Author

Xiaoyan Su

Date

July 1996

Document Type

Dissertation

Degree Name

Ph.D.

Department

Dept. of Materials Science and Engineering

Institution

Oregon Graduate Institute of Science & Technology

Abstract

A study of surface initiated rolling/sliding contact fatigue (RCF) behavior of two pearlitic and four low/medium carbon bainitic steels has focused on three aspects; RCF performance as a function of contact pressure, failure mechanisms, and ratchetting strain behavior including the effect of slide/roll ratio. A contact pressure range of 850 to 2300 MPa, water lubrication and a slide/roll ratio of 10% were used to establish RCF life. The relationship between rolling contact fatigue life and contact pressure is non-linear. The relative performance of the steels is consistent with their mechanical properties, with longer RCF life being associated with higher strength irrespective of microstructure. The six steels tested exhibited very similar RCF resistance as a function of normalized contact pressure, P[subscript]0/k. Two distinct crack growth modes have been identified. The shear band cracking mode, which is associated with the severely deformed surface layer, predominates above the theoretical shakedown limit of P[subscript]0/k = 4. The branched cracking mode predominates at lower P[subscript]0/k values where subsurface material is relatively undeformed. Failure mechanisms have been suggested respective to the two crack growth modes. The effect of both oil and water lubrication on crack growth has been investigated. Ratchetting strain data of a pearlitic steel are a non-linear function of both contact pressure and the number of contact cycles. The most important feature of the non linearity is its asymptotic strain rate behavior. A critical strain concept for crack initiation has been proposed. The variation in ratchetting strain consistently reflected the variation in tangential to normal traction ratios (T/N) at different slide/roll ratios, with a higher strain for higher T/N and convergence in strain rate corresponding to convergence in T/N ratios as contact cycles increased. The variation in T/N values is partially attributed to the influence of sliding distance on the coefficient of friction through building up a stable contact interface, and a stick-slip phenomenon involved at slide/roll ratios ≤ 1 %.

Identifier

doi:10.6083/M40C4SPM

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