Variability in Flight-By-Flight Crack Growth Rate Behavior
AIR FORCE FLIGHT DYNAMICS LAB WRIGHT-PATTERSON AFB OH
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The initial result of variable amplitude fatigue crack growth rate test program are presented. The test program was designed to study the influence that the stress intensity factor gradient dKda had on crack growth rate behavior in 7075-T6 aluminum alloy. Constant stress intensity factor conditions were used to study this behavior, using a twenty-four inch wide center-cracked panel. Based on earlier work by Hillberry on constant amplitude crack growth rate behavior, a three-parameter lognormal distribution function was selected to describe the crack growth rate data. Comparison of crack lengths showed no gradient effect. Parameters of the statistical distribution were compared using the secant and incremental polynomial methods of differentiation and for different flight intervals achieved by removing points prior to the differentiation process.