Response of epoxy adhesives when stressed to failure in milliseconds

This investigation showed, as did an earlier investigation, that the adhesives tested exhibit greater resistance to rapidly applied stresses than they do to slower or gradually applied stresses. Also, the adhesives were shown to lose strength with test temperature whether tested rapidly or slowly. Within a region bounded approximately by a high‐temperature second‐order transition point (the ASTM heat distortion temperature) and a low‐temperature second‐order transition point, the test data fits an Arrhenius‐type exponential equation, S = AeE/RT, where S is the stress in the joint at failure and the other terms have their usual significance. Energies calculated from this expression are lower for dynamic (fast) than for static (slower) rates of testing. Further, it was shown that little change in bond strength occurs until time to failure is below 100 msec. Plots of stress versus 1/T for a series of test times show a decided discontinuity at 1/T = 0.0039 or −16°C.