Investigation on Fatigue Crack Propagation Behaviour of U-Notched 316 L Specimen Under Strain-Controlled Mode

During the practical service condition, the strain-controlled fatigue is an important reason for component failure, which cannot be demonstrated by the traditional fatigue crack growth test under stress-controlled mode in accordance to ASTM E647. Therefore, this study is devoted to investigating the fatigue crack propagation behaviour of U-notched specimen under strain-controlled mode. The numerical study is firstly performed to clarify the stress/strain concentration effect and crack tip mechanical behaviour, which is then validated by strain-controlled fatigue tests. The compliance method is also adopted to calibrate the crack length obtained by optical measurement. A modified geometry factor for stress intensity factor (SIF) K considering short crack is proposed for comparison with a J-integral solution based on Electric Power Research Institute (EPRI) method. Digital image correlation (DIC) technology is also adopted to capture the strain field at crack tip to validate the numerical strain distribution. Moreover, different fracture parameters, including ΔK and ΔJ, are applied to characterize the crack driving force. It is shown that the strain concentration phenomenon at the notch root can reflect the accumulation of fatigue damage. The ΔK under small scale yielding (SSY) situation is not applicable due to the large plastic deformation occurring at the crack tip. Whereas, agreement is found between crack propagation rate and the fracture parameter ΔJ based on the elastic-plastic fracture mechanics (EPFM).