SEDL / STP / STP1231-EB / STP13972S

Measurement of Transformation Strain During Fatigue Testing

Neu, RW
visiting assistant professorvisiting scientist, University of IllinoisWright Laboratory, Materials Directorate, UrbanaWright-Patterson Air Force Base, ILOH

Sehitoglu, H
Professor, University of Illinois, Urbana, IL

Pages: 17    Published: Jan 1994

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Abstract

Dimensional changes from stress-induced transformations simultaneously influence the fatigue behavior, while thermal-induced transformations do not. These dimensional changes, including the anisotropy, are measured in the laboratory under various loading conditions. This technique can be used to determine the best balance between materials with good dimensional stability as well as good fatigue resistance. Axial strain-controlled monotonic and fatigue test results for different treatments of 4320 steel, both carburized and plain, tested at temperatures from -40°C to 255°C are discussed. Upon increase in volume, due to the stressinduced phase transformation in the carburized steel containing 35% retained austenite, a mean compressive stress developed, improving the fatigue life by a factor of 10. However, this steel was also the least dimensionally stable. A liquid nitrogen treatment reduced the amount of retained austenite to 14%, which made the steel more dimensionally stable, but decreased the fatigue life.