Butt joint strength: effect of residual stress and stress relaxation

Previously published butt joint strength data suggest that the residual stress generated by cooling has little effect on the joint strength. Stress relaxation test data are reported here for the adhesive used in those joint tests. This adhesive displays highly nonlinear, stress level-dependent viscoelasticity at stress levels approaching the adhesive'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°C below the adhesive's glass transition temperature. Consequently the influence of residual stress on the butt joint strength 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 interface corner where failure initiates, can decay significantly when given sufficient time. The temperature dependence of the interface corner fracture toughness Kfc, a material constant that has been shown to characterize the effect of bond thickness on butt joint strength, is also discussed. The observed variation in Kfc with temperature is compared with the variations exhibited by the adhesive's yield strength and plane strain fracture toughness.