Fatigue crack growth – Mode I cycles with periodic Mode II loading

The influence of periodic Mode II loading on Mode I fatigue crack growth is studied using a new type of loading device. The average rate of crack growth is found to be governed mainly by four parameters; ΔKI and the Mode I R-ratio, the magnitude of the Mode II load and the Mode II period, M (M = number of Mode I load cycles per Mode II load). The Mode II load has to be large enough to create residual tangential displacements of the crack faces otherwise no effect occurs at all. Two mechanisms are found, Mode II-induced crack closure that reduces the crack propagation rate (long range) and a mechanism that increases the growth rate temporarily at every Mode II load (short-range). For a specific Mode I load and Mode II magnitude, it is possible to find a minimum crack growth rate for a certain Mode II period, Mopt. At high R-ratios the Mode II-induced closure mechanism disappears, but the temporary increase in growth rate at every Mode II load is still active. A semi-analytical model for the average crack growth rate during sequential Mode I and Mode II loading is presented.