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Longitudinal tensile failure of unidirectional fibre-reinforced composites remains difficult to predict accurately. The key underlying mechanism is the tensile failure of individual fibres. This paper objectively measured the relevant input data and performed a detailed experimental validation of blind predictions of six state-of-the-art models using high-resolution in-situ synchrotron radiation computed tomography (SRCT) measurements on two carbon fibre/epoxy composites. Models without major conservative assumptions regarding stress redistributions around fibre breaks significantly overpredicted failure strains and strengths, but predictions of models with at least one such assumption were in better agreement for those properties. Moreover, all models failed to predict fibre break (and cluster) development accurately, suggesting that it is vital to improve experimental methods to characterise accurately the in-situ strength distribution of fibres within the composites. As a result of detailed measurements of all required input parameters and advanced SRCT experiments, this paper establishes a benchmark for future research on longitudinal tensile failure The research leading to these results has been conducted in the framework of the FiBreMoD project and has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 722626. Compressive tests on the neat epoxy resin were performed at the mechanical lab of the Institute of Mechanics, Materials and Civil Engineering (IMMC), Université Catholique de Louvain (UCLouvain) with the kind support of J. Chevalier and T. Pardoen. M.J. Emerson from QIM, Technical University of Denmark (DTU) is thankfully acknowledged for the handson training sessions and support of the Insegt Fibre segmentation toolbox. Support of C. Schlepütz from TOMCAT beamline at Swiss Light Source for optimising the acquisition settings during the beamtimes under proposal IDs 20161157 and 20171494 is thankfully acknowledged. J.M. Guerrero would like to acknowledge the grant BES-2016-078270 from the ‘Subprograma Estatal de Formación del MICINN’ cofinanced by the European Social Fund, and all authors from the University of Girona acknowledge the funding from the Spanish project RTI2018-097880-B-I00. The work of M. Hajikazemi forms part of the research programme of DPI, project 812T17. S. Pimenta acknowledges the funding from the Royal Academy of Engineering in the scope of her Research Fellowship on “Multiscale discontinuous composites for high-volume and sustainable applications” (2015-2019). R.P. Tavares and P.P. Camanho would like to thank the financial support provided by FCT - Fundação para a Ciência e a Tecnologia through National Funds in the scope of project MITP-TB/PFM/0005/2013

A novel modified dog-bone shape direct tensile test is proposed and compared with the wedge splitting test as well as Brazilian test. The acoustic emission and digital image correlation techniques are adopted for monitoring the entire fracture process. The tensile strength determined by Brazilian test and wedge splitting test are higher than that evaluated by the direct tensile test. For direct tensile test, the specimens lost their integrity immediately after reaching the tensile strength and its predominant fracture mode is tensile nature cracking. The wedge splitting test enables the stable crack propagation because of the development of remarkable fracture process zone, which contributes to the presence of intensive shear cracking. The fracture behaviour of Brazilian test has the material brittleness dependence. For brittleness reduced refractories, the quasi-stable failure process is noticed. The deviations of the tensile strength, fracture process and failure mode for the three Mode I tensile tests are attributed to the different mechanical principles, the geometry/size effect and heterogeneity effect.

Thermal induced cracks and the tensile strength variation of Eibenstock granite subjected to high-speed heating have been studied through lab testing. The Digital Image Correlation (DIC) technique was used for analysing the tensile behaviour of granite at high-speed heating rate and different temperatures. For samples heated to 800 °C with heating rates of 200 °C/min and 300 °C/min, thermal-induced macrocracks become visible to the naked eyes, while no obvious macrocracks are observed on samples heated according to ISO 834 curve. Compared to the influence of temperature, the selected heating rates only have a negligible impact on tensile strength and axial displacement. The DIC analysis shows the crack initiation and progressive crack propagation before the main crack is formed at failure. Compared to the samples heated up to 400 °C, the samples heated up to 800 °C show less brittle fracture behaviour, and the tensile strength is greatly reduced. The final failure patterns prove that different heating scenarios lead to different microcrack structures and consequently influence the failure mode at the macroscopic scale.

This paper investigates the role of dynamic stress concentrations, and of fracture mechanics-driven growth of critical clusters of fibres, on the longitudinal tensile failure of fibre-reinforced composites. For this purpose, we developed a semi-analytical fibre bundle model to simulate the longitudinal tensile failure of large composite bundles of continuous fibres. The model uses shear-lag to calculate the stress recovery along broken fibres, and an efficient field superposition method to calculate the stress concentration on the intact fibres, which has been validated against analytical and Finite Element (FE) results from the literature. The baseline version of the model uses static equilibrium stress states, and considers fibre failure driven by strength of materials (stress overload) as the only damage theory which can drive bundle failure. Like other models in the literature, the baseline model fails to capture the correct size effect (decreasing composite strength with bundle size) shown by experimental results. Two model variants have been developed which include dynamics stress concentrations and a fracture mechanics failure criterion respectively. To the knowledge of the authors, it is the first attempt in the literature to investigate these two effects in a fibre bundle model by direct simulation of large composite bundles. It is shown that, although the dynamic stress concentration significantly decreases the predicted bundle strength, it does not allow to predict the correct trend of the size effect. Finally, the results suggest that fracture mechanics may be the physical mechanism which is necessary to include to correctly predict the decreasing composite strength with bundle size shown by experimental results.

This work investigates the possibility of replication of natural rock specimens, which can be used to analyze rock mechanical behavior by subjecting a number of identical specimens to tensile tests and a variety of analysis methods. We compare the properties of fractures generated in artificial sandstone specimens to those generated in natural sandstone specimens. Artificial sandstone specimens, created using 3D additive manufacturing printing processes, were subject to tensile failure using the Brazilian test method and the results from these tests were compared to results from Brazilian tests conducted on natural sandstones. The specimens included two distinct types of synthetic rock, unaltered from the manufacturers typical process, and three natural sandstones. For each test, the loading history to failure of the specimens was recorded and the failure mode was confirmed using digital imaging techniques. In addition, three-dimensional images were taken of the fracture surfaces, which were then used to compare the geometric characteristics of all materials tested. The indirect tensile strength of the artificial sandstone specimens ranged between 1.0 and 2.8 MPa. Natural sandstone specimens with a wide range of indirect tensile strengths were tested for comparison. These included a strong sandstone, an intermediate sandstone, and a weak sandstone; which were found to have indirect tensile strength ranges of 10.5–25.5 MPa, 4.4–6.4 MPa, and 0.9–1.1 MPa, respectively. Digital image correlation confirmed that the artificial specimens generally failed in a tensile (mode I) fracture, similar to the natural specimens. Likewise, fracture surface roughness measures showed no clear distinction between weak natural and artificial sandstones. This indicates that there are distinct similarities between the fractures generated in the natural and artificial sandstones of comparable indirect tensile strengths. The three-dimensionally printed sandstone specimens are shown to exhibit indirect tensile strength, surface roughness and crack propagation behavior which resembles a weak natural sandstone.

This paper presents the determination of the crack resistance curve of the unidirectional carbon-epoxy composite material IM7-8552 for intralaminar fiber tensile failure under dynamic loading. The methodology, proposed by Catalanotti et al. (2014) for quasi-static loading conditions , was enhanced to high rate loading in the order of about 60 s −1 . Dynamic tests were performed using a split-Hopkinson tension bar, while quasi-static reference tests were conducted on a standard electromechanical testing machine. Double-edge notched tension specimens of different sizes were tested to obtain the size effect law, which in combination with the concepts of the energy release rate is used to measure the entire crack resistance curve for the fiber tensile failure mode. Digital image correlation is applied to further verify the validity of the experiments performed at both static and dynamic loading. The data reduction methodology applied in this paper is suitable for intralaminar fiber failure modes without significant delamination . Sufficient proof is given that quasi-static fracture mechanics theory can also be used for the data reduction of the dynamic tests. It is shown, that the intralaminar fracture toughness for fiber tensile failure of UD IM7-8552 increases with increasing rate of loading.

First, the Equivalent Material Concept (EMC), originally proposed by the author in 2012, is newly formulated in a way the effects of the plastic region ahead of the notch tip prior to the formation of crack at the notch tip on tensile strength of the equivalent material are taken into consideration. The new EMC is linked to the mean stress (MS) brittle fracture criterion to predict the load-carrying capacity (LCC) of some V-notched specimens reported in the literature, made of very ductile steel and subjected to pure mode I loading. It is revealed that the experimentally obtained LCCs could be well predicted by means of the new EMC-MS criterion, without needing elastic-plastic analyses or the failure criteria in the context of the elastic-plastic fracture mechanics.

From a macroscopic point of view, the dynamic tensile response of concrete is mainly due to the viscous behavior of the bulk material and inertia effects at multi-scale levels. It has been suggested that almost all of these mechanisms have to be considered as intrinsic material properties and explicitly included in the constitutive relations of the continuum model. It is discussed and demonstrated that the use of semi-empirical dynamic strength increase factor (DIF) functions to numerically describe rate effects do not characterize true constitutive relations of the material. A strain-rate dependent formulation is used to describe the strength and fracture energy increase of concrete under dynamic tensile loading conditions. However, instead of the commonly used e (instantaneous strain-rate) to update the constitutive law, an effective rate (R) is considered. With this new concept a time scale is introduced in the constitutive law which restrains the ‘evolution of rate’, to represent the inherent dynamic properties of concrete. This has a weak regularization effect and acts as a localization limiter. Mesh objectivity is recovered with the addition of a material length scale to the constitutive relations, here accomplished by an explicit stress-based nonlocal regularization scheme. Two sets of modified split Hopkinson bar tests are simulated for validation, using respectively notched and un-notched specimens. The results are objective and in good agreement with the experiments.

The influence of defect band on the tensile failure in high pressure die casting of AM60B magnesium alloy was studied by in situ scanning electron microscope. Results showed that the size and distribution of the externally solidified crystals (ESCs) had a significant influence on the tensile behavior induced by defect band in the specimen. In the specimen with few ESCs, the crack would initiate at the gas-shrinkage pore in the center of the specimen. Further crack propagation at the defect band was hindered by the surrounding dense microstructure, and the effect of defect band on the tensile failure was just reducing the efficient force bearing area. In the specimen with large and complex ESCs, the microstructure around the defect band would become coarse and comprised of large porosities with complex morphology. The defect band would act as crack initiation site and facilitate the crack propagation to both center and surface of the specimen, leading to the final fracture.

This study presents the use of the dynamic particle difference method (PDM) to analyze tensile failure in concrete subjected to high loading rates. In general, strong form-based meshfree methods suffer from limitations pertaining to the material modeling of concrete because concrete exhibits both softening and damage behaviors that initiate crack growth under an impact load. These methods are generally based on the direct discretization of the governing equations, such as Navier's equation, which involves second-order differentiation. However, conventional material models are based on the first-order derivatives of displacement. The newly developed dynamic PDM can effectively address the limitations of material modeling using a combination of first-order derivative approximations. This circumvents the requirement for high-order derivative approximations, which are essential in strong formulations, such as the finite difference method and point collocation method. The strain rate effect caused by an extremely high loading speed was successfully modeled by accurately reflecting the energy dissipations that result from the cohesive property of the concrete and brittle cracking. Although the developed method incorporates the elastic constitutive relation, it enables the effective modeling of these nonlinear effects. In addition, it can reduce the computational effort. The proportional damping algorithm simulates the effect of velocity in the equation of motion and the cohesion effect in the concrete material. The damage model and the visibility criterion adequately handle crack initiation and propagation in the concrete member. Furthermore, it is noteworthy that the final discrete forms of the dynamic PDM are similar to the integrands of the weak form in the conventional finite element formulation. We ascertained that the stiffness and mass proportional damping effects are related to the inertia and strain of the material, respectively. It was confirmed that the location and direction of crack propagation in concrete varied with the strain rate. Hence, the accuracy and robustness of the proposed method were successfully verified by simulation, and the strain-rate dependency of concrete fracture was efficiently simulated using the proposed method.

The focus of this paper is to understand the effect of voids on the damage mechanism of the composite under out-of-plane tensile load with different lay-up temperature conditions. A micro-scale model for ply-by-ply structure is established containing intralaminar and interlaminar domain in which voids are generated by a random irregular voids generation algorithm to explore the role of voids (intra-ply voids and inter-ply voids) on the transverse damage behavior of the composite. Then, a macro-scale model containing elements with and without void is established to capture crack propagation behavior in the specimen. Analysis of voids in variation layup temperatures are conducted experimentally and numerically. The results show that with the increasing lay-up temperature, the porosity increases from 1% to 4.5%, and the void distribution changes from the intra-ply voids dominant to the inter-ply voids dominant, while the out-of-plane tensile strength decreases by about 30%.

Powder metallurgical open-cell steel foams made of hollow struts are of great interest for impact resistance. Here we report experimental results of high-resolution characterisation of single struts, extracted from a commercially available steel foam. Synchrotron radiation X-ray microtomography reveals that the hollow struts are made of triplets of lens shaped rods, connected to each other along the long axis, appearing triangular in cross-sections. The struts have a non-uniform geometry and they include macro and micropores as well as high-density inclusion powder particles, poorly fused with the matrix. Micro-tensile testing showed that tensile failure is accompanied by longitudinal unzipping due to shear along the thinner material found near the hollow corners. Included, higher-density powder particles deflect crack propagation along the interface with the matrix, where much porosity is observed. Results of finite element simulations well match our mechanical tests, revealing plastic deformation due to bending and significant shear stresses arising between neighbouring rods within the same strut. The failure behaviour and the mechanical response of sintered struts in steel foams produced using non-uniformly coated polyurethane templates is the result of an interplay between the triplet geometry, sub-micron pores, precipitates and high-density inclusions. Keywords: Synchrotron radiation μCT, Metal foam, Micro-tensile testing, Finite-element simulation, Crack propagation, Microporosity

In this study, two experimental approaches are used to understand fracture mechanisms that govern transverse tensile failure of fiber-reinforced polymer composites at the microscale. These observations are used to directly measure, and indirectly estimate, the magnitude and scatter of the transverse tensile strength, YT, and the associated effective flaw size, a0. To this end, static three-point bend tests are performed on pristine and notched 90° unidirectional IM7/8552 carbon-epoxy samples. In the pristine specimen study, tensile microcracks are observed to initiate well before the ultimate failure load used to compute YT is reached. In the notched specimen study, a comparison is made between experimentally measured strengths with known notch lengths and a linear elastic fracture mechanics solution. The fracture mechanics solution significantly over-predicts the apparent YT for notch lengths less than a ply thickness, suggesting that this approach may not be appropriate for estimation of transverse tensile strength at the microscale. The observations made in this study suggest that YT may not be a true material property, but rather, a structural property dependent on specimen geometry and microstructural variability.

The material properties and failure modes of concrete depend on loading rate. For realistic simulation of concrete behavior under dynamic loading, it is essential that this rate dependency be considered. This paper proposes a three-dimensional lattice model—formed from rigid-body-spring elements—that incorporates the rate dependency of concrete strength, elastic modulus, and fracture energy. This is achieved by introducing a visco-elasto-plastic damage unit (formed from a combination of dashpots and Coulomb friction devices) into the rigid-body-spring elements. The results of simulations of elastic wave propagation and comparisons with experimental results reported in the literature validate the model for basic load patterns. Further, changes in the dynamic failure mode with differing loading rates, studied through simulation of the Split Hopkinson Pressure Bar test, are shown to be in agreement with experimental results obtained with respect to strength and fracture locations.

Understanding the effect of water saturation on dynamic failure of rocks is of great importance to tunnel excavation at water-rich coal mines and prevention of rock bursts by water injection. Dynamic Brazilian disc tests are performed to study mechanical behaviour of sandstones in this paper. The results indicate that water saturation significantly weakens the dynamic tensile strength of sandstones and increases the specimen strain at which the specimen fails. The damage degree of sandstones reduces gradually with increasing water contents. Failure of the sandstone specimen includes the crack initiation at the center of the specimen, macroscopic crack propagation, and stretch of the macroscopic crack through the specimen. In addition, parallel macroscopic crack propagation is found in the specimen with a low water content. From the observation of fracture sections, microstructures are compact in the specimen with high water contents. This is due to the swell of the kaolinite in the specimen after water saturation. The failure mechanism of microstructures is typical brittle failure in the specimen with a high water content, whereas ductile fracture is found in the specimen with a low water content. Different failure processes of microstructures lead to the differences between mechanical properties and macroscopic failure characteristics of the specimens with various water contents.

In this paper, a theoretical model is proposed to predict the initial failure location and the failure progression of the lattice structures made from the elastic-brittle bulk materials. The effects of size and topology on the tensile failure behavior of multiple representative lattice structures (2D auxetic, 2D diamond, 2D triangular1 and 2D triangular2), under various geometry design conditions (including cell topology, cell size and number of unit cells), are systematically investigated through the proposed analytical model. From the results, it was shown that the 2D bending-dominated structures with lower nodal connectivity (number of struts that meet in joints) (2D auxetic and 2D diamond) exhibited a relatively progressive crack propagation pattern, while the 2D stretching-dominated structures with higher nodal connectivity (2D triangular1 and 2D triangular2) appear to exhibit rather catastrophic brittle fracture failure. During the failure fracture propagation, the energy absorption of the 2D stretching-dominated structures were significantly higher than that of the 2D bending-dominated structures. Moreover, for all lattice designs, the tensile failure behaviors tend to converge to more consistent patterns when the unit cell numbers increase beyond certain limitations.

Tensile failure behavior of concrete invariably dominates the behavior of concrete specimens as well as structural elements and it is strongly affected by loading rate. The present study focuses on the effects of loading rate and heterogeneity of meso-/micro-structure on the failure pattern and the macroscopic mechanical properties of concrete. For simplicity, concrete is regarded as a two-phase composite composed of aggregate and mortar matrix at meso-scale. The damaged plasticity theory combined with strain-rate effect is employed to describe the dynamic mechanical behavior of mortar matrix, and the aggregate phase is assumed to be elastic. The dynamic tensile failure modes of a single-edge notched concrete specimen and the L specimen under different loading rates are numerically investigated. The simulation results indicate that dynamic failure pattern and the direction of crack propagation of concrete have pronounced loading rate sensitivity. With the increase of loading rate, the failure mode of concrete changes from mode-I to mixed mode. The more complex the meso-structure is, the higher the interaction intensity between the meso components has and the more complicated the crack paths are, resulting in a more obvious crack branching behavior. Furthermore, as loading rate increases much more branching cracks generate within concrete and the width of the damaged region increases, implying that the fracture process at relatively high strain rates requires more energy demand to reach failure. And this should be the main reason for the improvement of the dynamic tensile strength of concrete.

The emergence of a new class of cellular solids, i.e., nano- and micro-architected materials, poses the question of whether they can be characterized as a continuum solid. Extensive research has shown that these ultralight and strong structural metamaterials are particularly attractive for mechanically-demanding applications; yet their susceptibility to flaws, fracture behavior, and discrete-continuum duality remains relatively unexplored. In the course of this work, we report the fabrication and tensile-to-failure response of three-dimensional ceramic nanolattices, comprised of 50nm-thick alumina tubes that are arranged into periodic 5um-wide octet-truss unit cells, with and without pre-fabricated through-thickness center notches oriented at different angles to the loading direction. In-situ uniaxial tensile experiments revealed that for all notch orientations, failure always initiated at the notch root, as would be in a monolithic material, with the tube walls at nodal junctions fracturing first, followed by instantaneous crack propagation through the discrete lattice architecture along nodal planes orthogonal to the loading direction. Measured tensile strength of 27.4 MPa was highest for the unnotched samples and decreased systematically with the increase of notch orientation to its minimum of 7.2 MPa in the orthogonally-notched samples. We found the specific tensile strength of hollow-tube octet alumina nanolattices to be 4 times higher than what has been reported for architected and bulk materials at similar low densities. Three-dimensional finite element simulations closely reproduce the observed failure mechanism and trends in failure strength. A direct comparison is made between the experimental measurements, finite element simulations, and predictions of linear elastic fracture mechanics for a self-similar monolithic tensile samples made out of an ideally-brittle solid. Results are in good agreement with the scaling of failure strengths from classical mode I fracture criteria and suggest that trajectory of crack propagation can be adequately explained by considering the connectivity of the lattice architecture. These findings imply that the continuum nature of nano-architected materials offers predictability of failure stresses, which helps enable the development of advanced materials through informed architectural design.

This study numerically simulates strain-rate dependent transverse tensile failure of unidirectional composites. The authors’ previous study reported that the failure mode depends on the strain rate, with an interface-failure-dominant mode at a relatively high strain rate and a matrix-failure-dominant mode at relatively low strain rate. The present study aims to demonstrate this failure-mode transition by a periodic unit-cell simulation containing 20 fibers located randomly in the matrix. An elasto-viscoplastic constitutive equation that involves continuum damage mechanics regarding yielding and cavitation-induced brittle failure is used for the matrix. A cohesive zone model is employed for the fiber–matrix interface, considering mixed-mode interfacial failure. For the results, the relationship between failure modes and the strain rate is consistent with the authors’ previous studies.

The failure behaviour of heterogeneous rock specimens with prefabricated single crack under direct tensile loading is investigated numerically by using an elastoplastic cellular automaton (EPCA2D) model. The response of tensile failure behaviour of heterogeneous rocks is investigated when different crack lengths and inclinations are used. The results confirm that the tensile strength of rock specimens presents a tendency of reduction with the increment of crack length at same fixed crack inclination. However, this reduction changes non‐monotonically due to the heterogeneity of rocks. Only the precrack is longer enough can it be the main factor of rock failure. With the same crack length, but different crack inclinations, the tensile strength increases with the increment of crack inclination. With the increment of the angle between precrack and tensile loading direction, the effect of the precrack on tensile failure behaviour of rock decreases.

For the potential use of ultrafine grained materials for high performance kinetic energy penetrators of caliber projectile, the shear failure of 4130 steel having two different ultrafine grained (UFG) structures was examined under the tensile mode. 4130 steel with a lamellar UFG structure and with an equiaxed UFG structure was fabricated by equal channel angular pressing with routes A and Bc, respectively. Yield strength of UFG steel was enhanced almost twice compared to that of coarse grained counterpart while the former exhibited drastic ductility loss due to a lack of strain hardenability. Tensile failure of the steel having a lamellar UFG structure is manifested in sequence by formation of few sharp shear bands inclined by 45° to the stress axis, propagation of only one shear band of them, broad conjugate shear band formation and propagation, and failure in the shear mode along the first-formed shear band. In addition, large cracks parallel to the tensile stress were simultaneously developed due to its lamellar nature. The steel with an equiaxed UFG structure undergoing tensile rupture is characterized by a series of sequential processes of formation of one broad shear band inclined by 45° to the stress axis, its propagation, another broad conjugate shear band formation and propagation, and failure in the opening mode with the zigzag pattern. These characteristics were discussed with the aid of the models suggested for enhanced shear formability of nanocrystalline and/or UFG materials.

Bonded scarf repairs are used in composite structures when high strength recovery is desired or when there is a requirement for a smooth surface to satisfy aerodynamic requirements. Experimental and finite element study is carried out to understand the damage propagation and ultimate strength of scarf patch repaired CFRP laminates under uni-axial tensile load here. The ultimate strength and damage propagation in scarf repaired laminates has been investigated with respect to change in scarf angle and patch diameter. It is revealed from the results that damage is initiated in adhesive film at the bonded interface of 0° plies of parent laminate and patch in and then propagates in circumferential direction around scarf patch. It is found that some damage occurs in the parent laminate before the damage initiation in the adhesive film. After the adhesive film failure, the damage in parent laminate quickly propagates transversely to the free edge sides of the laminate and failure occurs. The results of this investigation provide further insight into the damage mechanisms in scarf repairs of composite structures under tensile load. This study may be helpful in improving the design and analysis techniques for scarf patch repair of composite structures.

We analyzed the strain distribution of lumber containing a knot under a tensile load. The local tensile strain near the knot was measured using the digital image correlation method. Fracture often initiated near the knot where the fiber orientation changed in a three-dimensional manner. The fiber direction in this zone was different from that in the clear part, coinciding with the thickness direction and not with the longitudinal direction of the specimen. Our results disagree with those of previous models that assumed the longitudinal direction of lumber as the direction of crack propagation. Strain analysis showed that a nonlinear region existed around the knot just before ultimate fracture occurred. The results indicated that nonlinear characterization is necessary to determine the failure mechanism of lumber containing a knot, despite the brittleness fracture at the macroscopic scale.

The influence of internal stress anisotropy resulting from anisotropic loading in a roll-to-roll (R2R) process, and polymer substrate anisotropy on the crack onset strain (COS) of thin oxide coatings was analyzed. Experimental data obtained for R2R processed films were compared with data obtained using an isotropic sheet-to-sheet (S2S) process with the same anisotropic substrate. In the R2R case the COS was found to increase by 20% between the transverse direction and the machine direction. In the S2S case the COS was found to be independent of orientation, except at a 45° in-plane orientation with respect to the machine direction, where it was 15% higher. The internal stress in the machine direction could not be determined, presumably due to deposition-induced curvature changes of the polymer substrate, and was therefore fitted to the COS data. Fracture mechanics analysis and finite element modeling of the experimental data showed that the influence of substrate anisotropy was marginal, and that it was the process-induced internal strain in the coating which controlled the COS. © 2010 Elsevier B.V.

Dry-snow slab avalanches are generally caused by a sequence of fracture processes, including failure initiation in a weak snow layer underlying a cohesive slab followed by crack propagation within the weak layer (WL) and tensile fracture through the slab. During past decades, theoretical and experimental work has gradually increased our knowledge of the fracture process in snow. However, our limited understanding of crack propagation and fracture arrest propensity prevents the evaluation of avalanche release sizes and thus impedes hazard assessment. To address this issue, slab tensile failure propensity is examined using a mechanically based statistical model of the slabWL system based on the finite element method. This model accounts for WL heterogeneity, stress redistribution by slab elasticity and possible tensile failure of the slab. Two types of avalanche release are distinguished in the simulations: (1) full-slope release if the heterogeneity is not sufficient to stop crack propagation and trigger a tensile failure within the slab; (2) partial-slope release if fracture arrest and slab tensile failure occur due to the WL heterogeneity. The probability of these two release types is presented as a function of the characteristics of WL heterogeneity and the slab. One of the main outcomes is that, for realistic values of the parameters, the tensile failure propensity is mainly influenced by slab properties. Hard and thick snow slabs are more prone to wide-scale crack propagation and thus lead to larger avalanches (full-slope release). In this case, the avalanche size is mainly influenced by topographical and morphological features such as rocks, trees, slope curvature and the spatial variability of the snow depth as often claimed in the literature., The Cryosphere, 9 (2), ISSN:1994-0416, ISSN:1994-0424

A Monte Carlo simulation is established to predict the failure strain of unidirectional fibre composites. The effect of matrix shear yielding of a high performance epoxy resin is introduced into the model through load sharing factors between the fibres adjacent to fibre-break(s). Strain concentration factors (SCF) of fibres are obtained using Finite Element Methods (FEM) in a three dimensional multi-fibre unit cell containing one, two and three adjoining fibre-break(s). The tensile strains of the surviving adjacent fibres are intensified as a function of their distances from the fracture. A statistical simulation is carried out to predict the failure strain of a single layer of unidirectional (UD) fibre composites with the thickness of the fibre ineffective length. Using the weakest link theory, the ultimate failure strain of a real size UD composite is predicted.

Carbon nanotubes (CNT) possess remarkable mechanical, thermal and electrical properties, which combined with their low density and high aspect ratio, make them a very attractive candidate as reinforcing materials for the development of an entirely new class of composites. However, to determine CNTs mechanical properties in a direct experimental way is a challenging and not economical task, because of the technical difficulties and the costs involved in the manipulation of nanoscale objects. Moreover, there is still a lack of the fundamental knowledge regarding the strength and failure behaviour of carbon nanotubes. Due to nanoscale, most of the continuum based classical fracture mechanics are not really suitable to describe the failure evolution. Failure of nanotubes has been mainly investigated using molecular dynamics theory. In this paper, we present an innovative method for modelling the failure of carbon nanotubes under uniaxial tensile loading. CNT can be thought as structural systems, where the primary bond between two nearest-neighbouring atoms forms the axially loaded-bearing components member and the individual atom acts as joints of the related load-bearing members. A Finite Element Model, based on the molecular mechanics theory, is proposed in this paper in order to investigate the fracture progress in Zig-Zag and Armchair carbon nanotube with defects under uniaxial tensile stress. The novelty of the proposed approach lies in the use of nonlinear axial and torsional springs to model the local interaction and breakage of bonds of CNT atoms under axial loads. The complete load–displacement relationship of Force/Displacement curve for a (5, 5) and a (9, 0) nanotube up to the complete fracture was obtained. Further, with a continuum assumption, it was possible to define a Stress/Strain curve with ultimate strength and strain. The results show that the effect of chirality on the mechanical properties and failure mode of CNTs was quite significant and cannot be neglected. Moreover, the results are in good agreement with experimental data and classical molecular dynamics simulation validating, therefore, the proposed modelling approach.

The paper outlines a technique for estimation of the size and shape of an inelastic zone evolving around a crack tip during the tensile failure of structures/structural members made of quasi-brittle building materials, particularly cementitious composites. The technique is based on an amalgamation of several concepts dealing with the failure of structural materials, i.e. multi-parameter linear elastic fracture mechanics, classical non-linear fracture models for concrete (equivalent elastic crack and cohesive crack models), and the plasticity approach. The benefit of this technique is expected to be seen in the field of the determination of fracture characteristics describing the tensile failure of quasi-brittle silicate-based composites. The method is demonstrated using an example of a (numerically simulated) fracture test involving the three-point bending of a notched beam and validated by experimentally obtained results of three-point bending and wedge-splitting tests taken from the literature.

Planar impact experiments were employed to induce dynamic tensile failure in Bedford limestone. Rock disks were impacted with aluminum and polymethyl methacralate (PMMA) flyer plates at velocities of 10 to 25 m/s. Tensile stress magnitudes and duration were chosen so as to induce a range of microcrack growth insufficient to cause complete spalling of the samples. Ultrasonic P- and S-wave velocities of recovered targets were compared to the velocities prior to impact. Velocity reduction, and by inference microcrack production, occurred in samples subjected to stresses above 35 MPa in the 1.3 microsec PMMA experiments and 60 MPa in the 0.5 microsec aluminum experiments. Using a simple model for the time-dependent stress-intensity factor at the tips of existing flaws, apparent fracture toughnesses of 2.4 and 2.5 MPa sq rt m are computed for the 1.3 and 0.5 microsec experiments. These are a factor of about 2 to 3 greater than quasi-static values. The greater dynamic fracture toughness observed may result from microcrack interaction during tensile failure. Data for water-saturated and dry targets are indistinguishable.

For decades, ballistic gelatin has been used as a tissue surrogate to test and evaluate bullets and firearms due to its similar viscosity to natural tissue. However, the high water content in ballistic gelatin makes it unstable at room temperature, and therefore causes it to have a poor shelf life. The development of polymer-based gels has shown promise as an alternative tissue surrogate. Polymer gels such as Perma-Gel are stable at room temperature and can be stored for long periods of time. Gels often fail due to tensile stresses during penetration. The failure behavior in tension is highly influenced by the presence of defects, such as cracks and voids, in the bulk material. A mode I experimental method was developed to obtain tensile failure criteria for the initiation and propagation behavior of these types of soft materials. Digital image correlation is used to determine the full-field surface strains around the crack tip to obtain a quantitative measure of the critical strain-field required for initiation and propagation of failure due to a defect. This systematic study utilizes these experimental techniques to determine the critical criteria for crack growth initiation and crack propagation of ballistic gelatins and a polymer gel as a function of loading rate. This paper presents experimental methodologies and results from Mode I fracture experiments including measured critical energy and strain-based criteria for failure initiation and growth, as well as their dependence on the rate of loading.