Modelling non-linear creep behaviour of an epoxy adhesive

Abstract

The creep behaviour of a rubber-toughened, two-part epoxy adhesive has been measured using specimens that have been stored in a desiccator and under ambient humidity. Tensile and compressive creep compliance curves for the dry material have been modelled using a stretched exponential function with parameters representing the short-term compliance, a mean retardation time for the relaxation process and a distribution of retardation times for the process. This function shows small departures from data at high stresses or long times. The retardation time parameter is observed to be dependent upon the magnitude of the stress giving rise to non-linear behaviour and enhanced creep deformation at moderate stress levels. At stress levels where behaviour is non-linear, creep curves under a uniaxial compressive stress are different from results measured under tension at the same stress. This is interpreted by relating the mean retardation time to an effective stress which is dependent upon the magnitudes of the shear and hydrostatic components of the applied stress. Results for material that has been stored under ambient humidity show a significantly higher creep rate than observed for the dry material. These results cannot be modelled using the function applied to the results for dry material. Attempts have been made to describe the creep behaviour of the adhesive with absorbed water in terms of two, overlapping relaxation processes such that the contribution from the short-term process is sensitive to the concentration of absorbed water. The creep results for the dry material have also been analysed using this two-process model with a smaller contribution from the short-term process, and this has produced a better description of behaviour at high stresses and long times than obtained with the function that models only a single process.