BUTT-JOINT STRENGTH - EFFECT OF RESIDUAL-STRESS AND STRESS-RELAXATION

Previously published butt joint strength data suggest that the residua l stress generated by cooling has little effect on the joint strength. Stress relaxation test data are reported here for the adhesive used i n those joint tests. This adhesive displays highly nonlinear, stress l evel-dependent viscoelasticity at stress levels approaching the adhesi ve's yield strength, and significant stress relaxation occurs once the adhesive yields. For example, when loaded to yield, stress levels can decrease by 30% over a period of 30 min even at temperatures of more than 100 degrees C below the adhesive's glass transition temperature. Consequently the influence of residual stress on the butt joint streng th could be much smaller than would be predicted by a linear analysis. The peak stresses in an adhesive joint, in the yield zone at the inte rface corner where failure initiates, can decay significantly when giv en sufficient time. The temperature dependence of the interface corner fracture toughness K-fc, a material constant that has been shown to c haracterize the effect of bond thickness on butt joint strength, is al so discussed. The observed variation in K-fc with temperature is compa red with the variations exhibited by the adhesive's yield strength and plane strain fracture toughness.