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.