Near threshold fatigue testing

Measurement of the near-threshold fatigue crack growth rate (FCGR) behavior provides a basis for the design and evaluation of components subjected to high cycle fatigue. Typically, the near-threshold fatigue regime describes crack growth rates below approximately 10[sup [minus]5] mm/cycle (4[times]10[sup [minus]7] inch/cycle). One such evaluation was recently performed by the authors for the binary alloy U-6Nb. The procedures developed for this evaluation are described in detail here to provide a general test method for near-threshold FCGR testing. In particular, we describe techniques for high-resolution measurements of crack length performed in-situ through a direct current, potential drop (DCPD) apparatus, and a method which eliminates crack closure effects through the use of loading cycles with constant maximum stress intensity.