Fig. 13: FCG curves in NR and TMCP steels

Assessment of FCGR path and mechanism in air

From the previous discussions, it is obvious that the material microstructure influenced the FCGRby crack path diversion, branching and formation of metal crumbs in-between branched and re-joined crack fronts. High FCG retardation was obtained when large angle diversion, large angle branching and change in the orientation of metal crumbs occurred. Some of the distinct micrographs showing phases and morphologies influencing the FCGR are presented in Fig. 14 for the NR steel and Fig. 15 for the TMCPsteel. The phases where obtained by scanning along the entire length of the fatigue crack from the initiation point to point of test stoppage.