Volume 22, Issue 6 (November 1994)
Back Face Strain Calibration for Crack Length Measurements
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 ΔK using compact fractures toughness specimens of 2024-T351 aluminum alloy. In addition, two segular fatigue life crack propagation test conditions were used in the analysis database. The calibration function A*(a/W) is presented in a tubular 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 ΔK 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 be accurate.