Fatigue initiation in adhesively-bonded single-lap joints

This paper presents detailed experimental fatigue data obtained from a selection of joints tested under cyclic loads. Some were tested to failure to determine fatigue load-life data. The other specimens were tested to induce different levels of damage. Multiple strain gauges (SGs) were used to record the change in backface strain during the testing, allowing measurement of damage in different locations. The overlap region was then sectioned and polished so it could be inspected using microscopy. The experimental results showed that damage first appeared in the fillet, appearing as a change in adhesive colour in a specific area. Cracks were observed in one specimen, which was tested for a higher number of cycles. A damage scale based on area affected, change in colour and size of cracks is proposed to classify the damaged sections. A result of this work is to tie the backface strain readings with the damage scale allowing damage to be assessed in a non-destructive manner.