Shear mode growth of short surface-breaking RCF cracks

Surface-breaking cracks occur in railheads due to unidirectional material flow caused by ratchetting from repeated contact loading. This type of crack is frequently observed on the gauge corner of high rails in curved track, where wheel sliding and high tangential forces occur. The present investigation examines the growth of a similar type of crack in a twin disc test specimen, using elastic–plastic finite element calculations and fracture mechanics theory. A two-dimensional finite element (FE) model was used to simulate the rolling contact fatigue twin disc test and crack behaviour for four short cracks: 0.05, 0.1, 0.2 and 0.4 mm. The results from the FE calculations were used in a Hobson-based, elastic–plastic fracture mechanics model. The Pineau criterion was employed to determine the governing crack growth mechanism, either shear or tensile. Net crack growth rate was defined as the difference between the rate of advance at the crack tip and crack mouth truncation by wear. Shear crack growth dominated for all four crack lengths and the net crack growth rate was greater than zero. The paths of crack advance were calculated for each crack; they show that the three shortest continue to grow parallel to the surface of the disc. The 0.4 mm crack, however, grows upward; thus, it may lead to spalling failure. The results are compared with others from similar twin disc tests and discussed in terms of their application to real train traffic situations where crack coalescence may occur.