Analysis of spall propagation in case hardened hybrid ball bearings

Bearings are critical to the overall performance and reliability of jet aircraft engines. Despite their optimized design, they cannot escape the damage induced by foreign object debris, improper handling, overloading, or rolling contact fatigue which can cause surface fatigue failures to occur in the form of small pits or spalls. Spalls will grow and propagate with continued engine operation and allow the main engine shaft to misalign leading to engine failure and possible loss of a multi-million dollar aircraft. Thus reducing the amount of time between initial spall formation and catastrophic engine failure is of great importance to pilot safety and mission success for military applications.;Spall propagation experiments carried out by the Air Force Research Labs show that M50, M50NiL, and 52100 bearing steels have different spall propagation characteristics. It is uncertain how certain aspects of bearing design such as initial residual stress, surface hardness, gradient in flow curve, and ball mass affect spall propagation rate. Both static and dynamic analyses will be performed here to simulate these contributions and the bearing operating conditions during spall widening and propagation.;The variation in plastic response of plastically graded, case hardened M50 NiL bearing steel was initially unknown and it was uncertain how the plastic response will affect the spall propagation that occurs within this case hardened region. A new method will be shown here that uses indentation experiments and finite element modeling to determine the plastic response of plastically graded, P675 and M50 NiL case hardened bearing steels. The method will use a material-dependent representative plastic strain that will relate indentation hardness measurements to flow stress, which will vary with depth for a graded material. The material dependent representative plastic strain will be validated for two nongraded materials: 303 stainless steel and the core region of P675.;An analysis of the critical stresses and plastic strains that develop within a spall edge due to multiple ball impacts will be performed using finite element modeling. The results of which will predict large amounts of plastic strain and tensile residual stresses to occur where cracks appear in the actual spalled bearings. It will be shown that the contribution from ball mass has the greatest affect on the magnitude and distribution of plastic strain within an impacted spall edge which would cause 52100 bearings have faster spall propagation characteristics than M50 and M50 NiL bearings. This behavior is observed in the spall propagation experiments performed by AFRL. The effects of initial residual compressive stress and gradient in flow curve will have secondary effects on spall propagation due to the geometry of the spall edge and the nonlinear subsurface trend in hardness for case hardened M50 NiL.