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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: 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: 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: 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: 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]

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: 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: 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: 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: 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]

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]