Long Term Strength of Structural Adhesive Joints

The long term static strength of adhesive joints is analyzed in terms of a modified Prot method and sustained load tests. Data from the failure times under different loading rates are used to predict the static stress that an adhesive joint will withstand for an infinite time, i.e., the endurance limit. Despite theoretical shortcomings, the method is found to give reasonable estimates of the endurance limit as determined by standard sustained load tests. The ratio of the short term lap shear strength to the endurance limit is found to be independent of adhesive modulus, temperature, and sample geometry. For engineering calculations on lap shear structural adhesive joints under a static load (at 23°C, 50% R.H.), the endurance limit may be assumed to be equal to 0.25 of the short term strength.