How defects in an adhesive layer influence the fatigue strength of bonded steel-sheet specimens

Fatigue specimens were manufactured with a number of artificial bond defects. The defects were different configurations of non-bonded areas within the nominally perfectly-bonded region. These defects included variants with a reduced degree of bond filling, bonds with a channel without adhesive, zig-zag-shaped bond edges, and finally bond areas with smooth corners. Fatigue testing was performed with the bond loaded in the peel mode. The stiffness was measured for the bonds and was found to vary significantly with defect type and size. The fatigue strength was also found to differ substantially for different bond defects. A finite element model combined with fracture mechanics was used to predict the stiffness drop during fatigue testing of the bonded specimens and to predict fatigue life. The effect of defects in the bond on the stiffness and life was predicted in a satisfactory way.