Fatigue Crack Growth from Compression Pre-Cracks

The compression pre-crack method can be used to develop near-threshold fatigue crack growth rates much quicker than the ASTM standard load shed method. The compression pre-crack method is faster than the standard method and eliminates load history effects that are of concern when the standard method is used. However, crack growth developed using the compression pre-cracking approach is susceptible to different load history effects. A suite of tests using the compression pre-crack method that were performed on AISI 9310 steel are presented in this paper. An approach for identifying the critical crack growth increments required to develop valid data is presented, and the effects of three potential mechanisms are evaluated. For these tests, the residual stress field resulting from the initial compression field appears to be the more important effect on crack growth behavior. Crack growth increments measured from the notch tip that are 2.5 times larger than the size of the plastic zone resulting from the compression pre-cracking are required in order to develop valid data on the fatigue crack growth rate.