Back Face Strain Calibration for Crack Length Measurements
Shaw WJD, Zhao W
(Received 0 0; accepted 0 0)

Abstract
A new calibration of back face strain (BFS) versus crack length a was developed. The calibration incorporates crack closure effects and is based upon a least squares multiple curvilinear regression analysis using seven specimens. Fatigue crack growth took place over a range of crack length to width ratios a/W from 0.2 to 0.8. The information was obtained under constant change in stress intensity factor DK using compact fracture toughness specimens of 2024-T351 aluminum alloy. In addition, two regular fatigue life crack propagation test conditions were used in the analysis database. The calibration function A*(a/W) is presented in a tabular form as well as in a polynomial equation format. The physical crack length was measured on both surfaces and later, after fracture, corrections of crack length were made for both curvature and plastic zone size. No effect of DK and specimen thickness on the calibration behavior was found to occur. The calibration equation was checked against crack lengths during fatigue crack growth behavior of both HSLA steel and mechanically alloyed aluminum materials and was found to he accurate.

Keywords:
back face strain, compliance, crack length, crack measurement, fatigue crack length

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Author Shaw WJD, Zhao W Title Back Face Strain Calibration for Crack Length Measurements Symposium , Committee on