Direct identification of nonlinear damage behavior of composite materials using the constitutive equation gap method

This paper aims at improving the reliability of the identification of parameters that govern a non-linear damage law for orthotropic materials, the non-linearity considered being due to the inherent damage induced by in-plane shear loading. The adopted identification technique is based on the minimization of the constitutive equation gap (CEG). It is built upon the quadratic deviation between the measured and computed strain field. The paper explains how an appropriate use of the CEG method can improve the identification of material model parameters from strain field measured by an optical technique of full-field measurement. The minimization problem is solved by the modified Levenberg-Marquardt method. In the present work, the experimental data are replaced by numerical ones provided by the finite element (FE) simulations. These latter are performed on an orthotropic glass/epoxy composite material. The obtained parameters based on these numerical data are considered as reference ones. A sensitivity analysis is proposed as an attempt to examine the influence of some parameters, among which the mesh refinement, the initial set of parameters, the experimental noise, and the orthotropy ratio. Two types of noise simulating the measurement errors are tested: a random noise simulated by a Gaussian white noise and a bias noise. The obtained results demonstrated the feasibility and applications of the proposed method. The use of the CEG method gives results of good quality.