A stability problem of composite beams with multiple delamination was tackled. A closed-form solution was found and buckling loads of composite beams with two delaminations were determined in order to obtain their compressive load-carrying capacity. Crack-opening mode was assumed for each detached delaminated region. Seven different regions having different transverse deformations resulted from assumed positions of delaminations. Developing the derived continuity condition equations reduced the number of algebraic equations required to solve the problem analytically. The results of the work were validated by comparing them to those reported in the literature. The effects of length, location, and distribution of multiple delaminations were considered in the comparison, and the results showed very good agreement. Buckling load decreases as delamination size increases. Buckling load for a beam with two delaminations is lower than that for the same beam with a single delamination.

Finite element (FE) contact and thermal macro/micro models have been developed to study the real thermal behavior of a fiber/matrix microstructure under sliding motion of a steel asperity. At first the contact parameters were evaluated using an approximate contact technique, followed by a transient thermal FE evaluation. The latter considers the heat partition between the steel asperity and the real fiber/matrix microenvironment of a normally oriented carbon fiber/polyether ether ketone composite. The temperature results obtained were compared with those representing a macroscopic approach.

An experimental and numerical mixed method based on the torsion test with and without warping of cross sections has been developed in order to characterize the shear behavior of a polymer material obtained layer by layer from a photosensitive resin (CiBA SL5170) polymerized by laser beam by the stereolithography technique. In this article, first the stereolithography technique is briefly presented, as well as the torsion device with and without warping of cross sections. Experimental and numerical results in static torsion of the polymer SL5170 are presented: shear modulus and strength of the material. From test results, a modeling of the superposition ratio of tensile and torsion stresses is proposed. It shows that the torsion of beams is complex. Torsion stresses and the tensile stress owed to the warping of section are superposed. In small-deformations state, material behavior behaves in a linear elastic way; experimental and numerical results are quantitatively and qualitatively coherent and conform to predictions of the elastic torsion theory. From experimental analysis of fatigue and elastoplastic damage behavior of the polymer, a phenomenological approach of the isotropic damage model is proposed. Based on the thermodynamics of irreversible processes, this local approach starts from a variant of the Chaboche-Lemaître model coupling plasticity and damage. This formulation is justified by the necessity to take into account the real stress state produced in the polymer during the torsion test.

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.

Damage in a composite typically begins at the constituent level and may, in fact, be limited to only one constituent in some situations. Accurate predictions of constituent damage at points in a laminate provide a genesis for progressively analyzing failure of a composite structure from start to finish. In this article we develop an efficient constituent-based failure analysis for composite structural laminates. Continuum-based (phase-averaged) constituent stress and strain fields are computed in a finite-element environment without a computational time penalty. Constituent stress-based failure criteria are developed and used to construct a progressive failure algorithm in which one constituent is allowed to fail while the other constituent remains intact, e.g., matrix cracking. The proposed failure algorithm was used to predict failure of a variety of laminates under uniaxial and biaxial loads. The results were shown to be superior to comparable single-continuum failure analyses and in good agreement with experimentally determined failure loads.

The increasing demand for fatigue life extension of both military and civilian aircraft has led to advances in repair technology for cracked metallic structures. Conventional structural repairs may significantly degrade the aircraft fatigue life and lower its aerodynamic performance. Adhesively bonded composite reinforcement is a new technology of great importance due to the remarkable advantages obtained, such as mechanical efficiency and repair time and cost reduction. In this article, bonded composite patch repairs were designed for quick application to aircraft under emergency conditions, such as aircraft battle damage repair (ABDR). A formulated method was developed, to be applied when damage has to be restored quickly, without restrictions to safety of flight. Different damage cases were investigated using finite-element analysis (FEA), taking into account specific parameters of the structure under repair. Based on the FEA results, a quick design procedure using composite patch repairs for the most frequent damage cases is proposed.

The free vibration of laminated shallow shells containing piezoelectric layers is investigated in this article. A finite-element formulation based on the transverse shear deformation theory has been used to determine the natural frequencies, mode shapes, and modal voltages of laminated composite shell structures containing piezoelectric layers. Comparisons of degenerate cases with published results show excellent agreement. The effect of electromechanical coupling on the predicted natural frequencies is discussed. Results are presented for different geometries, laminate configurations, and boundary conditions. The effects of shell shallowness and piezoelectric layer thickness are also studied. Higher natural frequencies are obtained when the full electromechanical coupling is considered. It is also found that the natural frequencies increase considerably with the shell curvature, particularly for thin piezoelectric layers.

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.