SEDL / STP / STP1122-EB / STP24150S

Fatigue Mechanics: An Assessment of a Unified Approach to Life Prediction

Newman, JC
Senior Scientist, NASA Langley Research Center, Hampton, VA

Phillips, EP
Research Engineer, NASA Langley Research Center, Hampton, VA

Swain, MH
Material Scientist, Lockhead Engineering and Sciences Co., Hampton, VA

Everett, RA
Research Engineer, U.S. Army Aviation Systems Command, Hampton, VA

Pages: 23    Published: Jan 1992

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Abstract

The separation between fatigue crack initiation and propagation has often been defined as a macrocrack, visible in a low-power microscope. However, in the last decade, research on the small-crack effect in numerous materials has indicated that crack propagation from a microstructural defect (5 to 20 μm in size) consumes a large percentage (50 to 90%) of the total fatigue life of materials, even near the fatigue limit. Thus the initiation of a “fatigue crack” may occur early in life from defects or irregularities, such as inclusions, voids, dislocations, or slip bands. In the present paper, an assessment has been made of a total-life analysis based solely on crack propagation. The analysis is based on observations of defect sizes at initiation sites and on fatigue-crack-growth rate data for small and large cracks. The assessment was based on data from 2024-T3 aluminum alloy, 2090-T8E41 aluminum-lithium alloy, annealed Ti-6Al-4V titanium alloy, and high-strength 4340 steel under either constant-amplitude or spectrum loading. Comparisons made between fatigue lives measured on notched specimens with those computed from the total-life analysis agreed well. The computed lives generally fell within the scatterband of experimental fatigue life data.