A model to predict stiffness reduction and stress redistribution due to damage of laminated polymer composites is presented. The material properties required by the model are limited to those already available for unidirectional composites. Classical lamination theory is generalized for the case of a continuously damaging material using concepts from continuous damage mechanics. The Tsai-Wu failure criteria can be recovered as a limiting case. The damage model is validated with experimental results for various laminates built with aramid/epoxy, T300/5208, and T300/914 carbon/epoxy.

New types of tridimensional composite materials have been developed during the last few years. These new materials consist of braided, weft or warp knitted, and 3D woven composites preforms, and they present excellent out-of-plane mechanical properties. In this article, tubular energy absorbers, constituted of carbon-glass hybrid braided composites, are analyzed by means of the finite-element method, with explicit time integration of the equations of motion. A new material model for (0°, - f °) braided composites is introduced in a commercial finite-element code by means of user-developed subroutines. Braided materials with carbon fiber in the 0° direction and glass fibers in the - f ° directions are studied. Moreover, different fiber orientations and geometry of the triggering devices are analyzed in order to obtain the optimum configuration in terms of specific energy absorption.

The bilinear elastic degradation called the knee phenomenon, observed well in the transverse tensile stress–strain curves of some metal-based composites, is modeled through both a simplified three-phase cylindrical model and a hexagonal-arrayed unidirectional composite. The interphase is modeled by spring layers which account for continuity of tractions, but allow radial and circumferential displacement jumps across the interphase that are linearly related to the normal and tangential tractions. Even though constituent materials are in the elastic range all the way through, the possible low stiffness of the interphase and the residual stresses induced by uniform cooling yield bilinear elastic behavior in the stress–effective strain curves. However, perfect bonding or low stiffness in the interphase with no residual stresses creates a linear curve. The effects of interphase stiffness, fiber volume fraction, temperature change, and transverse tensile load on both the micro- and macro-thermomechanical beha...

Experimental results for creep bending tests of tape and twill woven carbon fiber-reinforced polymer (CFRP) laminates are presented and discussed. The aim of the research program was to characterize and compare the long-term behavior of both laminates. These materials will be included as structural elements in the construction of supports for delicate and precise radiation detection elements, so they need to be highly stable under any environmental conditions. The time dependency of the fiber-dominated properties of the CFRP laminates is negligibly small. Therefore the initial analysis pointed to better creep behavior of the twill-woven CFRP laminate and also predicted null creep strains. The experimental results show a different picture: the tape laminates performed better than the woven laminates, which exhibit larger creep strains than the initial guess.

Solutions are presented for the stresses in a specially orthotropic infinite strip which is subjected to localized uniform normal loading on one edge while the other edge is either restrained against normal displacement only, or completely fixed. The solutions are used to investigate the diffusion of load into the strip and in particular the decay of normal stress across the width of the strip. For orthotropic strips representative of a broad range of balanced and symmetric angle-ply composite laminates, minimum strip widths are found that ensure at least 90% decay of the normal stress across the strip. In addition, in a few cases where, on the fixed edge the peak shear stress exceeds the normal stress in magnitude, minimum strip widths that ensure 90% decay of both stresses are found. To help in putting these results into perspective, and to illustrate the influence of material properties on load diffusion in orthotropic materials, closed-form solutions for the stresses in similarly loaded orthotropic half-planes are obtained. These solutions are used to generate illustrative stress contour plots for several representative laminates. Among the laminates, those composed of intermediate-angle plies, i.e., from about 30° to 60°, exhibit marked changes in normal stress contour shape with stress level. The stress contours are also used to find 90% decay distances in the half-planes. In all cases, the minimum strip widths for 90% decay of the normal stress exceed the 90% decay distances in the corresponding half-planes, in amounts ranging from only a few percent to about 50% of the half-plane decay distances. The 90% decay distances depend on both material properties and the boundary conditions on the supported edge.

The unusual characteristics of mixed-mode delamination fracture in the presence of large bridging zones of through-thickness reinforcement are explained through an idealization of the mixed-mode bending specimen. With simple but realistic constitutive laws assumed for the bridging mechanisms, beam theory offers insightful analytical solutions and illuminates the length scales that control the crack characteristics. Of particular significance is that the mode I energy release rate can vanish after a period of crack growth, due to the bridging effect of the through-thickness reinforcement. This crack tip closure phenomenon is one manifestation of bridging-induced spatial oscillations that sometimes appears in the crack displacement function, which can be understood qualitatively through the similarity of the fracture problem to the classical problem of a beam on a Winkler foundation. The possibility of crack closure and consequent crack arrest has important consequences for designing certifying tests for laminates with through-thickness reinforcement and predicting strength and lifetime.

An elastic-plastic, time-independent, macroscopic, homogenous model of an 8HS woven graphite/PMR-15 composite material has been developed that predicts the nonlinear response of the material subjected to shear-dominated biaxial loads. The model has been used to determine the response of woven composite off-axis and Iosipescu test specimens in nonlinear finite element analyses using a multilinear averaging technique. The numerically calculated response of the specimen was then compared to experimentally obtained data. It has been shown that the numerically calculated stress - strain diagrams of the off-axis specimens are very close to the experimentally obtained curves. It has also been shown that the numerically determined shear stress - strain and load-displacement curves of the woven Iosipescu specimens are close to the experimentally obtained curves up to the point of significant interlaminar damage initiation and propagation. The results obtained in this study clearly demonstrate that the nonlinear material behavior of the graphite/polyimide woven composites subjected to shear-dominated biaxial loading conditions cannot be ignored and should be considered in any stress analysis. The linear-elastic approach grossly overestimates the loads and stresses at failure of these materials in the off-axis and Iosipescu tests. It can be assumed that the same discrepancies will arise in the numerical analysis of the woven composites tested under other biaxial shear-dominated loading conditions using other biaxial test methods.

A micromechanical analysis of damage evolution (interfacial debonding) in particle-filled elastomers addresses the effect of the interactions between particles and of variation in filler size. The composite is treated as an assembly of two constituents in a finite-element model. It is shown that the interaction between particles controls the damage evolution: (1) For high volume fraction, a relatively small change in particle size has a surprisingly large effect on the local material response; (2) for large differences in particle sizes (e.g., bimodal distribution), damage occurs at interfaces between large particles and matrix, with limited damage occurring at small particles. While these effects of particle interaction and size variation are smoothed out in a large ensemble of particles, it is foreseeable that they are an important factor in a failure process such as macroscopic crack propagation, which spans scales considerably larger than the maximum particle size. Specifically, one thus expects that in the vicinity of a macroscopic crack the large particles become sites for small cracks which coalesce into larger ones and join up with the macro crack, while small particles operate primarily so as to locally stiffen the matrix without incurring significant damage in their vicinity.