Modelling crack propagation in structural adhesives under external creep loading

Bulk adhesive compact tension specimens have been subjected to creep loads which range from 45% to 80% of the initial static failure load. Measurements of crack growth, load-line displacement, and crack tip profile were taken. Tensile creep tests were carried out on the same material and the results were used to generate material models for the analytical solution of steady-state power law (SSPL) creep and the numerical (finite element analysis) solution which incorporates the transient and primary creep material behaviour. Standard solutions for SSPL creep of compact tension specimens were used to calculate fundamental parameters such as the C* integral, creep zone growth, and transition time. From these data it would seem likely that the crack growth is controlled by creep, rather than elastic, singular fields. Crack driving parameters from both elastic and creep fields appear equally valid but the latter appear more discriminating. Finite element analyses were carried out for both a stationary crack and a propagating crack. The former are relevant to the incubation period and allow the redistribution of stress and accumulation of creep strain to be assessed. The latter enable the predicted conditions in the domain surrounding the propagating crack to be investigated for appropriate crack driving parameters. There is strong evidence to suggest that the adhesive uni-axial creep rupture strain can be successfully used to predict creep crack growth.