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Effects of Kmax on closure-free, near-threshold fatigue crack growth are examined in ingot metallurgy aluminum alloys 2024 and 7050, and powder metallurgy aluminum alloy 8009. Significantly lower closure-free fatigue crack growth thresholds, as well as accelerated fatigue crack growth rates, were observed as Kmax increased. Results indicate that no single value of fatigue crack growth threshold exists, because near-threshold fatigue crack growth is dependent on both Kmax and ΔK. The near-threshold Kmax effect is linked to an increase in crack-tip void production. Experimental observations suggest that Kmax-accelerated, closure-free, near-threshold fatigue crack growth rates were caused by changes in crack-tip process zone damage mechanism(s) that are a result of increased crack-tip driving force.
fatigue crack growth, threshold, aluminum alloys, K, max, load ratio, R, fractography
Graduate Student, Virginia Tech, Blacksburg, VA
Mechanical Engineer, National Research Council, NASA-Langley Research CenterVolpe National Transportation Systems Center, HamptonCambridge, VAMA
Senior Scientist, NASA-Langley Research Center, Hampton, VA