FAILURE CRITERIA FOR COMPOSITE MATERIALS UNDER MULTIAXIAL STRESS STATES Essam Totry 1 , Carlos González 1, 2 , Javier LLorca 1, 2 1 Departmento de Ciencia de Materiales, Universidad Politécnica de Madrid. 2 Instituto Madrileño de Estudios Avanzados en Materiales (IMDEA-Materiales). E. T. S. de Ingenieros de Caminos. Ciudad Universitaria. 28040 – Madrid, Spain. KEYWORDS: Failure criteria, multiaxial loading, computational micromechanics. ABSTRACT The failure locus a fiber-reinforced composite lamina, made up of 50 vol. % of carbon fibers embedded in an epoxy matrix, is computed under multiaxial stress states involving transverse compression, in-plane and out-of-plane shear. The mechanical response was obtained by the finite element method of a representative volume element of the lamina. The actual deformation and failure mechanisms experimentally observed in the matrix, fibers and interfaces were included in the simulations through the appropriate constitutive equations. The corresponding failure loci were validated against experimental results (when available) and compared with those given by three failure criteria (Hashin, Puck and LaRC) which provide reasonable predictions in other multiaxial stress states. SIMULATION STRATEGY The macroscopic properties of a composite lamina until failure were obtained by means of the finite element analysis of a representative volume element (RVE) of the microstructure. The three-dimensional RVE of the lamina contained a random dispersion of circular fibers (which covered 50% of the area) embedded in a matrix, as shown in Fig. 1(a). It was subjected to normal compressive stresses in the transverse direction (either σ 22 or σ 33 ) and to shear stresses perpendicular (τ 23 ) and/or parallel to the fibers (τ 12 ). Simulations were carried out with Abaqus/Standard within the framework of the finite deformations theory with the initial unstressed state as reference. C fibers were modelled as linear, thermo-elastic and transversally isotropic solids. The polymeric matrix was assumed to behave as an isotropic, thermo-elasto-plastic solid. Plastic deformation was governed by the Mohr-Coulumb criterion and the total matrix strain was given by the addition of the thermo-elastic and plastic strain components. In addition, decohesion was included in the simulations by means of interface elements inserted at the fiber/matrix interface. The mechanical behaviour of these elements was expressed in terms of a traction-separation law which relates the displacement jump across the interface with the traction vector acting upon it. RESULTS AND CONCLUSIONS The RVE was subjected to different multiaxial stress states and the failure locus for each case was determined from the results of the numerical simulations. One representative example is the failure locus of the composite lamina under transverse compression (σ 33 ) and out-of-plane