On the origin of pop-in crack extension

Pop-in is a small amount of abrupt crack extension encountered during both monotonic and fatigue loading. It can drastically decrease the fatigue life and lead to early catastrophic failure. However, a complete explanation for the occurrence of pop-in has not been reported. In this paper, the fracture toughness, fatigue crack growth rate, microstructure and the segregation patterns using EPMA, have been studied in medium manganese rail steel and an experimental high strength Cr-V rail steel, both of which exhibit pop-in crack extension. The results clearly show that pop-in represents a deviation from the homogeneous behaviour of a material and it occurs in the rail steel in a region where a segregation band of Cr and/or Mn is present. The magnitude of the pop-in crack extension roughly corresponds to the thickness of the segregation band in a rail steel.