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The fatigue fracture surface micromorphology in the ultra-low growth rate regime has been examined in aluminum, copper, nickel, titanium, and iron based alloys. A transition from striated to faceted fracture surface appearance is found for all materials examined. For the aluminum alloys a faceted appearance was observed over a three decade growth rate regime down to 5 × 10−9 mm/cycle for which macroscopic test data are available. At intermediate crack growth rates (2 × 10−5 to 1 × 10−4 mm/cycle) where the striation mode usually dominates, evidence of the faceted morphology is also seen and persists to higher growth rates within this growth rate range the lower the stacking fault energy of the material. This is believed related to a preference for faceted growth in planar slip materials. Faceted growth is produced at low ΔKapp levels under uniform loading conditions and at low ΔKeff levels associated with complex load interactions. The fracture facets generated by these two conditions are not readily distinguishable. Finally, it is not yet possible to identify the increment of discontinuous crack extension on the fracture facets in the crack growth regime where discontinuous crack extension must occur.
crack propagation, fatigue (materials), mechanical properties, growth, fractography
Professor, Lehigh University, Bethlehem, Pa.
research engineer, Westinghouse Hanford Co., Richland, Wash.