compliant sizing is applied. SEM also reveals improved compatibilization effects when block copolymer instead of multiblock copolymer is used for the PP/PBT blend preparation. The pull-out test was applied to quantitatively evaluate the interface adhesion between the fibres and matrices. It showed a high value of the interfacial shear strength between basalt fibres modified with PP-compliant sizing and polymer blend compatibilized by block copolymer, thus confirming good adhesion. One possible explanation of such good mechanical properties can be related to the chemical interactions between functional groups, mainly maleic anhydride on basalt fibres and the polyolefin component (PP) of the polymer matrix.

We demonstrate a solution-based fabrication of centimeter-size free-standing films assembled from organic nanocrystals based on common organic dyes (perylene diimides, PDIs). These nanostructured films exhibit good mechanical stability, and thermal robustness superior to most plastics, retaining the crystalline microstructure and macroscopic shape upon heating up to 250-300 degrees C. The films show nonlinear optical response and can be used as ultrafiltration membranes. The macroscopic functional materials based on small molecules can be alternative or complementary to materials based on macromolecules.

One major challenge of functional material fabrication is combining flexibility, strength, and toughness. In several biological and artificial systems, these desired mechanical properties are achieved by hierarchical architectures and various forms of anisotropy, as found in bones and nacre. Here, it is reported that crystals of N-capped diphenylalanine, one of the most studied self-assembling systems in nanotechnology, exhibit well-ordered packing and diffraction of sub-angstrom resolution, yet display an exceptionally flexible nature. To explore this flexibility, the mechanical properties of individual crystals are evaluated, assisted by density functional theory calculations. High-resolution scanning electron microscopy reveals that the crystals are composed of layered self-assembled structures. The observed combination of strength, toughness, and flexibility can therefore be explained in terms of weak interactions between rigid layers. These crystals represent a novel class of self-assembled layered materials, which can be utilized for various technological applications, where a combination of usually contradictory mechanical properties is desired.

Purpose: The aim of this study is to determine the effect of variation of the exposure time of near-infrared irradiation on corneal stiffening after a bacteriochlorophyll derivative (WST11) with dextran (WST-D) application.Methods: One hundred four paired eyes of 3-month-old New Zealand White rabbits were included in this study. Fifty-two eyes (ex vivo n = 34, in vivo n = 18) were mechanically deepithelialized, treated topically with WST-D, and irradiated at 10 mW/(c)m(2) using a diode laser at 755 nm for 1, 5, or 30 minutes. Untreated fellow eyes served as controls. Corneoscleral rings were removed immediately after treatment (ex vivo), or 1 month after treatment (in vivo). Corneal strips were cut and underwent biomechanical stress-strain measurements.Results: Ex vivo, the mean tangent elastic modulus was significantly higher in the treatment groups than in the control groups for 1, 5, and 30 minutes of irradiation, respectively, 6.06 MPa, 95% confidence interval (CI, 4.5-7.6) versus 14.02 MPa, 95% CI (10.2-17.8), n = 11, 4.8 MPa, 95% CI (3.9-5.7) versus 15.03 MPa, 95% CI (12-18.1), n = 11, and 7.8 MPa, 95% CI (5.6-10.02) versus 16.2 MPa, 95% CI (13.6-18.9), n = 11; P

PURPOSE. To determine the long-term safety and efficacy of WST-D/near-infrared (NIR) corneal stiffening. METHODS. One eye of 23 New Zealand White rabbits was de-epithelialized mechanically followed by topical application of 2.5 mg/mL WST11, combined with dextran-500 (WST-D) for 20 minutes. Subsequently, samples were irradiated with a NIR (755 nm) laser at 10 mW/cm² for 30 minutes. Untreated fellow eyes served as controls. One week (n = 4), 1 month (n = 6), 4 months (n = 9), or 8 months (n = 4) after treatment rabbits were euthanized. Corneal strips were cut in superiorinferior direction for extensiometry testing (1, 4, and 8 months), and histologic sections were prepared for evaluation of keratocyte distribution (1 week and 8 months). RESULTS. Elastic modulus after treatment was significantly higher than in paired controls (16.0 ± 2.3 MPa versus 9.6 ± 3.6 MPa [P = 0.008], 18.1 ± 4.5 MPa versus 12.6 ± 2.3 MPa [P = 0.003], and 18.6 ± 3.6 MPa versus 14.2 ± 3.6 MPa [P = 0.010], at 1, 4, and 8 months, respectively). A significant decrease in keratocyte count at the anterior stroma was observed directly after treatment (1.5 ± 1.7 vs. 19.0 ± 4.1 [P = 0.002]). At 8 months keratocyte repopulation appeared completed, with similar distribution in treated and untreated corneas (15.9 ± 1.1 vs. 14.5 ± 2.5 [P = 0.562]). Corneal thickness was comparable between treated and untreated corneas at all time points. CONCLUSIONS. WST-D/NIR treatment resulted in significant and persistent long-term increase in corneal stiffness. Initial keratocyte apoptosis in the anterior stroma is followed by repopulation to normal level at 8 months after treatment. The safe nature of NIR light allows treatment of corneas of any thickness without endangering corneal endothelium or deeper ocular structures, potentially benefiting patients deemed unsuitable for riboflavin/UV-A cross-linking.

In this article, the authors present their point of view on whether an analogy can be drawn between nano and classical microcomposites. Their opinion corresponds with a 25-year-old opinion article by Calvert and is based on their own extensive studies and a large body of studies in the scientific literature. They propose that polymer nanocomposites are in fact quasi-homogeneous molecular blends, which ought to be regarded as molecular composites or self-reinforced polymers. Hence, the micromechanical models of classical composites may not generally apply to nanocomposites, where-instead-the interactions on a molecular scale between the nanoparticles and the polymer matrix control the properties. A few examples, including of nucleation and confinement by nanoparticles, are discussed.

The dielectric and electrical characteristics of the semiconductive WS₂ nanotubes/epoxy composites were studied as a function of the nanotubes concentration and the pressure applied during their molding. In addition, the ability of WS₂ nanotubes to serve as stress sensors in epoxy based nanocomposites, for health-monitoring applications, was studied. The nanocomposite elements were loaded in three-point bending configuration. The direct current was monitored simultaneously with stress-strain measurements. It was found that, in nanocomposites, above the percolation concentrations of the nanotubes, the electrical conductivity increases considerably with the applied load and hence WS₂ nanotubes can be potentially used as sensors for health monitoring of structural components.

Functional fillers in multilayered films provide opportunity in tailoring the mechanical properties through chemical cross-linking. In this study, Laponite-graphene oxide co-dispersion was used to incorporate graphene oxide (GO) easily into polyvinyl alcohol (PVA)/Laponite layer-by-layer (LBL) films. The LBL films were found to be uniform and the layer thickness increased linearly with number of depositions. The process was extended to a large number of depositions to investigate the macroscopic mechanical properties of the freestanding films. The LBL films showed remarkable improvements in mechanical properties as compared to neat PVA film. The GO-incorporated LBL films displayed higher enhancements in the tensile strength, ductility, and toughness as compared to that of PVA/Laponite LBL films, upon chemical cross-linking. This suggests the advantageous effects of GO incorporation. Interestingly, cross-linking of LBL films for longer time period (>1 h) and higher temperature (similar to 80 degrees C) was not found to be much beneficial. (C) 2016 Wiley Periodicals, Inc.

The simple process of a liquid wetting a solid surface is controlled by a plethora of factors-surface texture, liquid droplet size and shape, energetics of both liquid and solid surfaces, as well as their interface. Studying these events at the nanoscale provides insights into the molecular basis of wetting. Nanotube wetting studies are particularly challenging due to their unique shape and small size. Nonetheless, the success of nanotubes, particularly inorganic ones, as fillers in composite materials makes it essential to understand how common liquids wet them. Here, we present a comprehensive wetting study of individual tungsten disulfide nanotubes by water. We reveal the nature of interaction at the inert outer wall and show that remarkably high wetting forces are attained on small, open-ended nanotubes due to capillary aspiration into the hollow core. This study provides a theoretical and experimental paradigm for this intricate problem.

The simultaneous sharp rise in stiffness, strength, and toughness of electrospun nanofibers at small diameters is explained here as the result of the molecular orientation induced by the strong stretching of the electrospinning extensional flow. Differing from the common view that this phenomenon is related to the nanofibers size scale, we show by theoretical analysis that it is likely the result of an abrupt transition in polymer chain extension that occurs at high flow strain rates. Consequently, the molecular orientation and mechanical properties experience a matching transition, followed by a linear rise with the strain rate. The model compares well with published experimental data, supporting the assertion that the observed phenomena can be explained as the consequence of electrospinning conditions instead of size dependence. We show how the mechanical properties can be tuned by controlling the process as well as set the goal for future improvement in these properties.

The development of composite materials that are simultaneously strong and tough is one of the most active topics of current material science. Observations of biological structural materials show that adequate introduction of reinforcements and interfaces, or interphases, at different scales usually improves toughness, without reduction in strength. The prospect of interphase properties tuning may lead to further increases in material toughness. Here we use evaporation-driven self-assembly (EDSA) to deposit a thin network of multi-wall carbon nanotubes on ceramic surfaces, thereby generating an interphase reinforcing layer in a multiscale laminated ceramic composite. Both strength and toughness are improved by up to 90%, while keeping the overall volume fraction of nanotubes in a composite below 0.012%, making it a most effective toughening and reinforcement technique.

Defects in crystalline structure are commonly believed to degrade the ideal strength of carbon nanotubes. However, the fracture mechanisms induced by such defects, as well as the validity of solid mechanics theories at the nanoscale, are still under debate. We show that the fracture toughness of single-walled nanotubes (SWNTs) conforms to the classic theory of fracture mechanics, even for the smallest possible vacancy defect (∼2). By simulating tension of SWNTs containing common types of defects, we demonstrate how stress concentration at the defect boundary leads to brittle (unstable) fracturing at a relatively low strain, degrading the ideal strength of SWNTs by up to 60%. We find that, owing to the SWNT's truss-like structure, defects at this scale are not sharp and stress concentrations are finite and low. Moreover, stress concentration, a geometric property at the macroscale, is interrelated with the SWNT fracture toughness, a material property. The resulting SWNT fracture toughness is 2.7 MPa m0.5, typical of moderately brittle materials and applicable also to graphene.

The aim was to develop a hybrid three-dimensional-tissue engineering construct for chondrogenesis. The hypothesis was that they support chondrogenesis. A biodegradable, highly porous polycaprolactone-grate was produced by solid freeform fabrication. The polycaprolactone support was coated with a chitosan/polyethylene oxide nanofibre sheet produced by electrospinning. Transforming growth factor-β3-induced chondrogenesis was followed using the following markers: sex determining region Y/-box 9, runt-related transcription factor 2 and collagen II and X in quantitative real-time polymerase chain reaction, histology and immunostaining. A polycaprolactone-grate and an optimized chitosan/polyethylene oxide nanofibre sheet supported cellular aggregation, chondrogenesis and matrix formation. In tissue engineering constructs, the sheets were seeded first with mesenchymal stem cells and then piled up according to the lasagne principle. The advantages of such a construct are (1) the cells do not need to migrate to the tissue engineering construct and therefore pore size and interconnectivity problems are omitted and (2) the cell-tight nanofibre sheet and collagen-fibre network mimic a cell culture platform for mesenchymal stem cells/chondrocytes (preventing escape) and hinders in-growth of fibroblasts and fibrous scarring (preventing capture). This allows time for the slowly progressing, multiphase true cartilage regeneration.

A synergetic effect between graphene nanoplatelets (GNP) and single-walled carbon nanotubes (SWCNT) leading to an improvement of the electrical conductivity of poly(trimethylene terephthalate-. block-poly(tetramethylene oxide) (PTT-PTMO) based nanocomposites is demonstrated. PTT-PTMO based nanocomposites were prepared with varying concentration of SWCNT and GNP as conducting fillers, and their electrical conductivity and morphology were evaluated using Dielectric and Raman spectroscopies respectively. It has been shown that the addition of SWCNT and GNP enhanced the electrical conductivity of composites, particularly in the case of composites with 0.3 wt. % SWCNT and 0.1 wt. % GNP nanoparticles. These results suggest the existence of synergy arising from the combination of two conducting fillers with different geometrical shapes and aspect ratios as well as different dispersion characteristics in the PTT-PTMO thermoplastic elastomer matrix.

Biological structures consisting of strong boney elements interconnected by compliant but tough collagenous sutures are abundantly found in skulls and shells of, among others, armadillos, alligators, turtles and more. In the turtle shell, a unique arrangement of alternating rigid (rib) and flexible (suture) elements gives rise to superior mechanical performance when subjected to low and high strain-rate loadings. However, the resistance to repeated load cycling - fatigue - of the turtle shell has yet to be examined. Such repeated loading could approximately simulate the consecutive high-stress bending loads exerted during (a predator) biting or clawing. In the present study flexural high-stress cyclic loads were applied to rib and suture specimens, taken from the top dorsal part of the red-eared slider turtle shell, termed carapace. Subsequently, to obtain a more complete and integrated fatigue behavior of the carapace, specimens containing a complex alternating rib-suture-rib-suture-rib configuration were tested as well. Although the sutures were found to be the least resistant to repeated loads, a synergistic effect was observed for the complex specimens, displaying improved fatigue durability compared to the individual (suture or even rib) constituents. This study may assist in the design of future high-stress fatigue-resistant materials incorporating complex assemblies of rigid and flexible elements.

Ceramic composites found in nature, such as bone, nacre, and sponge spicule, often provide an effective resolution to a wellknown conflict between materials' strength and toughness. This arises, on the one hand, from their high ceramic content that ensures high strength of the material. On the other hand, various pathways are provided for stress dissipation, and thus toughness, due to their intricate hierarchical architectures. Such pathways include crack bridging, crack deflection, and delamination in the case of layered structures. On the basis of these inspiring ideas, we attempted here to create simultaneously strong and tough laminated alumina composite with high ceramic content. Composites were prepared from highgrade commercial alumina with spin-coated interlayers of ductile polymers (PMMA and PVA). The specimens' ultimate properties (strength, fracture toughness, and work of fracture) were measured by a four-point bending method. In some cases, fracture toughness of the composites was increased by up to an order of magnitude, reminiscent of the natural layered composites. It is proposed that this increase may be attributed to an interlocking mechanism, often encountered in biological composites. The significance of sample architecture and the role of the interfacial and bulk properties of the interlayer material are discussed.

Extremely fast growth of carbon nanotubes (CNTs), within just 5 s, was achieved by a facile microwave (MW) assisted heating technique, under ambient conditions. A mixture of graphite, ferrocene and carbon fiber was used as the precursor material. Growth characteristics were examined using different compositions of the precursor and MW powers. Both graphite and carbon fiber were found to be necessary for achieving fast growth of CNTs. MW heating of the precursor mixture at 100% power (1800 W) resulted in a yield of CNTs (26 +/- 5 wt%). Conversion efficiency of carbon from ferrocene to CNT at 100% MW power is about 82 wt% on average. When relative proportion of graphite in the mixture is high, yield decreases but conversion efficiency of carbon increases. Base growth mode is proposed as dominant growth process of the CNTs. Driven by the capillary effect of CNTs, catalyst particles are found to be encapsulated inside the tubes at different locations along their length. Solvent free, instant, easy, and cost effective growth process proposed here using ordinary MW oven is likely to be amenable to scale-up for industrial production of CNTs. (C) 2014 Elsevier Ltd. All rights reserved.

The toughness, strength and stiffness of nanocomposites are considered for the general case of hollow fillers with arbitrarily shaped cross-sections. The particular cases of nanotubes, thin wall general cylindrical fillers, and thin ribbons are examined. The toughness is expressed by the energy dissipated when the filler pulls out from the matrix during the composite fracture, taking into consideration the filler critical length. The study reveals how the properties of nanocomposites can be optimized by modulating the filler shape and dimensions, as well as the mechanical properties of the material and interface. The tradeoffs between toughness, strength and stiffness are analyzed in view of their different and sometimes opposite dependence on the material and geometric parameters. It is shown that when the filler is shorter than its critical length, typical of most current nanotubes, the toughness, strength and stiffness can be improved simultaneously by reducing the filler cross-sectional aspect ratio (wall thickness divided by diameter). The mechanical performance of composites reinforced by carbon nanotubes and microfibers is compared for several possible filler packing conformations, demonstrating the high potential of nanoreinforcement.

In the present work, MoS2 nanoparticles with fullerene-like structure, and most particularly those doped with minute amounts of rhenium atoms, are used as additive to medical gels in order to facilitate their entry into constricted openings of soft material rings. This procedure is used to mimic the entry of endoscopes to constricted openings of the human body, like urethra, etc. It is shown that the Re-doped nanoparticles reduce the traction force used to retrieve the metallic lead of the endoscope from the soft ring by a factor close to three times with respect to the original gel. The mechanism of the mitigation of both friction and adhesion forces in these systems by the nanoparticles is discussed.

In this study, a molecular dynamic model was employed to simulate oblique extraction of a multi-walled carbon nanotube (MWNT) from resin and to calculate the carbon nanotube's (CNTs) deformation and stress distribution. Numerical results reveal the occurrence of elongation and necking in a CNT prior to the movement of its embedded end and of local buckling in a segment near where a CNT exits resin under large angle oblique extraction. Numerical results for the 0, 15, 30, 45, 60 and 75 oblique extractions reveal the effects of the oblique angle on the force-displacement curve and the tensile stress distribution over selected CNT cross-sections. Based on the simulation results, it is noted that CNT breakage in large-angle oblique extraction is mainly attributed to stress concentration caused by local buckling and it has a detrimental effect on the CNT toughening capacity.

The utilization of highly branched polymer (e.g., epoxy resins) in engineering applications is often limited by their brittle nature (low fracture toughness). Loading the polymer matrix by fillers such as individual nanotubes is a promising alternative to enhance fracture toughness without compromising other mechanical properties. However, to fully understand the nanotubes toughening role and correctly characterize the nanocomposite failure mechanisms, a complete exfoliation of the nanotubes aggregates into individual nanotubes is essential. In this work, we embed only individual nanotubes in the polymer matrix using a novel dispersion method. The individual nanotube concentration in the composite is accurately determined. We achieve a record fracture toughness enhancement and, for the first time, demonstrate a coherent quantitative correlation between the fracture toughness and the surface roughness. Finally, comprehensive statistical investigation of the nanotube failure mechanisms shows that carbon nanotubes fail via fracture mechanism, while tungsten di-sulfide nanotubes via pullout mechanism. The failure mechanism could be predicted by the slope of the surface roughness vs. fracture toughness curve.

Biological tissues usually exhibit complex multiscale structural architectures. In many of these, and particularly in mineralized tissues, the basic building block is a staggered array-a composite material made of soft matrix and stiff reinforcing elements. Here we study the stiffness of non-overlapping staggered arrays, a case that has not previously been considered in the literature, and introduce closed-form analytical expressions for its Young's modulus. These expressions are then used to estimate the stiffness of natural staggered biocomposites such as low-mineralized collagen fibril and mineralized tendon. We then consider a two-scale composite scheme for evaluating the modulus of a specific hierarchical structure, the compact bone tissue, which is made of mineralized collagen fibrils with weakly overlapping staggered architecture. It is found that small variations in the staggered structure induce significant differences in the macroscopic stiffness, and, in particular, provide a possible explanation for the as yet unexplained stiffening effects observed in medium-mineralized tissues.

Recent progress made in the field of hierarchical biological materials is reviewed with an emphasis on the staggering characteristics at the smaller structural scale of a number of tissues. We show by means of selected examples that the small-scale architecture, and particularly the degree of staggering and overlap, plays a critical role in the macroscopic elastic behavior of those tissues. (c) 2013 Elsevier Inc. All rights reserved.

We quantify nanocomposite toughness through a reanalysis for nanotubes of the Cottrell-Kelly-Tyson (CKT) model, of the definition of critical length, and of the energy dissipation model for pull-out. The effect of the hollow cylindrical geometry of nanotubes is discussed, followed by an examination of proper ways to compare energy dissipation at the nano and micro levels.

The mechanical and structural properties of the sublayers of osteonal lamellae were studied. Young's modulus (E) of adjacent individual lamellae was measured by nanoindentation of parallel slices every 1-3 μm, in planes parallel and perpendicular to the osteon axis (OA). In planes parallel to the OA, the modulus of a lamella could vary significantly between sequential slices. Significant modulus variations were also sometimes found on opposing sides of the osteonal canal for the same lamella. These results are rationalized by considerations involving the microstructural organization of the collagen fibrils in the lamellae. Scanning electron microscope imaging of freeze fractured surfaces revealed that the substructure of a single lamella can vary significantly on the opposing sides of the osteonal axis. Using a serial surface view method, parallel planes were exposed every 8-10 nm using a dual-beam microscope. Analysis of the orientations of fibrils revealed that the structure is rotated plywood like, consisting of unidirectional sublayers of fibrils of several orientations, with occasional randomly oriented sublayers. The dependence of the measured mechanical properties of the lamellae on the indentation location may be explained by the observed structure, as well as by the curvature of the osteonal lamellae through simple geometrical-structural considerations. Mechanical advantages arising from the curved laminate structure are discussed.

We recently discovered that minute additions of sodium chloride (NaCl) during the preparation of electrospun polymethyl methacrylate (ES PMMA) nanofibers significantly raise their mechanical properties. In the present work, twisted yarns made of such ES PMMA nanofibers were prepared and the effects of NaCl mediation and yarn structure and twist on the resulting mechanical properties were studied. Improvements in these properties due to NaCl mediation are again observed for yarns, as they were for single ES fibers. Furthermore, both Young's modulus and the ultimate strength of the yarns show an increase with increasing angle of twist up to a maximum, followed by a decrease at higher twist angles.

To assess the effect of carbon nanotube (CNT) grafting on interfacial stress transfer in fiber composites, CNTs were grown upon individual carbon T-300 fibers by chemical vapor deposition. Continuously-monitored single fiber composite (SFC) fragmentation tests were performed on both pristine and CNT-decorated fibers embedded in epoxy. The critical fragment length, fiber tensile strength at critical length, and interfacial shear strength were evaluated. Despite the fiber strength degradation resulting from the harsh CNT growth conditions, the CNT-modified fibers lead to a twofold increase in interfacial shear strength which correlates with the nearly threefold increase in apparent fiber diameter resulting from CNT grafting. These observations corroborate recently published studies with other CNT-grafted fibers. An analysis of the relative contributions to the interfacial strength of the fiber diameter and strength due to surface treatment is presented. It is concluded that the common view whereby an experimentally observed shorter average fragment length leads to a stronger interfacial adhesion is not necessarily correct, if the treatment has changed the fiber tensile strength or its diameter.

Dentin and enamel are viewed here as multi-scale composites comprising a staggered micro-structure made of stiff platelets embedded in a more compliant matrix, and further assembled into macroscopic composite-like structures. Mechanical models are formulated for both tissues and their effective moduli are evaluated analytically. The resulting predictions are in very good agreement with Finite Elements (FE) simulations and experimental data from the literature. The models developed in this study demonstrate the possibility, in certain cases, to generate special mechanical effects linked to the structural complexity of these tissues.

Growing carbon nanotubes (CNTs) on the surface of fibers has the potential to modify fiber-matrix interfacial adhesion, enhance the composite delamination resistance, and possibly improve its toughness and any matrix-dominated elastic property as well. In the present work aligned CNTs were grown upon ceramic fibers (silica and alumina) by chemical vapor deposition (CVD) at temperatures of 650. °C and 750. °C. Continuously-monitored single fiber composite (SFC) fragmentation tests were performed on pristine as well as on CNT-grown fibers embedded in epoxy. The critical fragment length, fiber tensile strength at critical length, and interfacial shear strength were evaluated. Significant increases (up to 50%) are observed in the fiber tensile strength and in the interfacial adhesion (which was sometimes doubled) with all fiber types upon which CNTs are CVD-grown at 750. °C. We discuss the likely sources of these improvements as well as their implications.

Variations in Young's modulus of individual lamellae around a single bone osteon have been measured in three orthogonal planes by nanoindentation. The objective of these measurements was to establish a correlation between the mechanical properties and the microstructure of the osteonal lamellae. When indentation was performed in a plane perpendicular to the osteon axis (OA), the modulus of the lamella closest to the canal appears to be higher than the modulus of all other lamellae. No such difference was observed in planes parallel to the OA. However, in the parallel planes, an unexpected asymmetry in modulus was detected on opposing sides of the canal, potentially supporting the validity of the rotated plywood structure model of bone lamellae. Finally, based on the experimentally measured Young's modulus values, most osteonal lamellae appear to exhibit structural anisotropy.

The effective moduli of a multi-scale composite are evaluated by a bottom-up (hierarchical) modeling approach. We focus on a two-scale structure in which the small scale includes a platelet array inside a matrix, and the large scale contains fibers inside a composite matrix. We demonstrate that the principal moduli of the multi-scale composite can be fine-tuned by the platelet arrangement and orientation. As a case study, we consider the phenomenon of fiber micro-buckling within the multi-scale composite. It is found that the compressive micro-buckling strength can be considerably increased for specific platelet orientations. The multi-scale design approach presented here can be used to generate novel families of composite materials with tunable mechanical properties.

This work aims at evaluating the elastic modulus of hard biological tissues by considering their staggered platelet micro-structure. An analytical expression for the effective modulus along the stagger direction is formulated using three non-dimensional structural variables. Structures with a single staggered hierarchy (e.g. collagen fibril) are first studied and predictions are compared with the experimental results and finite element simulations from the literature. A more complicated configuration, such as an array of fibrils, is analyzed next. Finally, a mechanical model is proposed for tooth dentin, in which variations in the multi-scale structural hierarchy are shown to significantly affect the macroscopic mechanical properties.

Residual stress in polymers arises from the freezing of unstable molecular conformations. Residual stress is critical because its relaxation can cause shrinkage, defects, and fractures of polymer materials. The storage of stress is purposely enhanced to develop shape memory materials. Unfortunately, the storage of mechanical stress is still poorly controlled and understood. An approach to sense the storage of stress based on the spectroscopic response of carbon nanotubes is explored. The Raman response of nanotubes exhibits a variable sensitivity to strain when embedded in polymers that have experienced different thermal and mechanical treatments. This unique feature opens up new possibilities for the use of carbon nanotubes as mechanical nanosensors.

It is now possible to synthesize inorganic nanotubes (INT) of WS2 in a pure phase and substantial amounts. Due to their crystalline perfection, they are characterized by excellent mechanical behavior. This study is dedicated to the investigation of the effect of INT-WS2 on the mechanical, thermal, adhesion and tribological properties of epoxy based nanocomposites. Various concentrations up to 1.0 wt% of the INT were added to the epoxy matrix. First a method was developed to mix the nanotubes in the epoxy resin matrix. A combination of both magnetic stirring and ultrasonic mixing was used. The adhesion, fracture toughness and strain energy release rate were studied. The INT-WS2 were found to significantly improve all these properties. The wear of the nanotubes-reinforced epoxy was eight-times lower than that of pure epoxy. These results suggest numerous applications.

A generic mechanical model for bio-composites, including stiff platelets arranged in a staggered order inside a homogeneous soft matrix, is proposed. Equations are formulated in terms of displacements and are characterized by a set of non-dimensional parameters. The displacements, stress fields and effective modulus of the composite are formulated. Two analytical models are proposed, one which includes the shear deformations along the entire medium and another simplified model, which is applicable to a slender geometry and yields a compact expression for the effective modulus. The results from the models are validated by numerical finite element simulations and found to be compatible with each other for a wide range of geometrical and material properties. Finally, the models are solved for two bio-structures, nacre and a collagen fibril, and their solutions are discussed.

Over the past 30 years Raman spectroscopy has transformed the field of composite micromechanics and is being successfully applied to the field of nanotube-reinforced polymers. Under the effects of stress, careful study of the Raman spectrum of carbon nanotubes provides a unique insight into various physical phenomena in nanocomposites. This chapter demonstrates the power of the Raman signature of carbon nanotubes as a detector of bulk matrix defects, the occurrence of polymer phase transitions, and the nanotubes' own orientation change with respect to applied stress, stress profiles from Raman-insensitive fibers, and dual information about improvements in the stress transfer ability and about nanotube wall structure degradation due to the surface treatment itself. Remaining challenges are described.

We study the effect of the molecular nature of the interface between an epoxy matrix and multi-walled carbon nanotubes (CNTs) on the mechanical properties of the resultant nano-composites, with emphasis on toughness. A number of samples based on variously functionalized CNTs, namely, pristine, carboxylated, and aminated CNTs are examined, with different qualities of dispersion. Nano-composite toughness is found to increase with enhanced interfacial adhesion, an effect that is opposite to what is usually observed in traditional fiber-based composites. The classical pull-out energy model is shown to effectively explain this result. It is thus possible to tune the toughness of a nano-composite by adjusting the molecular nature of its interface and the CNT characteristics, namely its strength and its length relative to its critical length.

The effect of fiber diameter change on the toughness of short fiber polymer-based composites is studied by assuming that the main energy absorption upon fracture is the fiber pull-out mechanism. An expression is derived for a characteristic fiber length, ℓ^*, distinct from the critical length, above which an increase in fiber diameter always yields an increase in composite toughness. Experimental validation is provided through impact testing of composites made of short polyethylene terephthalate fibers in a polypropylene matrix.

We study the strain-induced shift of the D* Raman band of single-wall carbon nanotubes in polyvinyl alcohol-nanotube composite fibers. If embedded in structural components, such strain-sensitive fibers may be considered for potential applications as strain or stress sensors. Due to improved interfacial adhesion, stronger shifts of the D* Raman band are observed when carboxylic functional groups are present at the nanotube surface. This indicates that nanotube carboxylation would yield better efficacy for future sensing applications. However, we also observe that the improvements of interfacial adhesion do not lead to substantially better mechanical properties of the fibers. This effect is discussed by considering possible degradation of nanotubes during surface functionalization.

The electrical resistance of single and multi-walled carbon nanotubes buckypaper films was studied as a function of mechanical strain. The buckypaper strain sensors were encapsulated in epoxy matrices and the effect of carbon nanotube type and degree of strain on their resistance change and sensitivity were studied.

Investigation of composite materials response to hypervelocity impact by space debris has been carried out. In order to simulate hypervelocity impact, a unique laser driven flyer plate (LDFP) system was used, generating hypervelocity debris with velocities of up to 3 km/s. The materials studied in this research were Kevlar 29/epoxy and Spectra1000/epoxy thin film micro-composites (thickness of about 100 μm). Both Spectra and Kevlar fibers are used in long-duration spacecraft outer wall shielding to reduce the perforation threat. The micro-mechanical response of different composites was studied and correlated to the fiber, the matrix and the fiber/matrix interface properties. Visual and microscopic examinations of the damaged area identified fiber debonding as the prevailing failure mechanism. On the basis of a simple energy balance model it can be stated that for Spectra/epoxy composite the dominant mechanism is new surface creation, whereas for Spectra surface-treated fibers/epoxy the fiber pull out is the dominant mechanism. For Kevlar/epoxy fiber, pull out mechanism plays an important role.

The mechanical properties of individual WS2 nanotubes were investigated and directly related to their atomic structure details by in situ transmission electron microscope measurements. A brittle mode deformation was observed in bending tests of short (ca. 1 mu m in length) multilayer nanotubes. This mode can be related to the atomic structure of their shells. In addition, longer nanotubes (6-7 mu m in length) were deformed in situ scanning electron microscope, but no plastic deformation was detected. A "sword-in-sheath" fracture mechanism was revealed in tensile loading of a nanotube, and the sliding of inner shells inside the outermost shell was imaged "on-line". Furthermore, bending modulus of 217 GPa was obtained from measurements of the electric-field-induced resonance of these nanotubes.

Live cells create adhering contacts with substrates via focal adhesion (FA) sites associated with the termini of actin stress fibers. These FA 'anchors' enable cells to adhere firmly and locally to a substrate, and to generate traction that enables cell body displacement. Using time-lapse video microscopy, we have monitored the spontaneous anisotropic changes in FA size and shape, resulting from molecular reorganization of the adhesion sites in live, non-motile rat embryonic fibroblasts adhering to fibronectin-coated glass surfaces. The resulting experimental data on FA growth, saturation and decay is compared with predictions from a number of biophysical models. We find that the growth and saturation regimes for all FAs exhibit a consistent, recurring pattern, whereas the disassembly regime exhibits erratic behavior. We observe that the maximum size of FA areas depends on the FA growth rate: larger FA areas are formed as the growth rate increases. Using a composite mechanics model by which the evolution of the shear stress profile along the FA region can be calculated, we suggest that FA saturation is triggered either by the reaching of a minimum shear stress threshold at the FA back edge, or of a maximum difference between the maximum and minimum shear stress along the FA site.

Geoinspired synthetic chrysotile nanotubes both stoichiometric and 0.67 wt % Fe doped were characterized by transmission electron microscopy and electron diffraction. Bending tests of the synthetic chrysotile nanotubes were performed using the atomic force microscope. The nanotubes were found to exhibit elastic behaviour at small deformations (below ca. 20 nm). Young's modulus values of (159±125) GPa and (279±260) GPa were obtained from the force-deflection curves using the bending equation for a clamped beam under a concentrated load, for the stoichiometric and the Fe doped chrysotile nanotubes, respectively. The structural modifications induced by Fe doping altered the mechanical properties, with an apparent dependence of the latter on the number of constituting walls of the nanotubes.

The lamellar bone's strength is mainly affected by the organization of its mineralized collagen fibers and material composition. In the present study, Raman microspectroscopic and imaging analyses were employed to study a normal human femoral midshaft bone cube-like specimen with a spatial resolution of similar to 1-2 mu m. Identical bone lamellae in both longitudinal and transverse directions were analyzed, which allowed us to separate out orientation and composition dependent Raman lines, depending on the polarization directions. This approach gives information about lamellar bone orientation and variation in bone composition. It is shown that the nu(1) PO4 to amide I ratio mainly displays lamellar bone orientation; and nu(2) PO4 to amide III and CO3 to nu(2) PO4 ratios display variation in bone composition. The nu(2) PO4 to amide III ratio is higher in the interstitial bone region, whereas the CO3 to nu(2) PO4 ratio has lower values in the same region. The present study provides fresh insights into the organization of a lamellar bone tissue from two orthogonal orientations. (C) 2007 Elsevier Inc. All rights reserved.

A process for melt mixing of carbon nanotubes in polyacrylonitrile, which is aided by addition of plasticizer is proposed and employed to extrude/draw filaments at various draw ratios and test them for a range of properties. Microscopic observations show that the nanotubes are evenly dispersed throughout the filament and that they are preferentially aligned along its axis. The extrusion/drawing process results in two fold higher draw ratios of the nanocomposite filament compared with the unfilled control, and the first is shown by wide-angle X-ray diffraction and by polarized Raman spectroscopy to possess higher crystalline and morphological orientation. Thermal analysis and measurements of electric conductivity and mechanical properties attest to the condition that the carbon nanotubes are engaged in π-π interactions with the nitrile groups and/or with their polymerized conjugated imine system. These interactions interfere with and reduce the mutual dipole interactions of the nitrile groups of the polyacrylonitrile, allowing the nanocomposite filaments to be stretched to higher draw ratios. The potential reinforcing effect of the carbon nanotubes is cancelled out by the loss of the original dipole interactions, resulting in lower mechanical properties. However, the interaction of the carbon nanotubes with the conjugated imine system offers potentially interesting electric properties.

The surface of multi-wall carbon nanotubes (MWNTs) was functionalized by covalent linking of long alkyl chains. Such functionalization led to a much better tube dispersion in organic solvents than pristine nanotubes, favored the formation of homogenous nanocomposite films, and yielded good interfacial bonding between the nanotubes and two polymer matrices: a thermo-set (Epon 828/T-403) and a thermoplastic (PMMA). Tensile tests indicated, however, that the reinforcement was greatly affected by the type of polymer matrix used. Relative to pure PMMA, a 32% improvement in tensile modulus and a 28% increase in tensile strength were observed in PMMA-based nanocomposites using 1.0 wt% nanotube filler. Contrasting with this, no improvement in mechanical properties was observed in epoxy-based nanocomposites. The poorer mechanical performance of the latter system can be explained by a decrease of the crosslinking density of the epoxy matrix in the nanocomposites, relative to pure epoxy. Indeed we demonstrate that the presence of nanotubes promotes an increase in the activation energy of the curing reaction in epoxy, and a decrease of the degree of curing.

A one-step procedure to assemble nanoscale electrospun poly(methylmetacrylate) (PMMA) and multiwall carbon nanotube (MWCNT) reinforced PMMA fibers into twisted continuous ropes is presented. A post-treatment procedure following rope assembly is essential to maximize the mechanical properties of the ropes. A comparison between the mechanical properties of the individual nanoscale fibers and microscale ropes reveals that rope strength variability is advantageously smaller than single fiber strength variability, but also that the average rope strength is smaller than the single fiber strength. The incorporation of MWCNTs in PMMA ropes often leads to a significant increase in failure strain and toughness.

Spacecraft in low-Earth orbit (LEO) are exposed to a large flux of hypervelocity impacts with small particles of natural space micrometeoroids and man generated debris. At risk are the outer surfaces of the spacecraft usually made of polymers and composite materials. The objective of this work is to study the effect of hypervelocity impact on the fracture of fiber-matrix micro-composites. The hypervelocity impact is generated using a laser driven flyer plate technique. Organic fibers/epoxy micro-composite samples were prepared using Spectra 1000 or Kevlar 29 fibers and epoxy resin as a matrix. A combined effect of hypervelocity impact along with ionizing radiation on micro-composites has been examined. For this purpose untreated micro-composites and samples after exposure to gamma radiation were impacted by the aluminum laser driven flyers at 0.9 and 1.7 km/s. The micro-mechanical response of different composites was studied and was correlated to the fibers and fiber/matrix interface properties.

Fibrous arrays and composites are among the strongest structures created by man or found in nature. Such materials often fail in a slow, cumulative fashion, suppressing the sudden occurrence of rapid structural collapse. Our classical understanding of the statistical tensile strength and failure of unidirectional composites is usually based on a stochastic model where the key predictor is the size (N*) of a critical cluster of adjacent broken fibres, which inevitably leads to final composite failure. Here we show, via direct measurements using high-resolution synchrotron X-ray tomography, that in a quartz-epoxy composite the classical stochastic theory underpredicts - by a factor 3-5 - the size N* of the critical 'failed-fibre' cluster. A simple fracture mechanics argument which relates the critical fibre cluster size to the material strength is proposed to account for our data.

The nanoscale deformation and fracture mechanisms of parallel fibered bone are investigated using a novel combination of in-situ tensile testing to failure combined with high brilliance synchrotron X-ray scattering. The technique enables the simultaneous measurement of strain at two length scales - in the mineralized collagen fibrils (∼100 nm diameter) along with the macroscopic strain (∼1 mm diameter). Under constant rate tensile loading, we find that fibril strain saturates beyond the macroscopic yield point of bone at ∼0.5%, providing a correlation between the failure mechanisms at the nanoscale and the bulk structural properties. When bone stretched beyond the yield point is unloaded back to zero stress, the fibrils are contracted relative to their original state. We examine the findings in the context of a fiber - matrix shearing model at the nanometer level.

Microscale aggregate formation, resulting from high intrinsic filler attractions, is one of the major issues in nanocomposite preparation and processing. Herein, the dispersive effects achieved by a wide range of surface-active agents, as well as surface oxidation and functionalization, are investigated. The aim of our research is to form a uniform, multiwalled carbon nanotube (MWNT) distribution in water-soluble (poly(ethylene glycol)) and water-insoluble (polypropylene) polymers. In order to understand the surface-charge-related stability of the treated nanotubes solutions, zeta-potential measurements are applied. Quantification of the state of the MWNT dispersion is derived from particle-size analysis, while visual characterization is based on optical and electron microscopy. To estimate the nucleating ability of the surface-modified carbon nanotubes, the temperature of crystallization and the degree of crystallinity are calculated from differential scanning thermograms. Finally, we suggest general guidelines to produce uniform MWNT dispersions using a dispersive agent and/or surface treatment in water-soluble and water-insoluble polymers.

Individual multiwalled carbon nanotubes grown by chemical vapor deposition (CVD) were tensile tested within the chamber of an electron microscope using an atomic force microscope-based technique. Weibull-Poisson statistics could accurately model the nanotube tensile strength data. Weibull shape and scale parameters of 1.7 and 109 GPa were obtained. The former reflects a wide variability in strength similar to that observed for high-modulus graphite fibers, while the latter indicates that the irregular CVD-grown tube wall structure requires, in some cases, higher breaking forces than more regular tube wall structures. This apparent strengthening mechanism is most likely caused by an enhanced interaction between the walls of the nanotube.

Deformation mechanisms in bone matrix at the nanoscale control its exceptional mechanical properties, but the detailed nature of these processes is as yet unknown. In situ tensile testing with synchrotron X-ray scattering allowed us to study directly and quantitatively the deformation mechanisms at the nanometer level. We find that bone deformation is not homogeneous but distributed between a tensile deformation of the fibrils and a shearing in the interfibrillar matrix between them.

Nanocomposites based on semi-crystalline poly(vinyl alcohol) (PVA) and well-dispersed chemically functionalized single-walled carbon nanotubes are combined through simple mixing. The interaction between the nanotubes and the polymer matrix is studied using optical and thermal methods. Significant enhancement of the mechanical properties is obtained for the functionalized-nanotube-based composites. These results imply that promoting nanotube dispersion and strong interfacial bonding through adequate functionalization of nanotubes improves the load transfer from the matrix to the reinforcing phase.

Individual multiwalled carbon nanotubes were controllably wetted by polyethylene glycol, glycerol, and water. A Wilhelmy force balance approach was used to calculate contact angles at the nanotube-polyethylene glycol and nanotube-glycerol interfaces, allowing examination of the contact angle dependence on the nanotube diameter. Water, however, exhibited a significantly larger interaction with the nanotube, which could only be explained by allowing for internal wetting of the open carbon nanotube structure. This internal wetting angle is smaller than the external one.

The wetting properties and surface characteristics of individual carbon nanotubes are elucidated by immersing the nanotube into various organic liquid. The resultant force acting on the nanotube can be used to evaluate a liquid contact angle at the nanotube surface from classical methods. This technique was shown to be accurate enough to discern differences in wetting behavior due to both structural and chemical changes in the nanotube structure.

Pullout experiments were performed at the nanoscale using an atomic force microscope, to assess the interfacial adhesion between multi-walled carbon nanotubes and a matrix of polyethylene-butene. Fracture energy for the nanotube-polymer interface was calculated from the measured pullout forces and embedded lengths. The results suggest the existence of a relatively strong interface, with higher fracture energy for smaller diameter nanotubes.

Carbon nanotubes (CNTs), whether single- or multi-walled (SWNT or MWNT, respectively), have, in an unparalleled fashion, grabbed the attention of both researchers and business leaders within the polymer community. The vast potential afforded by the unprecedented combination of mechanical, electrical, and thermal properties within one nanoscale additive opens new vistas for commodity plastics, elastomers, adhesives, and coatings, as well as new specialty systems with never-before-realized combinations of material properties within a processible plastic or fiber1, 2, 3

Stepped conical structures have been produced at the surface of poly(ethylene glycol) by contacting a single, relatively short carbon nanotube attached to an AFM tip with the molten polymer surface, followed by polymer cooling. Cooling of the polymer melt in the nanotube vicinity is the most likely mechanism for these ziggurat-like structural formations. Simple heat transfer calculations confirm the effect of the nanotube length on the propensity for local solidification of the polymer.

Two methods are described for experimental determination of interfacial strengths of carbon nanotubes (CNT) in a polymer matrix. These SPMbased techniques allow realtime monitoring of the dynamics of a single pullout event. A comparison and contrast of these different techniques is presented here. Correlations found with the tube diameter give insights as to the nature of the CNTpolymer interaction.

The measurement of the force required to separate a carbon nanotube from a solid polymer matrix was discussed. The reproducible nanopullout experiments were performed by using atomic force microscopy during the analysis. The results showed that the polymer matrix in close vicinity of the carbon nanotube was able to withstand stresses, causing considerable yield in a bulk polymer specimen.

An experimental technique for probing the adhesion of carbon nanotubes to a polymer matrix was described. The nanotube-polymer interaction was quantified by detaching individual single-walled carbon nanotube bundles and multiwalled-carbon nanotube from an epoxy matrix using a scanning probe microscope tips. These experiments were used for the calculation of the nanotube-polymer interfacial shear strength.

The interfacial shear strength in single-wall nanotube-polymer composites is calculated using a traditional force balance approach, modified for a hollow tube, and the effect of varying some of the model parameters is examined and discussed. It is shown that high values of the interfacial shear strength (compared to those in current advanced fiber-based polymer composites) are in principle attainable. Defects in the hexagonal structure of a nanotube, which technically is a 'perfect' material, are expected to strongly reduce its strength and the model predicts that, as a consequence, a large variability should be experimentally observed in either the interfacial strength or the critical length of apparently identical nanotubes.

The existence of a size effect is an integral part of the physical phenomenon of brittle fracture. The literature dealing with the influence of fiber size on its ultimate strength has mainly focused on specimens whose failure is governed by a single category of flaw. In this work, we are discussing the size effect from a statistical point of view for fibers exhibiting multiple failure modes.

Carbon nanotube nanocomposites containing well-dispersed and aligned single- and multi-walled nanotubes have been fabricated by an extrusion method and their impact and hardness properties determined. The impact resistance of the polymer matrix has been improved considerably by the inclusion of the highly flexible and elastic nanotubes. The Knoop hardness reached a maximum at 90 deg to the orientation of the reinforcement, which demonstrated the direction of maximum strength, is in the extrusion flow direction. New techniques to investigate the adhesion of carbon nanotubes to a polymer matrix are described and recently produced data are presented. The measured interfacial shear strengths for multi-walled carbon nanotube nanocomposites is found to be very high, a result which is in agreement with earlier conjectures.

Single-wall nanotubes (SWNTs) embedded in polymer can be used as mechanical sensors because the position of the D^* Raman band of SWNTs is strongly dependent on the strain transferred from the matrix to the nanotubes. In order to detect the stress (or strain) information in specific directions, polarized Raman spectroscopy is used to select out the signal from the nanotubes that are parallel to the polarization direction. This method is demonstrated by measuring the stress distribution around a circular hole in the SWNT/polymer composites under uniaxial tension. Then the stress field in a polymer matrix in the vicinity of a single glass fiber is mapped on the micrometer scale. A stress concentration zone is observed around the fiber end. The importance of this technique and the measurements for composite design and micro-mechanical models is discussed briefly.

The stress field in a polymer matrix in the vicinity of a single glass fiber was mapped on the micron scale by using the strain-response of the Raman spectrum of single-wall carbon nanotubes (SWNTs) embedded in the matrix. A stress concentration zone is observed around the fiber end. Far away from the fiber, the applied stress value is recovered. The importance of this technique and the measurements for composite design and micro-mechanical models is discussed.

Raman spectral shifts of single-wall carbon nanotubes embedded in polymer systems were used to measure transitions in polymers. Glass-transition temperatures and secondary transitions were observed, and Raman spectroscopic data were compared with dynamic mechanical tests for a thermosetting and a thermoplastic polymer. The data confirm that the Raman spectral response of carbon nanotubes embedded in polymers is sensitive to polymer transitions.

We report for the first time details of the morphology of alpha isotactic polypropylene transcrystallinity induced by aramid fibers as determined by high spatial resolution X-ray diffraction. We suggest that the parent lamellae nucleate at the fiber surface with the crystallite a*-axes parallel to the fiber axis, twist one quarter turn about the parent a axis within an approximate distance of 25 mu -m and then continue to grow without further twisting. This result is unexpected since lamellar twisting has never been observed in pure a spherulitic polypropylene.

The relation between the mechanical properties and the lamellar morphology of the α-isotactic polypropylene transcrystalline interphase in a fiber composite was investigated by means of nanometric shear and directional indentation measurements, using scanning force microscopy. Measurements were performed along directions parallel and perpendicular to the transcrystalline growth direction. A key finding is the inversion of the shear modulus anisotropy ratio from 2.3 to 0.5 as the crystal grows away from the fiber. On the basis of this finding, an improved view of the hierarchical lamellar and molecular order of the transcrystalline layer is proposed.

The ability of the microbond technique to characterise changes in the physico-chemical structure of the interface between fibre and matrix has been checked using eight epoxyde/glass fibre systems differing by their matrix chemistry, fibre surface treatments, and fibre diameter. It has been shown that the widely used average IFSS method can lead to biased results. The test is now considered as giving mode I or mixed-mode properties of the interface and not only a mode II interfacial toughness or interfacial shear strength (IFSS). Energy approaches are thus be preferred to stress criterion models. The suitability of six theoretical models was checked. Difficulties were found in determining a parameter or method effectively representative of the physico-chemical structure of the interface. The model providing the most reliable results was that of Scheer and Nairn. Significant plastic flow of the polymeric droplet was observed, leading to a questioning of the hypotheses of ideal elastic components.

The components that contribute to Raman spectral shifts of single-wall carbon nanotubes (SWNT's) embedded in polymer systems have been identified. The temperature dependence of the Raman shift can be separated into the temperature dependence of the nanotubes, the cohesive energy density of the polymer, and the buildup of thermal strain. Discounting all components apart from the thermal strain from the Raman shift-temperature data, it is shown that the mechanical response of single-wall carbon nanotubes in tension and compression are identical. The stress-strain response of SWNT's can explain recent experimental data for carbon nanotube-composite systems.

Specific peaks of the Raman spectrum of single-wall nanotubes shift significantly upon immersion of the tubes in a liquid, relative to the corresponding peaks in air. This observation means that nanotubes are sensitive to molecular forces, and is interpreted by relating the corresponding molecular strain to a thermodynamic parameter, the cohesive energy density (or more loosely, the surface tension) for a range of liquids. We find that nanotubes deform by a different amount for each liquid. Calibration of this phenomenon enables the construction of a compressive stress-strain curve for carbon nanotubes.

Why does a bone fracture? We should be able to answer that in a nontrivial manner, but we cannot. This is an embarrassing admission in an age when treatments are available that can reverse the tendency toward bone loss and thus lessen the danger of fracture. Furthermore, if we knew much more about the relation between the material bone and its fracture properties, we might be able to optimize treatments to improve the quality of the material, or at least predict when the chance of a fracture increases, taking into account also the state of the material. We have been working on aspects of this problem for 15 years. We were recently much humbled by some observations we made,2 which illustrate just how little is still known about the relations between bone microstructure and its mechanical properties. As these results are probably of interest to many in the bone community, and especially researchers focussing on osteoporosis, we thought it would be appropriate to briefly summarize the results and highlight major open questions. [First paragraph]

Raman spectroscopy was used to compare the structural effects on single-walled carbon nanotubes of pressures due to the cohesive energy of liquid media with the effects of an externally applied macroscopic pressure. Results were very similar, showing that the interpretation of the cohesive energy density as an internal pressure is physically realistic.

High hydrostatic pressures were applied to single-wall carbon nanotubes by means of a diamond anvil cell (DAC), and micro-Raman spectroscopy was simultaneously used to monitor the pressure-induced shift of various nanotube bands. The data confirm recent results independently obtained from internal pressure experiments with various liquids, where the peak shifts were considered to arise from compressive forces imposed by the liquids on the nanotubes. It is also shown that the nanotube peak at 1580 cm^-1 (the G band) shifts linearly with pressure up to 20 000 atm and deviates from linearity at higher pressure. This deviation is found to be coincident with a drop in Raman intensity for the disorder-induced peak at 2610 cm^-1 (the overtone of the D* band), possibly corresponding to the occurrence of reversible flattening of the nanotubes. The independent results presented here confirm the potential of nanotubes as molecular sensors.

The flexural modulus and work-to-fracture properties of circumferential lamellar bone from baboon tibia are presented, based on experiments with miniature cantilever bending specimens. Data is provided for specimens in three orthogonal directions that show marked anisotropy. The advantages of such miniature specimens (about 150 μm in diameter and 2 mm long) include the possibility of sampling very small volumes within a heterogeneous structure such as osteonal bone, or studying biological materials that are not available in large enough volumes for conventional mechanical analysis.

A scanning force microscope was fitted with an elongated, blade-like tip, with which nanoindentations were performed in the transcrystalline isotactic polypropylene phase grown from the surface of high-modulus carbon fibers. The anisotropic Young's modulus was evaluated by measuring the force-penetration curve of the indentation and the tip topography, as a function of the indentation depth. The modulus is 1.6-3 times higher when the longer axis of the indenting tip is perpendicular to the transcrystalline growth direction than when it is parallel to that direction. We discuss possible options for the lamellar arrangement in a transcrystalline isotactic polypropylene layer and, based on the present experimental data, propose a most likely model.

We report the first observation of the formation of damage doublets in adjacent carbon nanotubes embedded in a polymer matrix. Such damage clusters are comparable to those arising in fiber-reinforced composite materials as a result of the redistribution of stress from a failed fiber to its unfailed adjacent neighbors. This observation may help shed light on fracture nucleation and growth in nanotube-based composites.

Compressive deformation was induced in single-wall carbon nanotubes (SWNTs) embedded into a polymer matrix by cooling a specimen to 81 K. The Raman D^*-band frequency shift was found to depend on the laser excitation wavelength. Overall, the observations were interpreted in terms of a possible correlation between elastic properties and geometry of carbon nanotubes.

Single-fiber composite experiments designed to assess the stress transfer ability of the interface between a commercial epoxy resin and optical fibers have been conducted. Unconventional fragmentation patterns were observed, which demonstrate the presence of dynamic energy release processes. Such dynamic contributions, which are not easily seen (if at all) in fragmentation tests with thinner fibers (such as carbon, glass, and kevlar), may have to be accounted for in the theoretical modeling of the test.

We report the observation of single nanotube fragmentation, under tensile stresses, using nanotube-containing thin polymeric films. Similar fragmentation tests with single fibers instead of nanotubes are routinely performed to study the fiber-matrix stress transfer ability in fiber composite materials, and thus the efficiency and quality of composite interfaces. The multiwall nanotube-matrix stress transfer efficiency is estimated to be at least one order of magnitude larger than in conventional fiber-based composites.

The directional Young's modulus of transcrystalline isotactic polypropylene grown from the surface of high modulus carbon fibers has been measured by two types of Knoop microindentation experiments. First, static indentation tests were performed and a new data reduction procedure was used to translate the Knoop hardness data into modulus values. The technique assumes that the extent of elastic recovery of a Knoop indent is linearly related to the modulus-to-hardness ratio. The Knoop tip is sensitive to material anisotropy, which allows us to generate data for the different lamellar directions. The hardness and Young's modulus of the transcrystalline layer are found to be higher by up to 30% when the longer diagonal of the probing Knoop tip is perpendicular to the transcrystalline growth direction. Second, continuous indentation tests are performed and a different data reduction technique is used, which leads to similar results. The microindentation approach used here is found to be particularly advantageous for the assessment of Young's modulus of small-scale polymeric regions or when only small polymeric specimens are available.

Experimental observations of various deformation and fracture modes under compression of single multiwalled carbon nanotubes, obtained as a result of embedment within a polymeric film, are reported. Based on a combination of experimental measurements and the theory of elastic stability, the compressive strengths of thin- and thick-walled nanotubes are found to be about 2 orders of magnitude higher than the compressive strength of any known fiber.

Unidirectional plane-parallel (UPP) platelet-or ribbon-reinforced composites have three orthogonal planes of symmetry. Their elastic behavior is characteristic of three-dimensional (3D) orthotropic materials, i.e. they possess nine independent elastic constants. Unlike the situation in unidirectional fiber composites, in which extreme behavior is observed along two orthogonal directions, the elastic constants (such as Young's modulus) along three orthogonal directions can differ significantly. An approach is proposed for the calculation of the Young's moduli of such 3D composite materials. The specific cases of unidirectional platelet- and ribbon-reinforced composites are presented as examples of particular relevance to advanced composite structures of present-day interest. While fiber-reinforced composites show lower specific strength and stiffness properties in directions perpendicular to the oriented fiber direction, UPP composites reinforced with platelets or ribbons offer superior stiffness properties in the plane of the lamina.

A theoretical model for built-in residual thermal stresses in three concentric, transversely isotropic, cylinders is presented. The scheme is an extension of earlier work by Nairn, who considered the case where only the central cylinder possesses such particular form of orthotropy, the two other (external) cylinders being isotropic. The generalization proposed here enables study of the thermal stresses in fiber-reinforced composite materials containing transversely isotropic interphases and matrices. In particular, the longitudinal thermal stress present in the fiber prior to a single-fiber fragmentation experiment is found to be compressive in nature, and in some cases is high enough to induce extensive fiber fragmentation. The number of such pre-existing breaks varies strongly as a function of the Weibull shape parameter of the fiber compressive strength distribution, and of the fiber volume fraction. Alternatively, the fiber Weibull shape parameter in compression may be predicted from fitting procedures, using experimental results that include the number of fiber breaks versus either the thermal decrement or the fiber volume fraction. General implications for high-fiber-content composites are discussed. The effect of interface thickness on the residual thermal stresses is discussed for the particular case of transcrystalline interphases, with assumed morphologies for α and β isotactic polypropylene.

A study of the thermomechanical stability of the fibre-matrix interphase in carbon/epoxy composites has been carried out. The thermodynamic work of adhesion has been evaluated at room temperature by wetting measursments. The interfacial shear stress transfer level 〈τ〉 for sized and desized carbon fibre has been measured as a function of temperature by means of a single-fibre fragmentation test. As the test temperature increased 〈τ〉 values were found to decrease, with values being higher for the desized carbon fibre. The dependence of interfacial shear stress transfer on bulk matrix mechanical properties (modulus and shear strength) has also been discussed. Dynamic mechanical measurements performed on single-bundle composites confirmed the better thermomechanical stability of the desized fibre interphase.

The residual stresses in both thermosetting and thermoplastic single-fibre composites have been experimentally evaluated by means of an original technique based on the continuous monitoring of the fragmentation test performed at various temperatures. The difference between the strain at the break of a single fibre in air and one embedded in a polymeric matrix has been measured as a function of temperature. By considering the compressive fibre modulus this strain difference has been converted into fibre compressive stresses related to the matrix thermal shrinkage after curing of the samples. In fact, as the test temperature increased, the thermal compressive stresses decreased until a zero value was obtained, corresponding to a so called "stress free temperature", equal to the curing temperature for amorphous thermosetting matrix composites or equal to the matrix melting temperature for semicrystalline-thermoplastic matrix composites. The experimental results have been compared with data obtained from a theoretical model and a good agreement was found especially if the temperature dependence of the matrix Young's modulus and matrix thermal expansion coefficient are accounted for in the computation.

A continuous fragmentation test, using thermal stresses, has been developed to determine the compressive strengths and Weibull parameters required to characterize the strength-length dependence of carbon fibres from a single test procedure. The onset of thermal stress in the fibre is determined in sit u and for amorphous systems is commensurate with the glass transition temperature of the microcomposite matrix. It is shown that the compressive strengths are considerably lower than the associated tensile strengths for all the fibres tested and that an electrochemical oxidation surface treatment and a polytetrafluoroethylene coating do not significantly affect the compressive strengths or compressive Weibull shape parameters with respect to the unmodified fibres. The mechanisms of stress transfer have been investigated and a compressive stress profile has been proposed that can determine the interfacial shear strength from fundamental scientific principles. The temperature dependence of the interfacial shear strength is investigated for carbon-fibre-polycarbonate microcomposites and the values obtained are concordant with a system that has weak interfacial bonding.

Functionalized isotactic polypropylenes have been synthetized in order to be used as coupling agents on glass fibers associated with a polypropylene matrix. This new way of interface toughening requires a grafting of such polymeric chains on the glass surface and a crystallization of these grafted chains at the same time in the same crystals as the polypropylene matrix (co-crystallization). Using a Ziegler-Natta polymerization., copolymers based on propene and two different types of dienes have been prepared. Such a synthesis allows one to change the functionality and the position of double bonds used to introduce the silane functions. A Speier hydrosilylation was used for this purpose with two kinds of silanes. Silane-functionalized oligopropenes were also prepared in order to compare the effect of the position of the silane functions on the glass/polypropylene adhesion : side chain or chain end positions. The characterization of these functionalized polypropylenes is described. Their ability to be grafted on a glass surface was checked by means of wetting measurements after extractions according to the hydrophobic nature of the polypropylene and the hydrophilic character of the glass. The co-crystallization of the pure and functionalized polypropylenes was demonstrated on blends. Two silane functionalized-PP were selected to study the glass fiber/PP adhesion by means of the microdroplet test.

By coating glass fibers with the appropriate nucleating agent, transcrystallinity can be generated in polypropylene/glass composities. Transcrystallinity can consist either of the alpha (monoclinic) or beta (hexagonal) crystal structure. Through the use of directional solidification, the transcrystalline morphology can be duplicated in polypropylene films on a level large enough for mechanical and morphological study. Permanganic etching and subsequent electron microscopy reveals that lamellar orientation in alpha transcrystallinity differs significantly from the beta form. Alpha transcrystallinity consists of lamellae which are edge-on relative to the polypropylene film thickness, while beta transcrystallinity consists of lamellae which are primarily flat-on. This difference in morphology results in significant variations in mechanical properties and damage mechanisms.

We present new theoretical and experimental results which demonstrate that the degree of fiber-matrix bonding can be quantified by means of the interface energy for the initiation of debonding, rather than by using a stress-based interfacial parameter. A one-dimensional model for the energy necessary to initiate/nucleate an interfacial crack from its associated transverse fiber break during a single fiber fragmentation test is proposed. The interface energy for the initiation of debonding is shown to be a function of the fiber and matrix geometrical and material characteristics, and of the initial debonding length. The validity of the approach is demonstrated in the case of fragmentation of sized and unsized E-glass fibers embedded in an UV-cured polymeric matrix.

Functionalized polypropenes have been synthesized to be used as coupling agents at the glass fiber/polypropene interface. The cocrystallization of this coupling agent and the matrix is studied by DSC and WAXS and in an other hand by a Monte Carlo simulation. The required characteristics of the functionalized polypropene cocrystalliize with the PP matrix are discussed. In a second part, the ability of the selected polymers to be grafted on a glass substrate is studied. The influence of the introduction of such coupling agents at the interface is studied in microcomposites.

Zioupos and Currey criticize our study in three different respects. The first point of contention arises from a misunderstanding of the text, which could probably have been more clearly written. We modelled lamellar bone that is folded into cylinders. This could be the midshaft region of the long bone of a small animal, mature Haversian bone, or that part of fibrolamellar (plexiform) bone that is cylindrical (the primary osteons). We did indeed compare our theoretical data to the experimental data of Bonfield and Grynpas (1977), who used bovine fibrolamellar bone. Zioupos and Currey are correct in pointing out that this is by no means all composed of cylindrical bone. We compromised in this way in order to compare our data to that of others, who also attempted to model the same set of experimental data. We clearly stated, however, that the true test must await more experimental measurements of the type performed by Bonfield and Grynpas (1977), but under conditions where the bone is carefully oriented in all three dimensions with respect to its structure (our study, p. 1318). [First paragraph]

The thermal residual stress induced in the fiber during specimen preparation, prior to a single-fiber fragmentation experiment, is studied using various theoretical schemes. It is found that the longitudinal component of this stress is compressive in nature, and that in some cases it is high enough to induce breaks in the fiber. The number of such pre-existing breaks may be calculated under certain conditions, one of them being that the Weibull shape parameter of the fiber compressive strength distribution be known. Experimental data produced with high-modulus graphite/polypropylene single-fiber composites are used to substantiate the arguments presented. Implications for high fiber content composites are discussed.

The work presented in the present paper focuses on the interface in Kevlar 49-epoxy composites. Experimental results for the interfacial shear strength obtained using the microbond test by means of different loading configurations (parallel plate loading, parallel conical plate loading, circular loading), are presented and compared. Contrasting with recent finite-element predictions proposed in the literature, the interfacial shear strength is found to be altogether insensitive to the type of microbond loading configuration. A comparison of the results obtained using two micromechanical tests (microbond and fragmentation) is performed. The interfacial shear strength results obtained by means of the fragmentation test are found to be higher by a factor of about 50% than those obtained by means of the microbond test. A possible explanation for this difference is proposed and discussed, and the value of the "true" interfacial shear strength is conjectured to fall between the values measured by these two tests. The effect of fibre surface chemistry modification (surface desizing) is probed by surface-sensitive techniques (XPS-ESCA and contact angle measurements from droplets) and by micromechanical testing techniques. Surface-sensitive techniques and micromechanical testing provide compatible information for the Kevlar-epoxy system studied here, and the knowledge of the chemical characteristics of the fibre surface can therefore be used as a means of predicting the interfacial shear strength.

This study deals with the effect of a transcrystalline LLDPE (linear low-density polyethylene) layer grown on Spectra 1000 UHMWPE (ultrahigh molecular weight polyethylene) fibres. Chemical similarity between the fibre and the surrounding melt does not promote transcrystallinity as no transcrystalline microstructure appears from the surface of as-received Spectra 1000 UHMWPE fibres. However, oxygen plasma treatment of the UHMWPE fibres yields a degree of surface roughness that appears to promote easy nucleation and growth of LLDPE transcrystallinity. The kinetics of transcrystalline growth were investigated quantitatively. The growth rate increased by a factor of about 12 for a 10°C increase in supercooling, and at 105°C the maximum observed thickness of the transcrystalline layer was about one fibre diameter. The induction time was found to decrease as the crystallization isotherm increased. We discuss the possibility of using surface energy parameters to define a better criterion for the nucleation of transcrystallinity from the UHMWPE fibre substrate. Preliminary data were generated for the interfacial mechanical shear strength by means of the microbond test. It is conjectured that the combined effects of a thermal treatment and the presence/absence of a transcrystalline layer might produce significant changes in the interfacial shear strength, as illustrated here by a 43% increase observed with specimens subjected to different thermal treatments.

The theme of microphenomena which was adopted for the conference implies that to understand microphenomena is to get into the very heart of the science of composite materials. The conference emphasised the microscopic scale of the study of composites by focusing on microphenomena and on microcomposites. Every engineering application of these unique materials induces in them a sequence of microprocesses that jointly determine their global behaviour. Only by thorough understanding of each of those microprocesses and of their mutual interactions could the appropriate engineering and design decisions be made. The best tool for investigating microprocesses is perhaps the study of microcomposites, which could be regarded as a 'test tube' by which fundamental scientific issues could be examined and compared with theories developed for ideal 'textbook' composites.

Microcomposites of single-pitch-based carbon fibre reinforced J-Polymer are employed to investigate the mechanical role of the fibre matrix transcrystalline interphase. The transcrystalline interphase in this semicrystalline thermoplastic system is varied by changing the crystallization kinetics, as determined by the thermal history. The kinetics of transcrystallization under isothermal conditions are presented as an example, and are used to form different transcrystalline interphase thickness. These affect the fibre fragmentation process that occurs while the specimens are cooled from the crystallization temperature to room temperature. This fragmentation process is attributed to residual thermal stresses, which can be calculated by assuming that it is controlled by Weibull statistics. Tensile loading of either longitudinal or transverse microcomposite specimens results in additional fragmentation, the extent of which is determined jointly by the thickness of the transcrystalline layer and by the yield strain of the matrix.

Transcrystalline microstructures are normally not observed at the interface between E-glass fibers and an isotactic polypropylene matrix, unless mechanical translation is applied to the fiber while it is in the supercooled polymer melt. We demonstrate here that transcrystallinity can form at the surface of E-glass fibers if appropriate nucleating agents are used to coat the fibers. These agents can nucleate either the α (monoclinic) or β (hexagonal) crystal forms of polypropylene. Single-fiber composite experiments were performed to assess the effect of transcrystallinity on matrix deformation. The preliminary results presented here reveal the occurrence of a previously unreported damage mechanism by which interlamellar fractures form preferentially at the interface well before any bulk matrix damage occurs. The density of this damage zone is higher in transcrystallinity of the β crystal form than of the α form, although it was found that in the α form the damage can propagate into the matrix. The occurrence of this damage mechanism suggests that toughness increases may potentially be obtained by careful design of the interfacial transcrystalline region in E-glass/polypropylene composites.

A discrete model of springs with bond-bending forces is proposed to simulate the fracture process in a composite of short stiff fibers in a softer matrix. Both components are assumed to be linear elastic up to failure. We find that the critical fiber length of a single fiber composite increases roughly linearly with the ratio of the fiber elastic modulus to matrix modulus. The finite size of the lattice in the direction perpendicular to the fiber orientation considerably alters the behavior of the critical length for large values of the modulus ratio. The simulations of the fracture process reveal different fracture behavior as a function of the fiber content and length. We calculate the Young's modulus, fracture stress, and the strain at maximum stress as a function of the fiber volume fraction and aspect ratio. The results are compared with the predictions of other theoretical studies and experiments.

Carbon/J-polymer single fibre composite samples were tested under tensile conditions with the fibre direction perpendicular to the tensile loading axis. The Poisson ratio effect induced a compression strain field in the fibre, resulting in a fragmentation phenomenon similar to that observed in a fibre subjected to tensile loading. This observation introduces a novel technique for the measurement of the compressive strength of single fibres, calculated either from the stress at first break, or from the Weibull scale parameter obtained from the fragmentation data produced at various stress levels. The special sample loading configuration used here also provides the first measurement of the effect of the length of the fibre on its compressive strength value.

This paper demonstrates the usefulness of careful experimental work with model composite materials, such as thin polymeric films in which single fibers are accurately positioned, in at least two respects: to assess the validity of a theory for a given physical property, and to accurately probe the effects of various parameters on the behavior of composites. Working with such model composites has obvious advantages, such as the full control of experimental parameters, the possibility of introducing perturbative effects in a controlled way, and the possibility of verifying theoretical models in the range of low fiber content. Indeed, macroscopic composite materials contain various types of defects and perturbative effects, such as fiber misalignment or slack, fiberpoor regions, voids, etc., which bias any quantitative assessment of mechanical and physical properties, and preclude the accurate verification of theoretical schemes. One difficulty in working with microcomposite models, also recalled here, is the need for an appropriate \u201cscalingup\u201d procedure to the level of macroscopic composites.

A brief reply to the Comment is provided. Both the procedure I suggested and the tables presented by Carroll [Textile Res. J. 47, 56 (1977)] allow easy and fast estimation of contact angles.

The characterization of the physicochemical nature of interfaces is a key problem in the field of advanced fibrous composites. The macroscopic regime contact angle, which reflects the energetics of wetting at the solid-liquid interface, is difficult to measure by usual methods in the case of very thin cylindrical fibers, but it may be calculated from the shape of a liquid droplet spread onto a cylindrical monofilament using a method developed by Yamaki and Katayama [J. Appl. Polym. Sci. 19, 2897 (1975)], and B. J. Carroll [J. Coll. Interf. Sci. 57, 488 (1976)]. Unfortunately, measurements of the contact angle based on this method are, so far, unable to provide an accuracy of better than about 5°. In the present article two simple extensions of the method of Yamaki and Katayama and Carroll, are presented, from which highly accurate values of the contact angle may be obtained. This is demonstrated experimentally from the spreading of glycerol droplets on carbon fibers and epoxy droplets on aramid fibers.

The quasi-static deformation and fracture modes of several types of fibrous composite materials are studied from a fundamental viewpoint using a new experimental approach. Microcomposite monolayers, consisting of single fibres accurately positioned into a thin poly-meric matrix, were manufactured using a specially developed technique, and tested for strength by means of a custom-made miniature tensile testing machine. The materials used were E-glass, and Kevlar 29, Kevlar 49 and Kevlar 149 para-aramid fibres, and a room-temperature curing epoxy resin. The tensile testing machine was fitted to the stage of a polarized light stereozoom microscope and the fracture process was recorded both via a standard 35 mm camera and a colour video camera. The fibre content of the first generation of micro-composite monolayers used in this work was low (

First results are presented concerning the elastic and ultimate mechanical behaviour of p(MMA) bone cement reinforced with as-received and surface-modified Spectra 900 polyethylene fibres. Even though the surface chemistry and reactivity of the fibres was modified, the surface oxidation and surface grafting treatments of the polyethylene fibres apparently did not significantly affect the mechanical properties of the polyethylene-reinforced p(MMA) bone cement or improve the interfacial bonding. This may be attributed to the rather unfavourable area-to-volume ratio of PE fibres for such treatments, as well as to the necessarily low content of PE fibres in the bone cement which does not allow a clear differentiation between the various samples.

A study of the fracture behavior of poly(methyl methacrylate) (PMMA) bone cement reinforced with short ultrahighmolecularweight polyethylene (Spectra 900) fibers is presented. Linear elastic and nonlinear elastic fracture mechanics results indicate that a significant reinforcing effect is obtained at fiber contents as low as 1% by weight, but beyond that concentration a plateau value is reached and the fracture toughness becomes insensitive to fiber content. The flexural strength and modulus are apparently not improved by the incorporation of polyethylene fibers in the acrylic cement, probably because of the presence of voids, the poor mixing practice and the weakness of the fiber/ matrix interfacial bond. The present polyethylene/PMMA composite presents several advantages as compared to other composite cements, but overall the mechanical performance of this system resembles that of Kevlar 29/PMMA cement, with a few differences. Scanning electron microscopy reveals characteristic micromechanisms of energy absorption in Spectra 900/PMMA bone cement. A scheme for the strength of random fiberreinforced composites, which is a simple extension of the Kelly and Tyson model for the strength of unidirectional composites, is presented and discussed. Young's modulus and the fracture toughness results are discussed in the framework of existing theories. More fundamental modeling treatments are needed in terms of fracture micromechanisms to understand and optimize the various mechanical properties with respect to structural parameters and cement preparation technique.

Recent results on the fracture behaviour of advanced fibers and model composite materials (microcomposites) are presented. The fiber data utilized are strength results for polydiacetylene whiskers and Kevlar 149 fibres. The probabilistic approach adopted for the effect of fibre diameter on strength is shown to be at least as appropriate as well-known empirical and LEFM-based schemes. A modified Poisson/Weibull scheme deals with a currently unsolved problem inherent to the classical Weibull distribution function, used as a Model for strength. Preliminary experimental results with model microcomposites (which consist of single fibers carefully placed within a matrix film using specially developed positioning techniques) are reviewed and, in particular, the usefulness of video/microphotographic techniques in the study of failure dynamics and fracture modes in composites is emphasized.

Heterophase or composite materials possessing a fibrillar structure arise frequently in nature as well as in a wealth of today's man-made objects and structures. The degree of adhesion between the fibrous component and the support matrix is known to be an important factor regarding the mechanical response of the material. Well-bonded highly fibrous heterophase materials possess improved elastic properties, as compared to the matrix, which are modeled by various theoretical treatments. For less frequently used weakly bonded composite materials, the theories are inadequate. Here we present a simple, approximate theoretical scheme for the case of low fibre content ( ≦ 20 pc by volume) composites with a weak interface among the constituents. Such composites are sometimes utilized for the fixation of prostheses in joint surgery. The proposed theoretical scheme appears to be appropriate as confirmed by experimental data.

A study of the fracture behaviour of Kevlar 29 reinforced dental cement is undertaken using both linear elastic and nonlinear elastic fracture mechanics techniques. Results from both approaches-of which the nonlinear elastic is believed to be more appropriate-indicate that a reinforcing effect is obtained for the fracture toughness even at very low fibre content. The flexural strength and modulus are apparently not improved, however, by the incorporation of Kevlar 29 fibres in the PMMA cement, probably because of the presence of voids, the poor fibre/matrix interfacial bonding and unsatisfying cement mixing practice. When compared to other PMMA composite cements, the present system appears to be probably more effective than carbon/PMMA, for example, in terms of fracture toughness. More experimental and analytical work is needed so as to optimize the mechanical properties with respect to structural parameters and cement preparation technique.