Mixed-Mode I/II fracture behavior of asymmetric composite joints

The partitioning of the total strain energy release rate, G(tot), into the Mode I, G(I), and Mode II, G(II), components is challenging, especially in asymmetric cracks. Although many researchers have studied the fracture behavior of composite materials under mixed-mode loading using symmetric and/or asymmetric mixed-mode bending (MMB) specimens, few studies exist in the literature regarding the experimental investigation and analysis of the fracture behavior of asymmetric MMB specimens comprised of orthotropic layered adherends. In this study, a new approach, designated as the "extended global method", was established the fracture mode partitioning. The "extended global method" was applied for the analysis of the experimental data. The mixed-mode fracture behavior of adhesively-bonded composite joints was experimentally investigated using asymmetric mixed-mode bending specimens. Finite element models were developed in order to validate the approach. The virtual crack closure technique was used for the calculation of the fracture components at the crack tip and an exponential traction-separation cohesive law was used to simulate the fiber bridging zone. The crack propagated along paths outside the symmetry plane and, therefore, mode partition could not be performed in commonly accepted ways followed for symmetric specimens. In addition, the experimental compliance method was used for calculating the fracture energy of the examined asymmetric mixed-mode bending specimens. Results obtained using the "extended global method" and the experimental compliance method were in good agreement with the results from FE models. The outcome of this study combined with the results obtained from pure Mode I and Mode II experiments can be used to establish failure criteria for the examined joints, that can be eventually be used for designing structural joints with the same adherends and adhesives. Copyright (C) 2016 The Authors. Published by Elsevier B.V.