Propagation and Closure Behavior of Small Fatigue Cracks under Two-Step Strain Amplitude

Cyclic strain controlled low-cycle fatigue tests were conducted on type 304 austenitic stainless steel under constant or two-step strain amplitude. The propagation behaviour of small fatigue cracks was observed with crack mouth opening behaviour. An interferometric displacement gage was used to measure the opening displacement of the surface cracks. The axial strain measured by the system was in very good agreement well with that measured by a differential transformer. The crack mouth opening displacement linearly increased with increasing crack length for small cracks. In the case of the two-step strain amplitude, the crack opening displacement was larger than that for constant strain amplitude when compared at the same crack length. The J-integral range calculated on the basis of the relationship between the stress and the crack mouth opening displacement was smaller than that obtained by the Dowling formula.