PSI - Issue 57

11

Yuri Kadin et al. / Procedia Structural Integrity 57 (2024) 236–249 Kadin et. al / Structural Integrity Procedia 00 (2023) 000 – 000

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 K II

 K III

 K /  K *

 K I

Crack front coordinate,  °

Fig. 14. Crack of horizontal orientation (  =0 ° ). Due to symmetry only one-half of  SIF distribution is presented; the second half (90 ° <  <180° ) is approximately the mirror reflection of the first half across the YZ -plane (see Fig. 10) at  =90° . Due to friction the symmetry can be violated, however in the current case the  SIF distribution is almost symmetric. As is shown by Figs. 13 (  =90º) and Figs. 15 (  =45º), the values of  K II and  K III are in the moderate range (below the fatigue threshold ), even though the loading conditions are extreme. Higher SIF values are obtained for the crack of horizontal orientation (  =0º): as is shown in Fig. 14,  K II can slightly exceed the threshold value (  K /  K * >1 for  <25º) which is fraught with some risk of fatigue failure. Note, that the opening mode K I is not vanishing in the case of crack at chamfer, however it is low compared to the shearing modes. This is typical for the RCF conditions, where the shear stress (and the modes K II and K III ) are the main drives of fatigue damage. For some conditions (e.g. compressive residual stress) and crack configurations the opening mode (activated by tension) can even vanish, as occurs in the case of vertical crack (see again Fig. 13). According to the current FE analysis the horizontal cracks are the most critical (see Figs. 14). Additionally, according to the experimental data horizontal cracks prone to be larger than vertical and inclined ones. Nevertheless, it does not mean that the rollers with large horizontal cracks at chamfer, are always critical. Recall, that the current parametric study was run assuming extremely high contact pressure, which means that in less demanding application the imperfect product still may perform. For this, an additional FE simulations were performed, considering the normal (moderate) loading conditions. As is shown in Fig. 16, the contact loading has strong effect on the crack criticality: the contact pressure reduction causes dramatic decrease of the  K II magnitude. This effect is primarily caused by the mitigation of high peak pressure at the contact truncation and the fact that the loaded contact remain far from the chamfer. This means that under the moderate conditions the truncation peak does not form, but the pressure at the roller edge drops to zero.