Predicting Fatigue Life of Axle Components: A Unique Modeling Approach 2020-01-0606
Loss of lubrication within an axle assembly due to the formation of through-thickness cracks in structural components can result in severe durability issues for the internal parts (gears, splines, bearings, etc.). One such example of a structural crack resulting in lubrication leakage can be observed in the cover pan of a Salisbury axle that has been subjected to cyclic fore-aft loading conditions (which are intended to replicate the loads acting on the axle during vehicle acceleration or deceleration). Investigation of the cover pan crack locations was performed using Magnetic Particle Inspection (MPI) and indicated the formation and propagation of multiple cracks adjacent to the cover pan bolt holes. Finite Element Analysis (FEA) was performed in order to simulate the fore-aft beaming test and was able to successfully identify the critical crack locations; however, crack initiation calculations yielded extremely conservative predictions, likely resulting from the large stress-riser induced by the pretensioned bolted joint. Additional physical testing intended to characterize the propagation of a surface crack initiating at a bolt-edge-contact location showed that an SAE 1010 steel plate, which is commonly used in cover pans, could withstand a significant number of load cycles prior to the formation of a through-thickness crack. To account for the through-thickness propagation of a crack originating at the edge of a bolted joint, this paper proposes a unique simulation framework which utilizes components of the Structural Stress Method (SSM). Unlike the typical implementation of the SSM, which was developed to predict the propagation and fracture of weldments, the analytical method described herein accounts for the mean stress state present in bolted cover pans. Validation of this approach using various axle designs and load magnitudes suggests that this method is capable of predicting cover pan leakages in a reliable manner.