Abstract This investigation was aimed to understand the different sub-stages seen by the crack during fatigue crack growth in Paris and threshold regimes using acoustic emission technique (AET). The AE signatures obtained from continuous monitoring during the entire test in different regimes of FCG curve have been examined. AE activity measured as counts per cycle and energy per cycle indicates discontinuities corresponding to (1) change from plane stress to plane strain condition, (2) onset of crack closure and (3) intergranular cracking in the transition regime.