Experimental behaviour and fatigue life prediction of bonded joints subjected to variable amplitude fatigue

The variable amplitude fatigue behaviour of adhesive joints has not been extensively considered in literature and considerable knowledge gaps regarding its underlying mechanisms still exist, hindering accurate life prediction. This study investigates the effect of loading sequence and number of load transitions on single lap joints (SLJs) bonded with a toughened epoxy adhesive. Life prediction is also conducted using established cumulative rules: Palmgren-Miner (PM), Broutman and Sahu (BS) and, Hashin and Rotem (HR). To overcome their limitations, a modified Cortan-Dolan approach is proposed. For this, SLJs were subjected to single transition (low-to-high and high-to-low load transitions at different stages of the fatigue life and with different magnitudes) and multi-block loading spectra with different numbers of transitions. Results showed an extension of the fatigue life for high-to-low transitions. This was attributed to a combination of load interaction effects, as evidenced by an elastoplastic numerical simulation, and non-linear damage accumulation. Conversely, low-to-high transitions exhibited a damaging effect. For single transition spectra, BS and HR models were seen to improve prediction over PM by indirectly considering these effects. The proposed approach further improved this prediction. Multi-block results showed no significant influence of the number of load transitions and were satisfactorily predicted using PM.