Mechanical behaviour of a roller bearing steel : Strength differential effect, low temperature creep and propagation of short cracks

Sammanfattning: Fatigue cracks in bearings either initiate from the surface or from an inclusion below the rolling contact surface. Then, short cracks start to propagate. Short crack grow at considerably faster rates than long cracks subjected to a nominally equivalent stress intensity factor range. One of the explanations for the difference in growth behaviour between short and long cracks is the development of plastic deformation at the advancing crack tip. In order to investigate this effect, the analysis of short crack propagation at bearing loads requires understanding of the fundamental material behaviour. This thesis presents the material characterisation of a bainitic high strength bearing steel, where the yield stress in tension was lower than in compression. This phenomenon is called strength differential effect (SDE). The work studies the influence of the SDE on the cyclic plastic properties, the elastic behaviour of the material, low temperature creep. These mechanical properties are quantified and modelled using continuum models.Paper A focused on the characterisation of the SDE which was modelled using a Drucker-Prager yield surface and a non-associated flow rule. The cyclic mechanical properties were quantified and modelled using combined non-linear hardening.In paper B the elastic behaviour of the material was studied; the material showed non-linear elastic behaviour in uniaxial tension and compression. The elastic modulus was higher in compression than in tension at high stress levels. On the other hand, the cyclic torsion experiments showed that the stress-strain elastic relation in shear was linear. A non-linear elastic model was proposed.Low temperature creep was studied in Paper C, where the creep strains were quantified in tension and compression. The material showed higher creep strains in tension than in compression for the same stress level and the influence of the SDE in low temperature creep was analysed.The short crack growth in the bainitic steel was analysed through simulations in Paper D. The material model described in Paper A was implemented in a material subroutine. The simulations captured the development of plastic strains as the short crack becomes long. The material model could qualitatively describe the experiment results, where the change in rate as the crack advanced from short to long was ascribed to the growing plastic zone ahead of the crack tip.