Computerized Single-Specimen J-R Curve Determination for Compact Tension and Three-Point Bend Specimens

Abstract

A computer-controlled test system was developed to measure J-R curves and JIc on compact tension and three-point bend specimens. The crack length was measured by unloading compliance. Improved crack length accuracy was obtained by amplifying the load and crack opening displacement (COD) signals by factors of 10 to 20 before the unloading. Crack length accuracy was further improved by collecting 500 data pairs on unloading using a fast data acquisition method. In the compact tension specimen, crack length and load-line displacement were measured with a load-line COD gage. In the three-point bend specimen, load-line deflection was measured using the crosshead deflection corrected for specimen and test fixture stiffness. An error analysis was performed to determine the crack length errors. The analysis showed the beneficial effect of using load and COD ranging to reduce the crack length errors. J-R curves were measured on 17-4-Ph stainless steel heat treated to 948 MPa yield. The J-R curves were measured on two specimen geometries, compact tension and three-point bend, using four starting a/w ratios from 0.65 to 0.80 and three side groove depths, 0%, 12.5%, and 25%. The test results showed that in specimens with side grooves of 12.5% and 25%, the measured JIc-values were independent upon both specimen geometry and starting a/w ratio. The material tearing modulus, Tmaterial, was found not to be a material property since its value depended upon specimen geometry.