Increase in tensile compliance with time during creep tests at different stress levels on the epoxy adhesive that has been stored under ambient humidity
Continuous lines = model with two relaxation processes
Broken lines = calculated contributions from the short-time process

Any adhesive joint under a long-term load of constant magnitude is liable to undergo deformation and failure. NPL are developing improved models for determining this creep behaviour of structural adhesives.

The results of creep testing have shown that adhesives, which have been stored under ambient conditions, generally exhibit significantly higher creep rates than for the dry material.

A new model developed at NPL describes the creep behaviour of a structural 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 for long times, than that obtained with the function which models only a single process.

The model has recently been extended to polypropylene, and takes into account the effects of physical ageing, a time-dependent process.

For more information, please contact Greg Dean