Composite materials are widely used in engineering applications. Among the different approaches proposed for the characterization of composite materials, the in-plane biaxial tensile test can be an interesting method instead of performing multiple uniaxial tensile tests. Indeed, those materials exhibit strong anisotropic mechanical behaviour especially when they are submitted to complex loadings. The major problem related to this test is the design of the cruciform specimen. In the literature, very different specimen shapes have been proposed for metallic and composite materials, depending on the mechanical behaviour to be characterized (elastic, plastic yield criterion ...).

In this study, the objective lies in the failure characterization of composite materials under a wide range of stress state. An in-plane cross specimen shape, previously defined by the authors [1], is validated by experiments in this study. The proposed shape ensures the following major requirements: (i) the failure occurs in the central zone of the specimen where the strain path can be controlled by the displacements imposed on each specimen arm, (ii) the stress (or strain) state must be homogeneous as far as possible in the central part of the specimen to make the post-treatment easier. The proposed specimen is composed of two aluminum tabs glued on each side of a constant thickness composite sample. The central area of the aluminum tabs is drilled and machined in order to film the specimen central zone during the test. The composite is a plain weave glass/epoxy laminate with a 1 mm constant thickness. The three parts are glued together using epoxy adhesive to form the final specimen. Experimental biaxial tensile tests are led for several displacement loading ratios from uniaxial to equi-biaxial at quasi-static conditions. Major and minor strains in the central zone (calculated by DIC technic) and measured tensile forces on the two specimen axes are obtained from these tests and constitute the experimental database.

According to the loading ratios, the minor and major stresses (or strains) at the onset of failure will define a failure envelop for the material. A comparison between the experimental failure envelop and the envelop generated by the well known Maximum Stress criterion is presented.

[1] A. Kobeissi, L. Leotoing, D. Guines and P. Rahme, Numerical investigation on the cruciform composite shape for the biaxial characterization test. ECCM18 – 18th European Conference on Composite Materials, Athens, Greece, 24-28thJune 2018.