Effects of K-max on fatigue crack growth threshold in aluminum alloys

Effects of K-max on closure-free, near-threshold fatigue crack growth are e xamined in ingot metallurgy aluminum alloys 2024 and 7050, and powder metal lurgy aluminum alloy 8009. Significantly lower closure-free fatigue crack g rowth thresholds, as well as accelerated fatigue crack growth rates, were o bserved as K-max increased. Results indicate that no single value of fatigu e crack growth threshold exists, because near-threshold fatigue crack growt h is dependent on both K-max and Delta K. The near-threshold K-max effect i s linked to an increase in crack-tip void production. Experimental observat ions suggest that K-max-accelerated, closure-free, near-threshold fatigue c rack growth rates were caused by changes in crack-tip process zone damage m echanism(s) that are a result of increased crack-tip driving force.