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In this work, quasistatic crack initiation under mixed mode loading in planar (two-dimensional plane stress) functionally graded materials (FGMs) is studied. The goal of this work is to directly compare experiments and simulations so as to evaluate the applicability of the maximum tangential stress (MTS) criterion in predicting crack kinking in FGMs. Initially, crack initiation in the homogeneous material, which forms the basis of Our FGM--polyethylene--is studied. The (generalized) maximum tangential stress is applied through the use of finite elements to determine crack initiation angles in the same graded configurations studied experimentally. Computational results of fracture parameters (stress intensity factors and T-stress), and crack initiation angles are compared to experimental results and good agreement is obtained. It is seen that the MTS criterion is applicable to FGM crack initiation prediction if the inherent material gradient length scale is larger than the fracture process zone. [DOI: 10.1115/1.2936238] Keywords: FGM, crack initiation angle, mixed mode fracture, digital image correlation

A spatially varying cohesive failure model is used to simulate quasi-static fracture in functionally graded polymers. A key aspect of this paper is that all mechanical properties and cohesive parameters entering the analysis are derived experimentally from full-scale fracture tests allowing for a fit of only the shape of the cohesive law to experimental data. The paper also summarizes the semi-implicit implementation of the cohesive model into a cohesive-volumetric finite element framework used to predict the quasi-static crack initiation and subsequent propagation in the presence of material gradients. [DOI: 10.1115/1.2151210]

The dominant and higher-order asymptotic stress and displacement fields surrounding a stationary crack embedded in a ductile functionally graded material subjected to anti-plane shear loading are derived. The plastic material gradient is assumed to be in the radial direction only and elastic effects are neglected. As in the elastic case, the leading (most singular) term in the asymptotic expansion is the same in the graded material as in the homogeneous one with the properties evaluated at the crack tip location. Assuming a power law for the plastic strains and another power law for the material spatial gradient, we derive the next term in the asymptotic expansion for the near-tip fields. The second term in the series may or may not differ from that of the homogeneous case depending on the particular material property variation. This result is a consequence of the interaction between the plasticity effects associated with a loading dependent length scale (the plastic zone size) and the inhomogeneity effects, which are also characterized by a separate length scale (the property gradient variation). [DOI: 10.1115/1.1876434]