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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: 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 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]

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 influence of nanoclay on the impact damage resistance of carbon fibre–epoxy composites has been investigated using the low-velocity impact and compression after impact (CAI) tests. The load–energy vs. time relations were analyzed to gain insight into the damage behaviours of the materials. The CFRPs containing organoclay brought about significant improvement in impact damage resistance and damage tolerance in the form of smaller damage area, higher residual strength and higher threshold energy level. The presence of nanoclay in the epoxy matrix induced the transition of failure mechanisms of CFRP laminates during the CAI test, from the brittle buckling mode to more ductile, multi-layer delamination mode. Addition of 3wt% clay was shown to be an optimal content for the highest damage resistance. [Copyright &y& Elsevier]

Abstract: In order to understand the nature of nonlinearity in the in-plane shear behaviour of fibre reinforce composites (FRC), experimental data of such materials under loading cycles, involving loading, unloading, reversed loading and unloading from reversed loading, are essential. Using the standard ASTM D 4255/D 4255M method, it will require the fixture to be subjected to tension as well as compression. It has been proven in this paper that the standard ASTM D 4255/D 4255M fixture is incapable of delivering such data as premature buckling is expected when the fixture is under tension, even though it claimed that the fixture could be loaded in tension as well as in compression. A cyclic rail shear fixture (CRSF) as a modified version of ASTM D 4255/D 4255M fixture has been proposed in this paper and it has been designed and manufactured. The new fixture, which maintains all the capabilities of its standard counterpart, is truly capable of sustaining loading in tension as well as in compression so that specimens can be tested for loading and reversed loading. A series of experiments have been carried out using the new fixture to characterise the nonlinear in-plane shear stress–strain relationship of unidirectional fibre reinforced composite specimens under various cyclic loading conditions. Some new experimental data of this nature are presented in this paper, which are novel in contents and crucial in guiding the formulation of theoretical models for the nonlinear behaviour of in-plane shear for general applications. [Copyright &y& Elsevier]