Fatigue crack growth analysis of a ductile structural acrylic adhesive under constant-amplitude load control at various loading conditions

Abstract

The fatigue behavior of adhesives depends on the type of adhesive and loading conditions. Highly ductile adhesives, such as structural acrylic adhesives, are excellent under static conditions; however, their fatigue crack growth (FCG) is not yet well understood. Especially when ductile adhesives are used to bond large structures, the joints are subjected to cyclic creep, a combination of fatigue and creep. Therefore, it is important to understand the contribution of creep to FCG clearly.

The fatigue double cantilever beam tests of a structural acrylic adhesive were conducted by varying mean loads to investigate the effect of creep on FCG. Calculating the equivalent crack length from compliance, the FCG was found to be divided into three stages, similar to the creep curve. The relationships between FCG rate and energy release rate (ERR) and between ERR and the number of cycles until the FCG onset, i.e., the FCG relationship and G−N curve, were investigated using several ERR parameters. A parameter $G_{\Delta P}^{1-\gamma }·G_{\text{mean}}^{\gamma }$, which is a combination of ERRs represented by $\Delta P=P_{\max }-P_{\min }$ and $P_{\text{mean}}=\left(P_{\max }+P_{\min }\right)/2$, was found to be the most suitable crack driving force, and was therefore used to discuss fatigue and creep contributions. For the adhesive tested, the creep contribution was slightly greater in the load control than in the displacement control. Furthermore, the effect of R ratio on the FCG relationships of $G_{\max }$ and $\Delta G$ was discussed in relation to the fatigue and creep contributions.