Shear propagation of subsurface cracks under rolling contact fatigue was investigated on quenched and tempered SAE 5135 gear steel. The formation of subsurface cracks almost parallel to the contact surface was observed, highlighting the de-attachment of micro-particles from the cracks surfaces due to shear. Subsequent wear and plasticization processes lead to fragmentation of these particles, which transform in a smaller incoherent structure, and to the formation of a gap between the crack faces. The gap is maximum in the central part of the crack, with a dimension of some microns, whereas it tends to zero approaching the crack tips. Moreover, wear reduces the roughness of the crack faces by smoothing the asperities.\ud\udA FEM model was developed to simulate the process of crack propagation in presence of asperities along the crack path, by taking in to account their shape variation due to wear. For this aim, the crack was divided in several zones, assigning different faces roughness to each of them according to the experimental evidence. The significant influence of the roughness severity in determining the crack driving force could be quantified by this model, putting in evidence its locking effect for varying crack length.
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机译：研究了经淬火和回火的SAE 5135齿轮钢在滚动接触疲劳下表面裂纹的剪切扩展。观察到几乎平行于接触表面的地下裂纹的形成，突出了由于剪切作用而使微粒从裂纹表面脱离。随后的磨损和塑化过程导致这些颗粒破碎，并以较小的不连贯结构转变，并在裂纹面之间形成间隙。间隙在裂纹的中心部分最大，尺寸为几微米，而趋于零，接近裂纹尖端。此外，磨损通过使凹凸不平而降低了裂纹面的粗糙度。\ ud \ ud通过考虑由于磨损引起的形状变化，开发了有限元模型来模拟存在裂纹沿裂纹路径时裂纹的扩展过程。 。为此，根据实验证据，将裂纹分为几个区域，并为每个区域分配不同的表面粗糙度。该模型可以量化粗糙度严重性对确定裂纹驱动力的重大影响，从而证明了其对于变化的裂纹长度的锁定效果。
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