Stretch zone formation in cyclic fracture of 20MnMoNi55 pressure vessel steel

Stretch zone formation during cyclic fracture in a nuclear pressure vessel steel has been examined. Cyclic fracture tests are carried out at different load ratios and incremental plastic displacements. Stretch zone features were manifested in local regions, however, their extent could be correlated to the test parameters employed. The mean stretch zone width decreased with R ratio and increased with incremental plastic displacement. Stretch zone dimension could be empirically correlated to a cyclic stretching coefficient proposed in this investigation. A comparison of cyclic fracture toughness obtained from stretch zone width and that measured experimentally was made, and the systematic difference was explained.