Biaxial-Tension Fatigue of Inconel 718
by
Daniel L. Morrow
Department of Mechanical and Industrial Engineering
Abstract
Many components used in the power generation and aircraft industries are subjected to severe multiaxial stresses. Some components, such as turbine discs, operate in stress quadrants I and III where their principal stresses are of the same sign. Little experimental work has been done to evaluate material fatigue characteristics for these loadings. This research characterized the multiaxial fatigue behavior of Inconel 718 in stress quadrants I and III.
Predictions of cracking behavior and fatigue life were based on multiaxial fatigue parameters generated from tension-torsion data and compared to experimental results. Observations of crack formation from replicas and post-failure sectioning of the specimens helped identify the appropriate damage parameters for this material.
All of the tests resulted in failure cracks that initiated and grew in shear. Roughly 90 percent of the fatigue life was spent initiating and growing these shear cracks to a surface length of 1.0 mm. Transition from shear to tensile crack growth occurred late in life if at all.
Fatigue life predictive capabilities of the five multiaxial fatigue theories considered improved as terms were added that affected shear crack initiation and growth. The poorest correlation for the biaxial-tension and biaxial mean stress tests came from the maximum principal strain theory, where the average life prediction was nonconservative by a factor of 26 and test results fell within a scatterband of a factor of ±41 in life.
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