A branch criterion for shallow angled rolling contact fatigue cracks in rails: Fid-Bau Portal

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

Wong, S.L. / Bold, P.E. / Brown, M.W. / Allen, R.J.

Squats are rolling fatigue cracks that initiate on the running surface or gauge corner of a rail, owing to repeated loading under rail-wheel contact stresses. Undoubtedly, there is plastic deformation beneath the wheel in the rail contact area. However, when the crack is long, it can be assumed that the plastic flow generated on the rail surface has little effect on the crack tip. Since the stress state around the crack tip can be described by an elastic stress field, analysis based on linear elastic theory is able to characterise the material behaviour. If a squat grows upwards, it will cause an irregular ride over a rough rail surface when the surface material flicks out. A more important problem with the squat is branching downwards, which may lead to fracture of the rail and danger of derailment. Using the ultrasonic technique one may inspect cracks to detect the formation of branches, but this underestimates branch crack length because of the difficulty of finding the right probe angle. Therefore the criterion governing crack branching needs to be known. A well-developed squat grows at about 10 deg 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 deg 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. 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.