Experimental research on fatigue crack growth behavior of Q690D high strength steel

The issue of fatigue and fracture in high strength steel (HSS) has always been a hot research direction due to long time service under cyclic loading. For studying the fatigue crack growth performance (FCGP) of Q690D HSS, fatigue crack growth rate (FCGR) tests on compact tensile (CT) specimens of two plate thicknesses were carried out at three stress ratios (SRs). The FCGR and thresholds were obtained, and general expressions for the FCGR and threshold suitable for different SRs were given. The research results show that this batch of Q690 HSS has good fatigue crack growth (FCG) resistance, and is better than the FCGR of ordinary steel in IIW-1823-07. The FCGR of Q690 steel increases with the increase of stress ratio (SR), and the fatigue crack growth threshold (FCGT) decreases linearly with the increase of SR. The values of the FCGR obtained from the grouped data are lower than the results from the single specimen data. The Paris parameters obtained based on single specimen data are biased towards safety. The macro-fracture in the stable crack growth zone is relatively flat. Secondary cracks and cleavage steps can be observed on the micro-fracture, showing obvious brittle fracture characteristics. The results of this research reveal the FCG characteristics of Q690D HSS material, which bridges the gap of existing research. It is of great significance for fatigue prevention and control, safety prediction and reliability assessment of HSS bridge structures.