Axial Cyclic Response of Unnotched and Notched Carburized Cylindrical Members under Constant Amplitude Completely Reversed Loading
by
Golan M. Newaz
Department of Theoretical and Applied Mechanics
Abstract
This investigation pertains to the evaluation of fatigue resistance, primarily at long life, of axially-loaded circumferentially-notched cylindrical surface-hardened (carburized) members (Kt = 1.46 and Kt= 2.22) prepared from AISI 8620H steel. Concurrently, the fatigue behavior of smooth carburized cylindrical member (Kt = 1.00) is also analyzed. Completely reversed constant amplitude load controlled tests were conducted. Sub-surface initiation was observed in all three sets of specimens in high cycle fatigue. Inclusion originated fatigue within the core was predominant in the unnotched members. For the notched carburized members the primary initiation site was found to be the case-core interface region. After detailed analysis of the data from the literature and this study, it is proposed that the "weak-link" effect in axial fatigue failure of smoth carburized members is influenced by the differential in fatigue resistances of the case and the core. The notched fatigue data strongly indicate that more benefit will be accrued from carburizing sharply notched members as opposed to shallow notched members for the same case depth. Finite element stress analysis has been essential for the determination of the states of stress and strain at subsurface locations of interest. It is suggested that a bimaterial model is adequate for the purpose of finite element stress analysis of carburized members. Multiaxial fatigue approaches proposed by Sines and Manson are utilized for the prediction of notched member life using the unnotched fatigue data. Unconservative estimates of life are achieved and these methods do not seem promising. However, good correlation is found using the Smith-Watson-Topper parameter, [E Smaxea]1/2. Finally, an analytical model is proposed for the determination of optimum case thickness of notched carburized members subjected to axial fatigue loading.
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