Search

Abstract: Experimental and numerical investigations were conducted into the damage growth and collapse behaviour of composite blade-stiffened structures. Four panel types were tested, consisting of two secondary-bonded skin–stiffener designs in both undamaged and pre-damaged configurations. The pre-damaged configurations were manufactured by replacing the skin–stiffener adhesive with a centrally located, full-width Teflon strip. All panels were loaded in compression to collapse, which was characterised by complex post-buckling deformation patterns and ply damage, particularly in the stiffener. For the pre-damaged panels, significant crack growth was seen in the skin–stiffener interface prior to collapse, which caused a reduction in load-carrying capacity. In the numerical analysis of the undamaged panels, collapse was predicted using a ply failure degradation model, and a global–local approach that monitored a strength-based criterion in the skin–stiffener interface. The pre-damaged models were analysed with ply degradation and a method for capturing interlaminar crack growth based on multi-point constraints controlled using the Virtual Crack Closure Technique. The numerical approach gave close correlation with experimental results, and allowed for an in-depth analysis of the damage growth and failure mechanisms contributing to panel collapse. The successful prediction of collapse under the combination of deep post-buckling deformations and several composite damage mechanisms has application for the next generation of composite aircraft designs. [Copyright &y& Elsevier]

Abstract: Theoretical analysis and numerical simulations are performed to investigate the buckling and postbuckling behaviors of stiff lamellae embedded in a compliant matrix under uniaxial compression. First, the analytical solution is derived for the critical compressive strain and wrinkle wavelength of a stiff layer sandwiched between two different soft layers, in which the effects of interfacial shear stresses and matrix thicknesses have been taken into account. During postbuckling, the system may keep the sinusoidal buckling shape or bifurcate into period-doubling morphology. A phase diagram is established, which enables us to easily predict the morphological evolution from the geometric and material parameters of the system. Then the above analysis is extended to two or more parallel stiff lamellae within a compliant matrix. Different buckling modes are found in such multilayer systems, i.e., short-wave mode, long-wave mode, and hierarchical mode. This study not only sheds light on the stability and morphological evolution of lamella composites and structures but also helps understand the morphogenesis of some biological tissues. [Copyright &y& Elsevier]

Abstract: The effect of a single circular delamination around the circular hole on the critical buckling load of woven fabric laminated composite plates has been investigated experimentally and numerically. The determination of the critical delamination diameter for laminated composite plates has been intended by using three dimensional buckling analysis. For the experiments, cross-ply laminated composite plates with delamination and without delamination have been produced. The experimental critical buckling loads of plates have been found by clamping from the two edges and then these results have been compared with the results obtained from the numerical analyses. For the numerical analyses, ANSYS 6.1 FEA program has been used. It has been seen that the results obtained the numerical buckling analyses are very close to the experimental results. In the light of this harmony, the numerical analyses for different delamination diameters and fiber orientations have been done and from these analyses, it has been seen that the important decreases occur in the critical buckling loads after a certain value of the delamination diameter and the different fiber orientations also affect to these loads. [Copyright &y& Elsevier]

Abstract: Due to the level of complexity of the governing equations for curved composite panels, few solutions, allowing the critical buckling loads and postbuckling behaviour of such structures to be determined, exist. Those that do are for basic problems based on single load types and simple boundary conditions. For other cases, such behaviour can only be determined by either grossly simplifying both the load and boundary conditions to those which can be predicted using these simplified equations (which may lead to overestimations in buckling load, and thereby premature failure or collapse), or by using alternative tools such as finite strip techniques or finite element analyses. This paper details a series of tests carried out to determine the behaviour of a number of optimised fibre composite panels of differing radii of curvature and aspect ratio, simply supported along two edges and built in along the other two, subject to a varying combination of shear and in-plane bending, for which no theoretical solution exists, and assesses the suitability of finite strip techniques and finite element analysis to predict this behaviour. [Copyright &y& Elsevier]

Abstract: Design rules based on theoretical solutions, which allow the prediction and understanding of the buckling and post-buckling behaviour of homogenous thin plates, exist for only a limited number of simple loading and boundary conditions due to the complexity of the equilibrium equations that describe the problem. When considering fibre composite plates, this issue is exacerbated by the inherent coupling between bending and extensional strains due to unsymmetric lamination, which results in three eighth order coupled partial differential equilibrium equations. This paper details a series of tests carried out to investigate the behaviour of a number of optimised fibre composite plates of differing geometry, simply supported along two edges and built in along the other two, subject to a varying combination of shear and in-plane bending, for which no theoretical solution exists, and assesses the suitability of analytical techniques and finite element analysis to predict this behaviour. [Copyright &y& Elsevier]

The study of axial compression buckling of isotropic cylinders has received much attention by various researchers over the years. It is commonly acknowledged that the presence of minute imperfections significantly reduces potential buckling loads in comparison with classical linear predictions. This approach, of including geometric imperfections, has been extended by a significant, yet fewer, number of researchers to composite cylindrical shells. As the current study shows, imperfections may not be the only major factor for the discrepancy between experimental and linear buckling loads. Flexural/twist anisotropy, present in most balanced, symmetric laminates with angle-ply layers, is shown to play a significant role in reducing buckling loads from those predicted by classical analysis. Indeed, the assumption of deflections in the form of a double sine series appears to be questionable for such laminates. A recently reported classically linear analysis that included the effects of flexural/twist coupling on buckling loads has been further developed to include the effects of extension/twist coupling. It is shown that buckling loads can be improved by making the laminate antisymmetric rather than symmetric, for the class of quasi-isotropic laminates, whilst retaining a spiral mode shape. [Copyright &y& Elsevier]

Abstract: Buckling of defective single-walled and double-walled carbon nanotubes (SWCNTs and DWCNTs, respectively) due to axial compressive loads has been studied by molecular dynamics simulations, and results compared with those of the perfect structures. It is found that single vacancy defect greatly weakens the carrying capacity of SWCNTs and DWCNTs, though it does slight harm to the effective elastic modulus of the tubes. The influence of defects on the buckling properties of nanotubes is related to the density of the defects, and the relative position of defects also plays an important role in buckling of DWCNTs. The van der Waals force among atoms in the inner and the outer tubes of short defective DWCNTs makes the critical buckling strain of DWCNTs greater than that of the inner tube. [Copyright &y& Elsevier]

Abstract: This article considers the problems of free vibrations, buckling, and static deflections of functionally graded plates in which material properties vary through the thickness. It is shown that, all other parameters remaining the same, the natural frequencies of functionally graded plates are always proportional to those of homogeneous isotropic plates and that the proportionality constant can be easily be predicted. Similar results are obtained for buckling and static deflection cases. Therefore, one can predict the behavior of functionally graded plates knowing that of similar homogeneous plates. Extensive results available in the literature can be used as well as solutions and software developed for isotropic materials, thereby greatly reducing the need for a direct analysis of such problems for functionally graded plates. [Copyright &y& Elsevier]

Abstract: The flatwise compressive behaviour of tetrachiral and hexachiral honeycombs is analysed, using analytical and Finite Element simulations, both with explicit and implicit formulations. The tetrachiral and hexachiral cells are composed by cylinders connected by four and six tangent ligaments respectively. The ligaments act as mixed stiffeners-elastic foundations during flatwise compressive loading, providing different buckling mode shapes during deformation. The models are compared with experimental results obtained using RP-based honeycombs tested according to ASTM C 393-00 and ASTM C365-00. [Copyright &y& Elsevier]

Abstract: The creep-buckling of hexagonal honeycombs with non-uniform cell-edge thickness is analyzed. Here, the non-uniform cross-section of cell edges is taken to be a plateau border and the solid making up the cell edges is assumed to follow power law creep. Theoretical expressions for describing the elastic buckling strength and the failure time for the onset of creep-buckling of hexagonal honeycombs with plateau borders are thus derived. The results indicate that the elastic buckling strength and the failure time for creep-buckling are significantly affected by the solid distribution in cell edges. Meanwhile, creep-buckling is more likely to occur when the applied compressive stress is close to the elastic buckling strength. However, creep-bending of cell edges becomes dominant if the applied compressive stress is much smaller than the elastic buckling strength. The transition from creep-buckling to creep-bending for hexagonal honeycombs with plateau borders depends on their relative density, the solid distribution in cell edges and the creep parameters of solid cell edges. [Copyright &y& Elsevier]

Abstract: The material properties of functionally graded materials (FGMs) possess inherent randomness due to their complicated fabrication process, in which the total control of various design parameters is often impossible. The present work investigates the effect of this randomness on the elastic buckling of FGM rectangular plates which are resting on an elastic foundation and subjected to uniform in-plane edge compressions. The interaction between the plate and foundation is included in the formulation with a two-parameter Pasternak model. The elastic material properties (including the Young’s modulus and Poisson’s ratio of each constituent material) and the foundation stiffness parameters are modeled as independent random variables. First-order shear deformation plate theory and a mean-centered first-order perturbation procedure are used to examine the stochastic characteristics of the buckling load. Typical results are presented for plates with aluminum/zirconia two-phase functionally graded material to show the influence of variation in material constants and foundation stiffness parameters, volume fraction index, edge in-plane forces, side-to-thickness ratio, and plate aspect ratio on the second-order statistics of buckling loads. [Copyright &y& Elsevier]

Abstract: The influence of winding pattern on the mechanical response of filament wound glass/epoxy cylinders exposed to external pressure is studied by testing cylindrical specimens having stacked layers with coincident patterns in a hyperbaric testing chamber. Different analytical models are evaluated to predict buckling pressure and modes of thin wall cylinders (diameter to thickness ratio d/h of 25) and satisfactory predictions are obtained which are in the same order of magnitude that those obtained in experimental results. Test results show no evident pattern influence on either strength (implosion pressure) or buckling behavior (buckling modes) of thin wall or thick wall (d/h of 10) cylinders. [Copyright &y& Elsevier]

Abstract: A postbuckling analysis is presented for a three-dimensional textile composite cylindrical shell of finite length subjected to axial compression in thermal environments. Based on a micro–macro-mechanical model, a three-dimensional textile composite may be as a cell system and the geometry of each cell is deeply dependent on its position in the cross-section of the cylindrical shell. The material properties of epoxy are expressed as a linear function of temperature. The governing equations are based on a higher order shear deformation shell theory with a von Kármán–Donnell-type of kinematic nonlinearity and including thermal effects. A singular perturbation technique is employed to determine the buckling loads and postbuckling equilibrium paths. The numerical illustrations concern the postbuckling behavior of perfect and imperfect, braided composite cylindrical shells with different values of geometric parameter and of fiber volume fraction in different cases of thermal environmental conditions. The results show that the shell has lower buckling load and postbuckling path when the temperature-dependent properties are taken into account. The results reveal that the temperature changes, the fiber volume fraction, and the shell geometric parameter have a significant effect on the buckling load and postbuckling behavior of textile composite cylindrical shells. [Copyright &y& Elsevier]

A postbuckling analysis is presented for a functionally graded cylindrical thin shell of finite length subjected to compressive axial loads and in thermal environments. Material properties are assumed to be temperature-dependent, and graded in the thickness direction according to a simple power law distribution in terms of the volume fractions of the constituents. The governing equations are based on the classical shell theory with von Ka´rma´n–Donnell-type of kinematic nonlinearity. The nonlinear prebuckling deformations and initial geometric imperfections of the shell are both taken into account. A boundary layer theory of shell buckling, which includes the effects of nonlinear prebuckling deformations, large deflections in the postbuckling range, and initial geometric imperfections of the shell, is extended to the case of functionally graded cylindrical shells. A singular perturbation technique is employed to determine the buckling loads and postbuckling equilibrium paths. The numerical illustrations concern the postbuckling response of axially-loaded, perfect and imperfect, cylindrical thin shells with two constituent materials and under different sets of thermal environments. The effects played by temperature rise, volume fraction distribution, shell geometric parameter, and initial geometric imperfections are studied. [Copyright &y& Elsevier]

A methodology for studying the implications of taking into account manufacturing at early design stages is presented. It is applied to a rectangular laminate made by RTM and compression (RTCM). The plate is designed for injection time, maximum mold pressure, stiffness and buckling. Semi-analytical, numerically inexpensive models of the processes and structure enable a thorough investigation of the couplings between process and structure by comparing four design formulations: in decoupled problems, either the process or the structure is optimized; then, the process is optimized with targetted low or high structural performance. A globalized Nelder–Mead optimization algorithm is used. The rigour of the method and the relative simplicity of the application case provide a clear description of how maximum mold pressures, injection times and final structures properties are traded-off. [Copyright &y& Elsevier]

Abstract: The onset of fracture process in a layered composite with coplanar interface cracks under compression along layers is studied within the scope of 3-D theory of stability using the model of piecewise-homogeneous medium. Each layer consists of a matrix reinforced by continuous parallel fibres and is modelled by a compressible, linear elastic, transversely isotropic material. The eigenvalue problem with respect to the controlled parameters, namely, the crack length, the crack spacing and the 0°-plies layer volume fraction is solved for the shear and the extensional modes utilizing the finite element analysis by commercially available FE code. The results are obtained for 0°-plies volume fraction V 0° =0.1…0.9 for the typical dispositions of cracks. The effect of the crack length, the crack spacing and the 0°-plies volume fraction on the critical microbuckling strain in the composite was determined. [Copyright &y& Elsevier]

Abstract: This paper reports a multiscale modeling of the compressive behavior of carbon nanotube/polymer composites. The nanotube is modeled at the atomistic scale, and the matrix deformation is analyzed by the continuum finite element method. The nanotube and polymer matrix are assumed to be bonded by van der Waals interactions at the interface. The stress distributions at the nanotube/polymer interface under isostrain and isostress loading conditions have been examined. The buckling forces of nanotube/polymer composites for different nanotube lengths and diameters are computed. The results indicate that continuous nanotubes can most effectively enhance the composite buckling resistance. [Copyright &y& Elsevier]

Abstract: Two-dimensional (2D) hierarchical cellular materials made up of sandwich walls are predicted to have superior mechanical properties compared with solid-wall cellular materials. Equations of the stiffness, the buckling strength, the plastic collapse strength, the brittle failure strength and the fracture toughness were deduced. The enhancement of the mechanical properties of 2nd order hierarchical honeycombs is substantial (even an order of magnitude). The hierarchical honeycomb is much more damage tolerant and insensitive to wavy imperfections of the cell wall. Sandwich struts also enhance the buckling strength of the stretching-dominated 2nd order lattice grid material. Made up of sandwich struts, the hierarchical honeycomb has comparable mechanical properties with the stretching-dominated lattice grid material. [Copyright &y& Elsevier]

Abstract Kink-band nucleation and propagation has been monitored over time in three dimensions (3D) by time-lapse X-ray computed tomography (CT) in the compressive zone of a blunt notched unidirectional (UD) T800 carbon fibre/epoxy composite under in situ four-point bending (FPB). The kink bands that develop from micro-buckling are classified into two types, namely type 1 and type 2 by analogy with Euler buckling. Type 1 (shear) kink bands accommodate a lateral displacement of the fibres either side of the kink band; whereas type 2 kinks comprise conjugate pairs forming chevrons (accompanied by a tilt if the bands are wedge-shaped). In the central plane of the sample the kink bands lie in the plane of bending, whereas near the side surfaces the lack of lateral constraint means that type 2 kink band pairs protrude out of the surface (normal to the bending plane) from the notch corners down the sides of the sample. Moreover, a comparison of CT scans during loading and after unloading shows that geometries measured post-mortem on cross-sections may not be wholly representative, with the angle of the broken fibre segments within the kink bands reduced by 10–20° and the curvature of buckled fibres almost halved. The novelty of this work relates to the observation of the nucleation and propagation of fibre kink bands in three dimensions and the definition and quantification of kink band variables that could lead to more accurate modelling and simulation of compressive behaviour. [ABSTRACT FROM AUTHOR]

Abstract: An existing procedure based on the combined use of the Virtual Crack Closure Technique and of a fail release approach for the analysis of delamination growth phenomena in composite structures has been enhanced with a front-tracing algorithm and suitable expressions for the evaluation of the Strain Energy Release Rate when dealing with non-smoothed delamination fronts. The enhanced procedure has been implemented into a commercial finite element software by means of user subroutines and applied to the analysis of a composite stiffened panel with an embedded delamination under compressive load. The effectiveness and robustness of the enhanced procedure have been assessed by comparing literature experimental data and numerical results obtained by using different mesh densities in the damaged area (global/local approach). [Copyright &y& Elsevier]

The weakest part of fiber reinforced polymer composites (FRP) sandwich structures is commonly the interfacial zone between layers. Under service loading, delamination failure will be most likely to initiate from these interfaces. Typical failure analysis for such sandwich structures usually depends on the modeling details of the interface. In this paper, the sandwich panel was modeled as separated layers with appropriate constrains imposed between them. These constrains guaranteed a smooth stress transfer from the skins to the core. The proposed FEM model was applied to simulate the failure behavior of a FRP sandwich panel that is used in bus body. The simulation results were compared with other numerical predictions and the experiment. It was concluded that this model is very efficient computationally for analyzing the failure issues of FRP sandwich structures. [Copyright &y& Elsevier]

Abstract: The compressive behavior of multiply delaminated composite plates was numerically evaluated considering both structural instability and delamination growth in order to clarify the reason for the significant reduction in the compressive strength of composite laminates that results from an impact. Embedded circular delaminations were placed at regular intervals in the thickness direction at the plate center. A three-dimensional block element was used to discretize the laminates, and a cohesive element was placed at the same planes as the delaminations. The cohesive element had a bilinear relationship between traction and relative displacement, and the element failed when the energy stored in it exceeded a threshold, that is, the critical energy release rate of the interface. Through the present numerical study we found that the applied load dropped after slight increase of the load when the delaminations started growing, and the delamination growth in the buckled laminate governed the compressive strength of the damaged composite laminates. [Copyright &y& Elsevier]

Abstract: A composite laminated plate containing an internal delamination and subject to compressive loads is considered. The plate is modelled as the union of two sublaminates partly bonded together by an elastic interface, in turn, represented by a continuous distribution of linear elastic springs acting in both the normal and tangential directions with respect to the interface plane. Based on a complete explicit solution, already presented in previous works by the authors, the expressions for the I and II (opening and sliding) modal contributions to the potential energy release rate, G, and the mode-mixity angle, ψ, are determined. In the present paper, the model is further developed to describe the mechanical response of delaminated plates under fatigue loads. To this aim, we apply a mode-dependent fatigue growth law, which accounts for the simultaneous presence of both opening and sliding crack propagation modes. The results can be used to easily predict the number of cycles needed for a delamination to extend to a given length or for complete failure, for any values of the minimum and maximum applied loads. [Copyright &y& Elsevier]

The fire stability of carbon fiber reinforced polymer (CFRP) shell structures was investigated using an intermediate-scale test setup. The shell specimens are representative of typical load-bearing CFRPs in modern civil aviation. The CFRP shell specimens were exposed to a fully developed fire with direct flame impingement to one side at a heat flux of 182 kW/m 2 . Specimens were simultaneously loaded with constant compressive force equal to 40% of the ultimate failure load. CFRP shells and four different fire retarding configurations, using integrated protective layers, were investigated. Unprotected CFRP specimens failed after just 27 s. Specimens with integrated protective layers with low heat conductivity and high burn-through resistance showed the most promising results. An integrated titanium foil decelerated the decomposition of the epoxy matrix and increased the time to failure by 68% compared to the unprotected CFRP shell. [ABSTRACT FROM AUTHOR]

Abstract: Wrinkling is one of the most common flaws that occur during textile composite reinforcement forming processes. These wrinkles are frequent because of the possible relative motion of fibres making up the reinforcement, leading to a very weak textile bending stiffness. It is necessary to simulate their onset but also their growth and their shape in order to verify that they do not extend to the useful part of the preform. In this paper the simulation of textile composite reinforcement forming and wrinkling is based on a simplified form of virtual internal work defined according to tensions, in-plane shear and bending moments on a unit woven cell. The role of the three rigidities (tensile, in-plane shear and bending) in wrinkling simulations is analysed. If in-plane shear stiffness plays a main role for onset of wrinkles in double-curved shape forming, there is no direct relation between shear angle and wrinkling. Wrinkling is a global phenomenon depending on all strains and stiffnesses and on boundary conditions. The bending stiffness mainly determines the shape of the wrinkles and it is not possible to perform a wrinkle simulation using a membrane approach. [Copyright &y& Elsevier]

Abstract The sudden compressive failure of unidirectional (UD) fibre reinforced composites at loads well below their tensile strengths is a cause of practical concern. In this respect and more generally, analytical and numerical models that describe composite behaviour have been hard to verify due to a lack of experimental observation, particularly in 3D. The aim of this paper is to combine fast in-situ X-ray computed tomography (CT) with advanced image analysis to capture the changes in fibre orientation in 3D during uninterrupted progressive loading in compression of a UD glass fibre reinforced polymer (GFRP). By analysing and establishing correspondence between a sequence of time-lapse X-ray CT images of the composite, we are able for the first time to follow each fibre and quantify the progressive deflection that takes place during axial compression in the steps leading up to fibre micro-buckling and kinking. Even at just 25 % of the failure load, fibres have started to tilt in approximately the direction of the ultimate kink band. The rate of tilting increases as the composite approaches the collapse load. More generally, our approach can be applied to investigate the behaviour of a wide range of fibrous materials under changing loading conditions. [ABSTRACT FROM AUTHOR]

Abstract: Tapered laminated structures have considerable potential for creating significant weight savings in engineering applications. The objective of the present work is to investigate the compressive response of curved composite plates with longitudinal internal ply-drop-off configurations. A nine-node tapered curved composite finite element is developed based on six well-established first-order shear deformation shell theories. The buckling loads obtained using Finite Element Method (FEM) are compared with that of the existing analytical and experimental results. A comprehensive parametric study is carried out and design guidelines for the tapered curved plates are established based on this study. [Copyright &y& Elsevier]

Abstract: A theoretical investigation on the strength and stiffness of carbon nanotubes (CNTs) under combined shortening and twisting strains is presented. CNTs with similar length-to-diameter aspect ratios, L/D, but different atomic structures (zig-zag, armchair and chiral) have been selected. Molecular dynamics (MD) simulations have been performed to study the critical buckling behaviour and the pre-critical and post-critical stiffness of CNTs under combined shortening–twisting conditions. The main results are presented in the form of interaction diagrams between the critical strain and the critical angle of twist per unit of length. An interaction equation is proposed and validated by comparison with the MD results. If shortening is more dominant than twisting, the strain energy at the onset of buckling drops considerably with the increase of the twisting–shortening rate. If twisting is more influential than shortening, the energy at the onset of buckling decreases very slowly with the twisting–shortening rate. We also found an interaction factor of 1.5 for CNTs under combined shortening–twisting, which is much lower than the value 2.0 commonly adopted for circular tubes at macro-scale. We conclude that CNTs are much more sensitive to buckling under shortening–twisting interaction than macro-scale tubes. [Copyright &y& Elsevier]

Abstract: Numerical procedures based on the combined use of the Virtual Crack Closure Technique and of a fail release approach have been widely used to simulate delamination growth phenomena of composite material structures. This paper starts explaining why this kind of methodologies might not be robust due to mesh and load step size dependency and introduces a novel approach able to cope with the problems identified. Finally the effectiveness and robustness of the proposed procedure, implemented into a commercial finite element software by means of user subroutines, are assessed by comparing the obtained numerical results for a delamination growth phenomenon against literature experimental data on a stiffened panel with a circular embedded delamination under compressive load. [Copyright &y& Elsevier]

Abstract: A Pareto-based multiobjective evolutionary algorithm is presented for stacking sequence optimization of composite structural parts. Special attention has been paid to engineering design guidelines for stacking sequence design. These guidelines are included into the formulation of the optimization problem as constraints or additional objectives. A new initialization strategy is proposed following mechanical considerations. The method is applied to the optimal design of a composite plate for weight minimization and maximization of the buckling margins under three hundred load cases that make also the originality of this work. It is shown that the introduction of new ply orientations compared to the classical 0°, ±45° and 90° plies can lead to significantly improved optimal designs. [Copyright &y& Elsevier]

Abstract: This study develops a new type of hybrid material that is a composite of a plain carbon steel micro-truss and a structural nanocrystalline Ni coating. The plain carbon steel micro-truss was made by a simple stretch–bend sheet forming method. It created a low density cellular material (∼5% relative density), combining the low embodied energy and cost of the starting precursor material with the structural efficiency of pyramidal micro-truss architecture. The nanocrystalline Ni structural coating was designed to provide both corrosion protection and inelastic buckling resistance. Because the ultra-high strength material was optimally located at the furthest distance from the neutral bending axis, only a thin coating of nanocrystalline Ni (∼50μm) is needed to double the inelastic buckling resistance of the 1.13mm×0.63mm plain carbon steel struts. [Copyright &y& Elsevier]

Abstract: Textile composites used as structural components have attracted more and more attention due to their superior properties and efficient stamping operations. However, formability of textile composite sheets, restricted by failure mechanisms such as wrinkling, remains as a crucial and challenging issue. Although in stamping of real parts, wrinkling depends on many factors, this paper mainly focuses on the in-plane characterization of pure shear deformation and its contribution to wrinkling. A comprehensive experimental study on woven fabric composites is presented. Modified picture frame tests were conducted up to wrinkling. The effect of test conditions was investigated, and the reduction of yarn width was found to be a key for wrinkling. The onset of wrinkling was determined by a densification method and by a laser scanning technique. Moreover, the cross-sectional profiles of fabric samples during the test were traced, helping to build up a theoretical model of the composite sheets during their large shear deformation. [Copyright &y& Elsevier]

Abstract: Carbon nanotubes (CNT) are potentially promising fibers for ultra high strength composites. In order to fully harness the outstanding mechanical properties of carbon nanotubes as fiber reinforcements, it is essential to understand the nature of load transfer between fiber and matrix under various types of loading conditions that include tension, compression, torsion and a combination thereof. In this paper, we study the compressive behavior (buckling and post-buckling) of carbon nanotubes in the neat form, when they are embedded in polyethylene matrix and with interface chemical modifications using molecular dynamics simulations based on Tersoff–Brenner potential. It is observed that the critical load for buckling increases only very marginally for nanotubes embedded in polythene matrix (with non-bonded interface) compared to neat CNTs. When CNTs are chemically bonded to the matrix, the compressive behavior occurs in two phases; pre- and post-buckling phases. First, the critical stress for buckling is found to reduce because the change in chemical bonding induces deviation from perfect cylindrical structure. In the post-buckling phase, however, the nanotubes behave similar to short fibers and deform by crushing. The results are compared with continuum solutions, wherever applicable. It is shown that the continuum solutions should be applied carefully whenever the effect of nanoscale interfaces becomes a factor. [Copyright &y& Elsevier]

Abstract: Delaminations cause a reduction in the compressive strength of a composite material and multiple delaminations cause greater strength reduction than single ones. Since realistic damage such as that resulting from an impact consists of multiple delaminations, it is of interest to understand the mechanism of strength reduction for laminates containing multiple delaminations. In this work, compression testing of glass fibre reinforced plastic specimens containing single and multiple embedded delaminations was carried out. Finite element modelling was used to gain further understanding of the mechanisms of compressive failure. Reasonable agreement between finite element predictions and experimental measurements were found for the whole range of delamination geometries that were tested. It is demonstrated that the maximum reduction in strength for a multiple occurs when the delaminations split the laminate into sub-laminates of similar thickness. [Copyright &y& Elsevier]

Epoxy matrix with high modulus was synergistically designed by organic amide acid (AA) and inorganic nano silica (SiO 2) as well as stiffened interphase of carbon fiber composite, and the effects of matrix modulus on longitudinal compressive strength and failure mechanism of composites were investigated. The elastic and shear modulus of matrix was enhanced by increased chemical cross-linking cites of AA and mechanical restraint of SiO 2 , contributing to improved interfacial properties from the construction of modulus intermedia layer in carbon fiber composite. Using combination model of elastic-buckling and plastic-kinking models, compressive strength of composites was simulated and predicted, which showed a close agreement to measured results and positive relationship with matrix modulus. Schematic mechanism of compression failure in CFRP composites were proposed, and stiffened matrix and favorable interphase could restrict fiber-buckling and facilitate kink-band formation to avoid interface delamination during elastic-plastic process, resulting in increased ultimate compressive strength. Image 1 [ABSTRACT FROM AUTHOR]

Abstract: All-metallic corrugate core sandwich panels as primary loading structures may rapidly soften under compressive loading due mainly to core member buckling once the peak compressive stress is reached, resulting in reduced load-carrying capability. Inserting close-celled aluminum foam into the corrugate core has been envisioned as a feasible way to enhance the load capacity. The enhancement due to foam filling were firstly explored experimentally under quasi-static out-of-plane compression and the underlying mechanisms subsequently numerically studied using finite element simulations. The foam filled corrugated panel was found to have strength and energy absorption much greater than the sum of those of an empty corrugated sandwich panel and the aluminum foam alone. It was demonstrated that the core members in the foam-filled panel were considerably stabilized by the filling foam against lateral deflection. In particular, the elastoplastic buckling wavelength of the core members was significantly reduced and the transition from axial deformation to bending of the core member was much delayed, both of which contributing to the enhanced strength and energy adsorption capability of the foam filled panel. [Copyright &y& Elsevier]

Abstract: Stretchable, elastomeric composite conductor made of multi-walled carbon nanotubes (MWNTs) and polydimethylsiloxane (PDMS) has been fabricated by simple mixing. Electrical percolation threshold, amount of filler at which a sharp decrease of resistance occurs, has been determined to be ∼0.6wt.% of MWNTs. The percolation threshold composition has also been confirmed from swelling experiments of the composite; the equilibrium swelling ratio slightly increases up to ∼0.6wt.%, then decreases at higher amount of filler MWNTs. Upon cyclic stretching/release of the composite, a fully reversible electrical behavior has been observed for composites having filler content below the percolation threshold value. On the other hand, hysteretic behavior was observed for higher filler amount than the threshold value, due to rearrangement of percolative paths upon the first cycle of stretching/release. Finally, mechanical moduli of the composites have been measured and compared by buckling and microtensile test. The buckling-based measurement has led to systematically higher (∼20%) value of moduli than those from microtensile measurement, due to the internal microstructure of the composite. The elastic conductor may help the implementation of various stretchable electronic devices. [Copyright &y& Elsevier]

Abstract: This paper describes an experimental study that examines the effect of specimen size on the axial compressive strength of IM7/8552 carbon fibre/epoxy unidirectional laminates (UD). Laminate gauge length, width and thickness were increased by a scaling factor of 2 and 4 from the baseline specimen size of 10mm×10mm×2mm. In all cases, strength decreased as specimen size increased, with a maximum reduction of 45%; no significant changes were observed for the axial modulus. Optical micrographs show that the failure mechanism is fibre microbuckling accompanied by matrix cracking and splitting. The location of failure in most specimens, and especially the thicker ones, is where the tabs terminate and the gauge section begins suggesting that the high local stresses developed due to geometric discontinuity contribute to premature failure and, hence, reduced compressive strength. Two generic quasi-isotropic multi-directional (MD) lay-ups were also tested in compression, one with blocked plies [45 n /90 n /−45 n /0 n ]s and the other with distributed plies [45/90/−45/0] ns with n =2, 4 and 8. The material used and test fixture was identical to that of the unidirectional specimens with three different gauge sections (30mm×30mm, 60mm×60mm and 120mm×120mm) to establish any size effects. Strength results showed no evidence of a size effect when the specimens are scaled up using distributed plies and compared to the 2mm thick specimens. All blocked specimens had similar compressive strengths to the sublaminate ones apart of the 8mm specimens that showed a 30% reduction due to extensive matrix cracking introduced during the specimen’s cutting process. The calculated unidirectional failure stress (of the 0° ply within the multidirectional laminate) of about 1710MPa is slightly higher than the average measured value of 1570MPa of the 2mm thick baseline unidirectional specimen, suggesting that the reduced unidirectional strength observed for the thicker specimens is a testing artefact. It appears that the unidirectional compressive strength in thicker specimens (>2mm) is found to be limited by the stress concentration developed at the end-tabs and manufacturing induced defects such as fibre misalignment, ply waviness and voids. [Copyright &y& Elsevier]

Titanium and carbon fibre pins have been inserted into the polymethacrylimide foam core of a sandwich panel (with carbon fibre face sheets) in order to increase the through-thickness strength. The elevation in compressive strength has been measured both quasi-statically and dynamically using a direct Kolsky bar, and the sensitivity of strength to the relative density and thickness of foam have been determined. An X-ray CAT scan machine was used to examine the deformed shape of the pins during interrupted compression testing of the sandwich specimens. It was found that the foam core stabilises the pins against elastic buckling, and the pin-reinforced core has a strength and energy absorption capacity in excess of the individual contributions from the foam and unsupported pins. It is shown that the compressive strength is governed by elastic buckling of the pins, with the foam core behaving as an elastic Winkler foundation in supporting the pins. The peak strength of the pin-reinforced core is increased by a factor of about four when the speed of loading is increased from the quasi-static rate of about 10−6 ms−1 to the dynamic value of 10 ms−1; it is concluded that the micro-inertia of the pins stabilises them against elastic buckling and leads to the observed elevation in strength. [Copyright &y& Elsevier]

Abstract: A fuselage representative carbon fibre-reinforced multi-stiffener panel is analysed under compressive loading. An intact and pre-damaged configuration is loaded into the postbuckling region and further on until collapse occurs. An analysis tool is applied that includes an approach for predicting interlaminar damage initiation and degradation models for capturing interlaminar damage growth as well as in-plane damage mechanisms. Analysis of the intact panel configuration predicts collapse due to fibre fracture in the stiffeners close to the panel clamps, which agrees well with the results from experimental testing. The pre-damaged configuration was proposed containing Teflon-coated layers to generate the initial debonds in the skin-stiffener interface. The outcome of the simulation of this configuration shows that crack growth is not predicted to occur, which agrees with the observations of the experiment. A parametric study is conducted to investigate the effect of the skin-stiffener debond parameters such as length, width and location on crack growth and the collapse behaviour of the panel. It is found that the sensitivity of the panel design to the damage parameters is highly dependent on the postbuckling mode shape or displacement pattern, and particularly the extent to which this influences the conditions at the crack front. More broadly, the analysis tool is shown to be capable of capturing the critical damage mechanisms leading to structural collapse of stiffened composite structures in the postbuckling region. [Copyright &y& Elsevier]

Low velocity axial impact crush behaviours and energy absorption characteristics of thin-walled aluminium conical (AC) and E-glass/epoxy composite over-wrapped aluminium conical hybrid frustrated shells (CWAC) have been studied through experimental and numerical procedures. In this regard, 15°–24° semi-apical angled aluminium and hybrid conical test specimens of required thickness and fibre ply orientations were fabricated through hand layup method. The axial impact tests were performed at different strain rates on each AC and CWAC samples with the help of low velocity impact test setup in order to assess the energy absorption capacity of each specimen against impact loading. The influence of inertia, strain rate, addition of GFRP laminates over aluminium shell and GFRP fibre ply orientation towards the crashworthiness performances of various categories of conical frusta were analyzed. The low velocity impact tests were also simulated with the help of finite element analysis (FEA) techniques of ABAQUS® software in order to predict and compare the crashworthiness of each category of specimen model with experimental results. The obtained crashworthiness test results and collapse behaviours of FEA analysis are found in good agreement with the experimental results. [ABSTRACT FROM AUTHOR]

Abstract: A refined plate theory for functionally graded plates resting on elastic foundation is developed in this paper. The theory accounts for a quadratic variation of the transverse shear strains across the thickness, and satisfies the zero traction boundary conditions on the top and bottom surfaces of the plate without using shear correction factors. The number of independent unknowns of present theory is four, as against five in other shear deformation theories. The material properties of plate are assumed to vary according to power law distribution of the volume fraction of the constituents. The elastic foundation is modeled as two-parameter Pasternak foundation. Equations of motion are derived using Hamilton’s principle. The closed-form solutions of rectangular plates are obtained. Numerical results are presented to verify the accuracy of present theory. [Copyright &y& Elsevier]

Abstract: 3D multi-layer and multi-representative unit cell (RUC) models are presented in order to capture the failure mechanisms of Z-pinned laminated textile composites presented in part 1 [Huang H, Waas A. Compressive response of Z-pinned woven glass fiber textile composite laminates: experiments, this issue] of this two part sequel. Simulations of 1, 9, 16, and 25-RUC models are compared to establish cell number effects in representing the textile composites for strength predictions. Further, simulations using multi-layer representations of the textile laminate are conducted to account for unintended stacking effects that occur during the manufacturing cycle. From the results of these simulations, the 3-layer model that has 16-RUCs in each layer is found to be the most adequate representation of the 3D multi-layer and multi-RUC models. Simulations show that stacking effects (layers not compacting and consolidating exactly as intended, resulting in a phase shift) during the manufacturing of the laminates, influence the outcome of the predicted compression strength. [Copyright &y& Elsevier]

Abstract: A shear loaded, stringer reinforced composite panel is analyzed to evaluate the fidelity of computational fracture mechanics analyses of complex structures. Shear loading causes the panel to buckle. The resulting out-of-plane deformations initiate skin/stringer separation at the location of an embedded defect. The panel and surrounding load fixture were modeled with shell elements. A small section of the stringer foot, web and noodle as well as the panel skin near the delamination front were modeled with a local 3D solid model. Across the width of the stringer foot, the mixed-mode strain energy release rates were calculated using the virtual crack closure technique. A failure index was calculated by correlating the results with a mixed-mode failure criterion of the graphite/epoxy material. The objective was to study the effect of the fidelity of the local 3D finite element model on the computed mixed-mode strain energy release rates and the failure index. [Copyright &y& Elsevier]

Abstract: The interaction between fiber kink banding and splitting in the compressive response of fiber composites is investigated by adopting a micromechanics based 2D finite element representation of the composite. The effect of scaling of the computational model (size of model) on the compressive strength and post critical response is also studied. The size of the computational model is scaled in a systematic manner by increasing the number of fibers in the model, but maintaining a fixed aspect ratio and fiber volume fraction. The response of the model changes with change in model size, and therefore, a converged model size (baseline) for predicting compressive strength is first established. Since the strains experienced within the kink band are very large, the baseline model is extended to account for splitting as a potential mode of failure by adopting a discrete cohesive zone model. A parametric study is carried out to investigate the interaction between kinking and splitting in limiting the compressive strength of the composite. [Copyright &y& Elsevier]

Abstract: Analyzing failure mechanics of fiber-reinforced media that correspond to different length scales with the goal of up-scaling is addressed in this paper with respect to compressive kinking instability in fiber reinforced laminae. Using the concentric cylinder model (CCM) as a representative element of a lamina, J2 deformation theory along with Hill’s anisotropic plasticity theory is used to extend the constitutive model for the fiber-matrix cylinder, and hence the lamina, into the nonlinear regime. The model requires the elastic properties of fiber and matrix, the fiber volume fraction and the nonlinear shear response of the polymer matrix. An 8-layer laminate is up-scaled using this method by homogenizing the off-axis laminae and retaining micro-mechanics in the 0° laminae, and results are compared against a discrete fiber-matrix micro-mechanical model of the laminate. The effect of homogenizing the laminae on the compressive response of the laminate is investigated. [Copyright &y& Elsevier]

Abstract: In the first of this two part sequel, experimental results pertaining to the compressive response and failure of Z-pinned S-Glass fiber, plain-weave laminated composites are presented. These experiments are motivated by a need to understand the effect of Z-pinning on the strength and stiffness of these composites. A series of experiments are performed based upon density of the Z-pins and the diameter of the Z-pins. It is concluded that the damage zone around a Z-pin plays an important role in influencing the stiffness and strength of the Z-pin composite. In part 2 of this sequel, a 3D finite element (FE) based numerical model (based upon the composite microstructure acquired from scanning electron micrograph-SEM images) are used to capture details of the observed failure mechanisms and to provide predictions of the stiffness and strength of the composite. [Copyright &y& Elsevier]