Multiple specimen J-R curves were developed for groups of 25.4-mm-thick (1T) compact specimens with different a/W values and depth of side grooving. The purpose of this investigation was to determine Jc (J at the onset of crack extension) for each group. Judicious selection of points on the load versus load-line deflection record at which to unload and heat-tint specimens permitted direct observation of approximate onset of crack extension. It was found that the present recommended procedure for determining Jc from multiple-specimen R-curves, which is being considered for standardization, consistently yielded nonconservative Jc-values. A more basic approach to analyzing multiple-specimen R-curves is presented, applied, and discussed. This analysis determined Jc-values that closely corresponded to actual observed onset of crack extension.
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A load-line extensometer using linear variable differential transformer (LVDT) displacement transducers was developed and evaluated for use in multispecimen and single specimen J value versus crack growth (J-R) curve testing. The LVDT extensometer accuracy and repeatability was compared with that of clip gage extensometers commonly used in fracture toughness evaluatons. Data on load versus displacement and J value versus crack growth for ASTM A533, Grade B, Class 1 (Unified Numbering System [UNS] K12539) steel obtained by using both extensometers were also compared.
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