Effect of residual stress on fatigue life

Residual stress play a very important role in determining the fatigue life of a component. Residual stress in a component is the result of one or many of the following reasons namely non homogeneous plastic deformation due to metal forming and machining operations or uniform heating and cooling and non uniform volumetric changes associated with phase transformation during heat treatment ,welding etc.The nature of residual stress on critical areas such as component's external surface and stress concentrated zones like fillets depend, to a larger extent, on the manufacturing processes and the process sequence followed.

The influence of manufacturing process of a component and their sequence thereof on the residual stress distribution and thereafter on the fatigue life of an automotive component was studied by conducting representative fatigue tests on the components prepared under different process conditions. The manufacturing process involved are induction hardening and tempering, extrusion and machining. The effect of each manufacturing step on the fatigue life was quantified by altering the process sequence and conducting fatigue tests.

Finite element analysis was employed to understand the nature of distribution of macro residual stress due to a critical process step namely extrusion. X-ray diffraction measurements were made to find the surface residual stress after each of the process. The effect of stress concentration due to a machined fillet and the surface finish near that zone was also studied experimentally.

An appropriate fatigue life estimation model was employed to estimate the fatigue life of the component taking residual stress into account.

Based on the experimental results and theoretical predictions, recommendations were given on the sequence of the manufacturing steps to be adapted and the fillet radius to be used for improved fatigue life.
[1] M. Smith, F. Fischer, M. Romois, O.S. Es-said, Engg. Fail. Anal. 14 (2007) 138-146
[2] G.A. Webster, A.N.Ezeilo, Int. J. Fatigue. 23 (2001) S375-S383.
[3] J. Papuga, Int. J. Fatigue. 33 (2011) 153-165.

Fatigue Test Setup

                                       Fatigue test setup

Influence of machining process

                            Cycles to failure Vs sample condition