Computational prediction of strain energy release rates of delamination in composite materials

Abstract

Delamination is one of the most frequent failure modes of composites. Its importance is evident since delamination may appear even when there is no apparent damage in the structure. The residual strength and stiffness of the delaminated composite decreases significantly in consequence of that. A study of the delaminations in the damaged structure must be made to determine their critical dimensions for propagation via the stress intensity factor (SIF), K, or the strain energy release rate (SERR), G. Preliminary calculations were carried out in isotropic materials. Modes I, II and mixed mode situations were analyzed. Since three types of interfaces for crack propagation were studied (orientations 0/0, 0/90 and 90/90), some problems arise in determining the SIF because of its oscillatory behavior. Hence the SERR study was used for this type of material, comparing numerical results with experimental DCB (double cantilever beam), ENF (end-notched flexure), ADCB (asymmetric DCB) and MMB (mixed-mode bending) test features. The SERR was calculated inn two different ways: one was derived from the experimental procedure and the other relies on the Rybicki-Kanninen method. All the numerical computations were performed in a commercial FEM code. The conclusions were: a) the 3D meshes used are able to predict the SERR and the SIF; b) the calculation of SERR is nearly mesh-independent, contrary to the calculation of the SIF; c) the correlation between experimental data and numerical results depends on the stacking sequences and on the type of interfaces for crack propagation and d) GII increases more rapidly then GI with a refinement of the mesh near the crack tip.