Experimental and Numerical Analysis of Sub-Surface Cracks in Railway Wheels

Shelling is the term used to describe the loss of part of the tread of a railway wheel as the result of the initiation and growth of a fatigue crack from an internal defect in the rim. Shelling makes the wheel unserviceable and to prevent it, railway wheels are periodically subjected to non-destructive testing. However in order to assess the criticality of any cracks found, it is necessary to know the modes I, II and III stress intensity factors of the crack. In this paper a versatile approach for the analysis of internal cracks in wheels under Hertzian loads is described. It is based on the analytical calculation of the displacement field in the wheel and on imposing it as boundary condition in a finite element analysis of the wheel zone close to the crack. Experimental tests were executed on photoelastic models to validate the results. A new technique was developed to naturally obtain an internal crack in the epoxy resin model. The stress-freezing technique was employed to elaborate the photoelastic fringes and to obtain the stress intensity factors. The numerical and the experimental results are critically discussed and their comparison shows an interesting and encouraging agreement.