Title

ELEVATED TEMPERATURE BIAXIAL FATIGUE

Date of Completion

January 1986

Keywords

Engineering, Mechanical

Degree

Ph.D.

Abstract

A experimental program for studying elevated temperature biaxial fatigue of a nickel based alloy Hastelloy-X has been completed. A new high temperature fatigue test facility with unique capabilities has been developed. Effort was directed toward understanding multiaxial fatigue and correlating the experimental data to the existing theories of fatigue failure. The difficult task of predicting fatigue lives for non-proportional loading was used as an ultimate test for various life prediction methods being considered. The primary means of reaching improved understanding of elevated temperature multiaxial fatigue was through several critical non-proportional loading experiments. The direction of cracking observed on failed specimens was also recorded and used to guide the further development of failure theories. Cyclic deformation responses were permanently recorded digitally during each test to further understanding the constitutive response of this material. It was discovered that the cracking mode switched from primarily cracking on the maximum shear planes at room temperature to cracking on the maximum normal strain planes at 1200(DEGREES)F. In contrast to some other metals, loading path in nonproportional loading had little effect on fatigue lives. Strain rate had a small effect on fatigue lives at 1200(DEGREES)F. Of the various correlating parameters the modified plastic work and octahedral shear stress were the most successful. New experimental evidence is presented that strongly supports the notion that the component of stress opening or closing cracks on maximum shear planes influences "initiation" fatigue life through friction effects as it is known to do in Mode II and III macrocrack propagation. ^