On the physical interpretation of the R-ratio effect and the LEFM parameters used for fatigue crack growth in adhesive bonds

The available models for the prediction of fatigue crack growth in adhesive bonds rely on the similitude principle. In most cases, one of three similitude parameters based on the strain energy release rate (SERR) is used; i.e. Gmax,(ΔG)2, or ΔG. In all cases it is usually observed that keeping the similitude parameter constant, and changing the R-ratio, results in a different crack growth rate. In this paper it is shown that this apparent 'R-ratio' effect is caused because the selected similitude parameter does not define a unique load cycle; a single value of the similitude parameter could correspond to infinitely many load cycles. The strain energy dissipation approach is used to show that the resistance to fatigue crack growth is related to the maximum applied load. The amount of energy available for crack growth is shown to be related to the applied cyclic work. With these relationships the R-ratio effects reported in literature can be qualitatively explained, purely in terms of the actual applied load cycle. Although it is possible that the material behaviour also depends on the R-ratio, the magnitude of these effects can only properly be determined if the applied load cycle is correctly described first.