Complex fracture parameters for an interface crack between twohardening materials: a photoelastic study

Abstract

In this paper, complex stress intensity factors (SIFs) at an interface crack are determined for a range of applied loads, crack lengths and remote mode mixes using automated photoelasticity. The specimen geometries comprise epoxy resin and aluminium alloy halves bonded together, and are loaded in either compact tension in mixed-mode conditions or in three-point bend under mode I conditions. In the experiments, full-field isochromatic data were obtained from the epoxy half using an established phase-stepping technique. A reworked approach to the determination of the SIFs was developed by combining a least-squares over-deterministic method for fitting crack-tip stress equations to the data and a weighting factor that ensures that only data in the singularity zone are used. For comparison, some of the specimens were tested using a linear-elastic finite element (FE) analysis and/or by experiment using homogeneous test specimens. Excellent agreement between the experimental and numerical SIF moduli was achieved for remote mode I loadings. However, for good agreement to be made between the phase angle results requires an additional phase term to be added to the FE solution at each load to account for the development of a crack-tip plastic zone. Further, results for the SIFs from remote mixed-mode loadings of the compact tension specimen only have a meaningful interpretation in light of small-scale yielding conditions. It is shown, qualitatively, that the experiments verify some of the predictions made in the literature of asymptotic behaviour at interface crack tips from results of elasto-plastic FE analyses.