Mixed-mode fatigue crack growth in FM73 bonded joints

Fatigue tests have been carried out to investigate mixed mode fatigue crack growth behaviour in FM73 epoxy adhesive using double cantilever beam (DCB) specimens. The DCB configuration used consisted of equal thickness mild steel adherends bonded with FM73 adhesive. The joints were tested under pure mode I and a range of fatigue mixed-mode conditions using a relatively simple, variable-mode loading fixture developed in previous work [1]. The fatigue testing was carried out in displacement control, with an initial load ratio (R) of 0.1. The fatigue load decreased as the fatigue crack grew and this load was recorded. Crack growth was monitored and measured using a video microscope. The results suggest that crack initiation in the test specimens is controlled by the mode I strain energy release rate, GI component. The fatigue crack growth rates were characterised using a Paris law approach, from which it appears that the total strain energy release rate range, ΔGTotal, is a more dominant factor in controlling crack growth than the mode I component of strain energy release rate range, ΔGI. For a quantitative description of the mixed-mode fatigue crack growth, generalised forms of the Paris relation are developed.