Size evolution of the surface short fatigue cracks of 1Cr18Ni9Ti pipe-weld metal with a local viewpoint

The size evolution of surface short cracks in 1Cr18Ni9Ti pipe-weld metal is investigated in this paper. A local viewpoint is used to explain the observed evolution of cracking phenomena. The so-called ‘effectively short fatigue crack criterion’ is used to characterize distributed populations of cracks. Special attention is paid to the initiation zone of dominant effectively short fatigue cracks (DESFCs) and then the zones ahead of the DESFC tips. The results reveal that the evolutionary size exhibits significant characteristics of microstructurally short crack (MSC) and physically short crack (PSC) stages. Fatigue damage is contributed largely to the initiation, and irregular growth, of effectively short fatigue cracks (ESFCs) in the MSC stage. In the PSC stage, the damage is due mainly to the DESFC growth, and partially to the growth of ESFCs and the coalescence of ESFCs themselves and with the DESFC. The damage processes involve gradual transitions from a non-ordered/chaotic status in the fatigue initiation regime, to an independently random status at the boundary between the MSC and PSC stages. Finally, a subsequent transition occurs to a history-dependent random status.