Noticeable low temperature creep was established for a bainitic and a martensitic microstructure of the 100CrMnMo8 high strength roller bearing steel. The response revealed primary creep that differed between the microstructures, following a power law for martensite and the logarithmic description for bainite. The detected creep was pressure sensitive, higher in tension than in compression for the same stress level, following the strength differential effect (SDE) at material yielding. Two models were proposed where the stress variable for the pressure effect was based on the Drucker-Prager yield function and deviatoric creep strains were derived from a non-associated von Mises potential. Model parameters were determined from experimental series on the respective microstructure. When the models were evaluated against the experiments the accuracy was of the same order as the effects of different heat treatment batches and different load application rates. The importance of different material parameters in the descriptions was discussed.

Plasticity effects on fatigue growth were simulated for a physically short crack. The material description comprised the Drucker-Prager yield surface, non-associated flow rule and non-linear combined hardening. The simulated development of the growth limiting parameter agreed with the experimental crack behaviour with early rapid propagation followed by a transition to slow R-controlled growth. The crack was open to the tip without any crack face closure throughout all load cycles. Instead compressive residual stresses developed at the unloaded tip which supplied an explanation to the slow rate of the propagated short crack in this bainitic high strength bearing steel. The material's strength differential effect was the key difference explaining why compressive residual stresses instead of crack face closure was responsible for the short crack effect in this material.

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.

Monotonic and cyclic deformations were studied for a high strength bainitic roller bearing steel. The temperature of 75 °C corresponded to normal roller bearing conditions. The materials showed hydrostatic influence on yielding, but no or marginal influence of plastic deformation on density change. Therefore, a linear elastic constitutive model with pressure dependent yielding, non-associated flow rule, combined non-linear kinematic and isotropic hardening was necessary to characterize the cyclic behaviour. A stepwise process is detailed for determining the material parameters of the pressure dependent model, where particular attention was placed on the hardening parameters. One set of parameters was sufficient to describe all tested load ranges including compressive ratchetting. Some comparative tests were performed at room temperature, 150 °C and on martensitic specimens at 75 °C. The temperature influence was limited to the isotropic hardening parameters.

A small but not negligible non-linear elastic behaviour was detected when investigating cyclic uniaxial push-pull experiments on a high strength bainitic steel. Cyclic torsion experiments led to the conclusion that the shear modulus was relatively constant. A non-linear elastic model was implemented where the bulk modulus was extended with a second order term related to the elastic dilatation and where the shear modulus was constant. The material presented a strength differential effect (SDE), with larger yield stress in compression than in tension. Consequently, the non-linear elastic model was combined with a plasticity model that incorporated a Drucker-Prager yield surface, non-associated flow rule and combined non-linear hardening. Expressions that include non-linear elasticity were derived for the elastic-plastic hardening and the compliance tensors. The extended material model predicted the elastic-plastic results from cyclic push-pull experiments. Also, a phenomenological analysis of the cyclic elastic response showed isotropic damage in the elastic moduli. The steady-state damage increased linearly with the cyclic plastic strain range.