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An approach for analyzing fatigue failures by investigating the underlying microstructure of the fatigue crack origin sites is presented. A precision sectioning technique, developed by the authors, makes it possible to slice fatigue specimens through the crack initiation sites thus permitting concurrent fractographic and metallographic examination. This failure analysis was found useful in determining the reason for low fatigue lives of titanium alloy powder compacts; nonmetallic inclusions were found to be the prime source for early fatigue initiation. In superalloy powder compacts, residual porosity was related to the fatigue crack origins. In β-processed or cast titanium alloy products, the shear of large colonies of transformed α-platelets and fracture along grain boundary α were identified as the main initiation mechanisms. The identification and the elimination of the metallurgical crack initiating features has the potential for improving the fatigue life and reducing the scatter of the fatigue results, since the largest proportion of fatigue life is spent in crack initiation.
fractography, fractures (materials), fatigue (materials), fatigue life, crack initiation, crack nucleation, titanium, titanium powder, titanium castings, titanium microstructure, heat resistant alloys, superalloy powder, nonmetallic inclusions, precision sectioning
University of Cincinnati, Cincinnati, Ohio
Air Force Materials Laboratory, Wright-Patterson Air Force Base, Ohio