A finite element analysis of stable crack growth in an aluminium alloy

Extensive stable cracking has been observed in large test pieces of 25 mm thick weldable AlMgZn alloy which is used in the construction of a portable bridge. Standard fracture specimens produced valid K_IC values, with short cracks exhibiting unstable fracture. Finite element analysis of the large specimens determined a valid J_R-curve that can increase the effective K_C by several times the K_IC value. The R-curve has an unusual ‘concave’ shape that is associated with the change from initially flat fracture to fully slant fracture. The early stages of the R-curve are affected by in-plane constraint that can be indexed by the T-stress. The R-curve can be used to explain the stability of long cracks in full-scale tests on a bridge prototype, compared with the instability of short cracks in small, standard test pieces.