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Three test conditions were chosen to conduct multiple constant amplitude fatigue tests on a powder metallurgy (P/M) aluminum alloy. These test conditions were chosen to produce mean target fatigue lives near 1, 50, and 200 kilocycles. A statistical rule was used to select specimens for fractographic examination. Two product forms, a forging and an extrusion, were tested in both the L and L-T orientation. The forging has approximately an 8% lower yield strength and a 20% higher reduction in area than the extrusion. At the 1-kilocycle target fatigue life, the more ductile forging has a statistically better fatigue performance. Beyond 10 kilocycles, the higher strength extrusion appears to have a longer life, although this was not statistically verified. It is suspected this is because of the increased variance at these longer fatigue lives. Further testing is required to verify this point. Additional testing of the extrusion showed that if the data are fitted to a Weibull model, the P/M extrusion has a vastly different failure rate than reported for ingot metallurgy (I/M) products. A decreasing failure rate for the P/M extrusion was verified with a nonparametric test. The difference in failure rates may be related to the finer grain size and constituent particle size found in P/M aluminum products. Additional study is needed to verify this hypothesis.
technical specialist, Arco Metals Co., Arlington Heights, IL
Senior staff fellow, Division of Computer Research and Technology, National Institutes of Health, Bethesda, MD
Stock #: JTE10692J