A Novel Mean Stress-Independent Fatigue Model for Bonded Joints With Ductile Adhesives

A mean stress-independent lap shear fatigue model is proposed for Aluminum 2024 alloy Single Lap Joints (SLJ) that are bonded with ductile, polyurethane-based, adhesive. Fatigue data is generated in the High Cycle Fatigue range, for 4 values of mean stress and multiple levels of alternating load. Individual S-N curves are constructed, for each value of mean stress, and the results are statistically analyzed. A separate logarithmic regression of the entire pool of data is performed, considering maximum stress and fatigue life of the samples. The test data align along the same regression line. As a result, the amount of tests to perform for durability characterization of single lap joints bonded with ductile adhesive can potentially be reduced. High sampling frequency information of the displacement of the specimens during fatigue tests is periodically acquired and filtered. The creep-like curve describing the evolution of the mean displacement during the fatigue tests suggests that cold flow phenomena could trigger fatigue failure, rather than the propagation of a crack.