Investigations of Material Response to Fatigue Phenomena in Contacting Bodies
Investigating the fatigue performance of machine components has been of significant interest to improve reliability and reduce the maintenance costs. In the current work, analytical as well as experimental approaches are used to investigate material response to contact fatigue damage. In particular, two fatigue phenomena namely; fretting fatigue and rolling contact fatigue (RCF) are studied. Fretting fatigue is a damage mechanism observed in machine components subjected to fretting in tandem with fluctuating bulk stresses. A fretting test fixture was developed to investigate fretting fatigue behavior of AISI 4140 vs. Ti-6Al-4V in a cylinder-on-flat contact configuration. The critical damage value for AISI 4140 was extracted using the method of variation of elasticity modulus. The fretting fatigue lives obtained from the proposed computational fatigue damage model were found to be in good agreement with the experimental results. The RCF investigation focuses on developing a modified 2D numerical model to simulate RCF damage in line contact configuration. First, a new computationally efficient approach is developed to investigate sub-surface initiated spalling in large bearings. Previously developed continuum damage mechanics based 2D fatigue model was modified to incorporate stress mapping procedure and dynamic remeshing tool to make the model computationally efficient. The new approach was validated against the previous numerical model for small rolling contacts. The scatter in the RCF lives and the progression of fatigue spalling for large bearings obtained from the model show good agreement with experimental results available in open literature. The ratio of L10 lives for different sized bearings computed from the model correlate well with the formula derived from the basic life rating for radial roller bearing as per ISO 281. Furthermore, the RCF model was extended to incorporate elastic-plastic material in order to investigate RCF of case carburized steels. A series of micro-indentation tests were conducted to obtain the hardness gradient in the case carburized 8620 steel. The hardness gradient in the material was modeled by changing the yield strength as a function of depth. The residual stress distribution due to carburization process was modeled by modifying the damage evolution law. The model was used to compare the rolling contact fatigue (RCF) lives of through hardened and case carburized bearing steel with different case depths. Based on the model results, the optimum case depths to maximize the RCF lives of the case carburized bearings at different loading conditions were obtained. This model was then modified to investigate RCF in refurbished case carburized bearings. Refurbishing process was simulated by removing a layer of material from the original surface after a set number of fatigue cycles. The original material properties, residual stresses and the fatigue damage accumulated prior to refurbishing in the remaining material were preserved. The refurbished geometry was then subjected to additional fatigue cycles until damage was detected. According to model results, more fatigue cycles prior to refurbishing enhance the total fatigue life of refurbished bearings. It was also found that beneficial impact of refurbishing on RCF lives of case carburized bearings depends on the relative values of case depth, contact half width, refurbishing depth.
Sadeghi, Purdue University.
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