The influence of fillet rolling on fatigue strength - experiment and calculation

Fillet rolling is a method that significantly improves the fatigue strength of members. Residual compressive stresses induced in the surface layer during the fillet rolling process are able to retard or prevent crack propagation. Large gains in fatigue strength up to 200% can be achieved. A procedure was developed to enable the fatigue strength calculation of fillet-rolled members. Two main topics had to be considered: The application of the FEM to simulate the rolling process and to calculate the residual stresses and the application of fracture mechanics to assess the effect of the compressive residual stresses on crack propagation. The influence of different process parameters can be investigated. After the simulation of fillet rolling the redistribution of the residual stresses dependent on cyclic loading can be determined by a FEM calculation. All the calculated results can be compared to measured results obtained from the X-ray diffraction method. Good correlation has been obtained for both the magnitude and the distribution pattern of the residual stresses. Fracture mechanics concepts were used to determine the fatigue strength and the depth of crack arrest. Stress intensity factors can be calculated both for the residual stresses and the loading stresses. Adding these two stress intensity factors an effective stress intensity factor can be calculated that characterises the ability of a crack to propagate. Comparing this effective stress intensity factor with the threshold value of fatigue crack propagation the fatigue strength of the rolled member can be found. The calculated fatigue strengths are slightly smaller than experimentally determined. The paper shows the FEM-simulation of the fillet-rolling process, the calculation of the redistribution and the results of fatigue strength calculation of notched components and crankshafts.