Title page for ETD etd-02252003-110749

Type of Document Master's Thesis
Author Knipling, Keith Edward
Author's Email Address k-knipling@northwestern.edu
URN etd-02252003-110749
Title High-cycle fatigue / low-cycle fatigue interactions in Ti-6Al-4V
Degree Master of Science
Department Materials Science and Engineering
Advisory Committee
Advisor Name Title
Dowling, Norman E. Committee Chair
Kampe, Stephen L. Committee Member
Reynolds, William T. Jr. Committee Member
  • High-cycle fatigue; low-cycle fatigue; Ti-6Al-4V;
Date of Defense 2002-09-16
Availability restricted
The largest single cause of failure in fan and compressor components in the cold frontal sections of commercial and military gas turbine engines has been attributed to high cycle fatigue (HCF). Additionally, both high-cycle fatigue (HCF) and low-cycle fatigue (LCF) loadings are widely recognized as unavoidable during operation of these components and because the classic Linear Damage Rule (LDR) neglects to account for the synergistic interaction between these damage contributors, dangerous over predictions of lifetime can result.

Combined low-cycle fatigue / high-cycle fatigue (HCF/LCF) loadings were investigated in smooth Ti-6Al-4V. The specimens were subjected to a variable amplitude block loading history comprised of completely-reversed (R = -1) tension-compression overloads followed by constant-amplitude zero-tension (R = 0) minor cycles. Axial specimens were excised from forgings representative of turbine engine fan blade forgings, and consisted of approximately 60% primary á in a matrix of lamellar á + â.

Data are reported for smooth specimens of Ti-6Al-4V subjected to both constant amplitude and variable amplitude loadings. The axial specimens were prepared according to two distinct specimen conditions: low stress ground and longitudinally-polished (LSG+LP) and stress-relieved and chemically milled (SR+CM) conditions. Significantly longer lives were observed for the LSG+LP specimen condition under both constant and variable amplitude loading, due to the presence of a beneficial compressive surface residual stress. The presence of this residual stress was confirmed by x-ray diffraction, and its magnitude was of the order of 180 MPa (~20% of the yield stress). In either specimen condition, no appreciable effect of periodic overloads on the life of subsequent minor cycles was observed.

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