Elastic-plastic fracture mechanics assessment of low constraint aluminium test specimens

Recent studies have shown that the near crack-tip stress field at a given J value is dependent on geometry. This dependence has been linked to the degree of constraint in the geometry, with low constraint geometries losing J dominance at very low deformation levels. New approaches centred on the use of a two-parameter description (e.g. J-T and J-Q) of the crack-tip stress-strain state have emerged. However, there is a serious lack of experimental and numerical results for low constraint geometries to quantify the T-stress and Q-value in the literature. This paper describes details of an experimental and numerical program carried out on low and high constraint geometries (CCT and TPB) fabricated from an aluminium alloy. The results show that the experimental and numerical fracture toughness values (J,) agree within +/-10 percent. The T-stress and Q-value two-parameter methodologies are successful at indexing the fracture toughness, ordering the data into a systematic trend of decreasing fracture toughness with increasing T or Q, albeit with some scatter. This allows the use of practical two-parameter failure criteria, in the form of J-T and J-Q loci, to predict the behaviour of cracked components, without the conservatism associated with the use of high constraint test geometries.