Corrected procedure for crack lengths calculation by the unloading compliance technique for Charpy size specimens

The problems with the crack length determination by the unloading compliance method are well known for Charpy size specimens. The final crack lengths calculated for bent specimens do not fulfill ASTM 1820 accuracy requirements. Some investigations have been performed to resolve this problem by correcting the compliance value, but satisfying results were not obtained. In the presented work the measured specimen compliance was taken as a correct value and the calculation procedure was modified. On the basis of experimentally obtained compliances of bent specimens and optically measured crack lengths a calculation procedure enabling accurate crack length calculation up to 5 mm of plastic deflection was developed. Applying the new procedure to a sample of 238 measured crack lengths, more than 80% of the values fulfilled the ASTM 1820 accuracy requirement, while presently used procedure provided only about 30% of valid results. The newly proposed procedure can be also prospectively used in modified form for specimens of a size being different from Charpy size.