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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: This work reports on an experimental and finite element method (FEM) parametric study of adhesively-bonded single and double-strap repairs on carbon-epoxy structures under buckling unrestrained compression. The influence of the overlap length and patch thickness was evaluated. This loading gains a particular significance from the additional characteristic mechanisms of structures under compression, such as fibres microbuckling, for buckling restrained structures, or global buckling of the assembly, if no transverse restriction exists. The FEM analysis is based on the use of cohesive elements including mixed-mode criteria to simulate a cohesive fracture of the adhesive layer. Trapezoidal laws in pure modes I and II were used to account for the ductility of most structural adhesives. These laws were estimated for the adhesive used from double cantilever beam (DCB) and end-notched flexure (ENF) tests, respectively, using an inverse technique. The pure mode III cohesive law was equalled to the pure mode II one. Compression failure in the laminates was predicted using a stress-based criterion. The accurate FEM predictions open a good prospect for the reduction of the extensive experimentation in the design of carbon-epoxy repairs. Design principles were also established for these repairs under buckling. [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: 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: This paper examines the compressive behaviour of plies with fibres previously fractured during impact. The analysis is conducted using finite element (FE) modelling in ABAQUS 6.7. Two- and three- dimensional models are used to consider the possibility of fibre penetration and “brooming” of fractured fibres, or fibre buckling. A parametric study of the influence of the input parameters of the Drucker Prager plasticity model was also conducted, to enable a better understanding of the model. This was used to capture the triaxial stress state in the matrix surrounding the fibres. The results suggest that fibre buckling is more likely to occur due to the geometry of fibres and fibre spacing in carbon fibre composites, but fibre penetration could still occur in regions of low fibre volume fraction. [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]