A branch criterion for shallow angled rolling contact fatigue cracks in rails

Abstract A well-developed ‘squat’ grows at about 10° to the upper surface of the rail. It is observed that this shallow angled rolling contact fatigue crack may change its growth direction; either to branch downwards and grow at about 55° to the rail surface or to branch upwards. Various research workers have shown that the crack is subjected to a characteristic sequence of tensile and shear stresses. The downward branches are more detrimental to the integrity of rails because they can lead to fracture. Therefore it is necessary to identify the condition that transforms the stable shallow-angled crack to a branch. Experimental results on a cruciform specimen subject to the characteristic load sequence show that the crack growth direction is determined by the effective mode I stress intensity factor range and degree of overlap in mixed mode loading.