The Creep Behaviour of Structural Adhesive Joints II

Abstract The creep behaviour of two structural adhesives (a phenolic resin/polyvinyl formal composite and a modified epoxy novolac) has been studied in their glassy state when used to bond high tensile steel lap joints. Curves of adhesive shear creep strain vs. In t consisted of delay periods (times after application of load but before creep is discernible) followed by essentially logarithmic creep. Creep curves were thus classified by recording delay time, td , extrapolated creep rate at zero strain, γ(t0), and the gradient of the logarithmic creep line, k. By comparing these values for different joint geometries, it has been established that the high stress concentrations found in lap joints under elastic conditions become much reduced as creep progresses. A model is proposed to account for the transition from delay time to creep behaviour.