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A quasi-conforming triangular sandwich plate element with 7 degrees of freedom (DOF) per node is established based on a simplified Zig-Zag model and a transverse shear deformation theory. The accuracy of the proposed element has been validated to be equivalent to the element based on the complete Zig-Zag model (11 DOF per note). The path-following method, proposed by Huang and Atluri, is improved and applied to follow complex postbuckling paths with secondary bifurcation points and progressive failure. A finite element (FE) modeling is completed to study the shear postbuckling behavior and failure of composite sandwich plates, based on the energy criterion of stability and the strength criterion of each layer. The basic characteristics of the shear postbuckling and failure are illustrated. The complexity and computation difficulty caused by secondary bifurcations and multi-paths are discussed. Numerical examples show that the proposed method is efficient in improving the convergence of iterations. [ABSTRACT FROM AUTHOR]

Abstract: A novel corrugated composite core, referred to as a hierarchical corrugation, has been developed and tested experimentally. Hierarchical corrugations exhibit a range of different failure modes depending on the geometrical properties and the material properties of the structures. In order to understand the different failure modes the analytical strength model, developed in part 1 of this paper, was used to make collapse mechanism maps for the different corrugation configurations. If designed correctly, the hierarchical structures can have more than 7 times higher weight specific strength compared to its monolithic counter part. The difference in strength arises mainly from the increase in buckling resistance of the sandwich core members compared to the monolithic version. The highest difference in strength is seen for core configurations with low overall density. As the density of the core increases, the monolithic core members get stockier and more resistant to buckling and thus the benefits of the hierarchical structure reduces. [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: The compression after impact (CAI) strength of fully orthotropic composite laminates with up to 21 plies is presented, as analysed by an existing strip model. Candidate layups, which can be symmetric, anti-symmetric or non-symmetric, are preselected to exhibit no elastic coupling response, with manufacturing rules applied. These criteria, along with the use of a simple surrogate sublaminate buckling model, were chosen to allow analysis of all feasible laminates in the design space without excessive computation time. Results indicate that although the inclusion of non-symmetric layups in the design space does not give benefits with respect to maximum achievable damage tolerance, these laminates can exhibit damage tolerance close to that of an anti-symmetric design for some ply counts, and better than symmetric solutions in most cases. It is also noted that in some instances increasing the number of plies in a laminate can actually reduce the highest achievable threshold load for damage tolerance, as a result of the large influence Poisson’s ratio has on sublaminate buckling. Average errors in the surrogate model were low in all cases, with maximum non-conservative errors less than 1%. The surrogate buckling model reduced computational time by over 99% when compared to the fully exhaustive search. [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]

Very thin carbon fiber composite shells can withstand large bending curvatures without failure. The resulting high tensile and compressive strains require accurate modeling of the fiber-dominated non-linear effects to predict the mechanical response. To date, no universal modeling technique can precisely capture the behavior of such structures. In this work, successful representation of composite's response was achieved by utilizing single fiber tension and compression experimental data, implemented to extend a basal-plane-realignment based non-linear carbon fiber material model. Numerical techniques were adopted to model the bending behavior of unidirectional carbon fiber composites that was recorded in a comprehensive experimental campaign. Observations show that high material non-linearity leads to a non-negligible neutral-axis shift and drastic reduction of bending modulus due to compressive softening. Tensile fiber failure is the driving mechanism in thin shells flexure allowing for elastic compressive strains of up to 3% without micro-buckling. As a result, a remarkable flexibility in thin shells is realized. With increasing thickness, the elastic flexibility is reduced as the failure-driving mode switches to compressive micro-buckling. [ABSTRACT FROM AUTHOR]

Abstract: Biological materials, such as bone and nacre, are nanocomposites of protein and mineral with superior mechanical properties. The basic building blocks of these materials feature a generic nanocomposite structure with staggered alignment of mineral platelets in protein matrix. Because of the structural complexity of the generic structure, its displacement and stress fields are currently still unknown. In this study, a perturbation method was applied for analytically solving the displacement and stress fields of the nanocomposite structure under uniaxial tension. The effect of the elastic modulus, aspect ratio and volume fraction of mineral and protein on the displacement and stress fields in the nanocomposite structure was studied. A non-dimensional parameter γ was then suggested for characterizing the stress and strain fields in this nanostructure. We showed that the assumption of uniform shear stress distribution at the mineral–protein interface in the TSC model is valid when γ is less than 4 which is broadly applicable to most biological materials. The analytical solutions of displacement and stress fields obtained in this study provide a solid basis for further analyses of mechanical properties, such as the buckling and the fracture behaviors of biological materials. [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]