Crack Wake Influence Theory and Elastic Crack Closure Measurement

Abstract

Experimental and analytical evidence indicates that “closure” or interference of crack faces does not entirely isolate the crack tip from damaging strains. To account for the contribution of the cyclic crack tip strain below the opening load, a new analysis technique has been developed for estimating ΔKeff. This technique is based on crack wake influence theory (the relationship between the effect of crack wake interference on a displacement or strain measurement location and the corresponding effect at the crack tip). Solving the distribution and position functions provides a method of equating remote compliance measurements to reduction in crack tip elastic strain. The experimental method involves determining the ratio of the compliance of the shielded crack to that of the fully open crack (CR). This relationship remains elusive until it is recognized that 1 — CR is directly related to the influence of the closure mechanism on both the measurement location and the crack tip. The verification of this method includes experimental measurements of a large center crack panel with multiple displacement and strain gage measurement locations. The standard crack has been replaced with a milled slot, allowing precision blocks to be inserted at specific positions to simulate crack wake interference. Finite element analysis and theory of elasticity are linked to the experiment to better understand the measurement requirements for crack tip closure. The estimation of ΔKeff using the ratio of influences from the theory of elasticity can be directly compared with other approximation techniques such as the adjusted compliance ratio or opening load.