Determining singular and non-singular Williams' expansion terms from full-field measurements: Consideration of structural effects on fracture behavior

When a cohesive crack propagates within a fiber reinforced composite, the crack tip will potentially be subjected to complex far-field stress. In quasi-brittle or ductile materials, the latter may affect the fracture behavior itself hence predictions simply using Griffith theory may deviate from experience. In this context, we investigated the influence of higher order linear elastic fracture mechanics parameters T and B (translating the nature of far-field stress) on KIC during mode I crack propagation. Full-field measurements are considered using both Digital Image Correlation (DIC) and Williams' series expansion to obtain KIC, T and B. The objective is to determine KIC by taking into account structural effects with T and B. Metrological aspects are initially assessed to evaluate the error bars for the method when considering realistic experimental biases. In particular, the influence of the projection zone, camera sensor noise and the speckle-like DIC pattern on the evaluation of KIC, T and B were investigated. Quasi-static loading fracture experiments in mode I were ultimately performed on epoxy resin (Hexply (R) M21) specimens with two different geometries (Tapered Double Cantilever Beam and triangular). It was shown that non-singular terms strongly depend on specimen geometry, inducing some structural effects in the vicinity of the crack tip. The impact of these effects on estimated KIC is ultimately discussed in the light of the results of the metrological assessment.