Propagation rate transients in J-controlled fatigue characterization of adhesives

Abstract

Fatigue analyses of adhesively bonded composite laminates were conducted to investigate discrepancies in experimentally measured rates of crack propagation in double cantilever beam specimens subjected to increasing or decreasing load histories. A cohesive fatigue damage model that accounts for the quasi-static traction-separation response and bridging was used in a finite element model of the test to identify transient effects caused by the evolution of the process zone in the adhesive, as well as the long-range, low-intensity bridging induced by the knit supporting carrier inside the adhesive. In addition to the quasi-static cohesive properties, the fatigue model requires two parameters that were obtained by fitting the rates of propagation at two experimental data points. The results indicate that the analyses accurately reproduce transient and steady-state rates of propagation observed in the experimental results, as well as the threshold of propagation. Transient rates of propagation consistently exceed those in steady state, which underscores the need to understand transients and account for them in fatigue characterization. The results also highlight that a Paris curve for steady-state propagation can be obtained most quickly using a decreasing load function, but that this curve is an unconservative bound to the rates of propagation. A lateral shift in the Paris curve is proposed for a conservative characterization that removes the effect of bridging and other R-curve effects.