Optical-Microscopic Observation of Crack Propagation Process in Cold-Rolled Low-Carbon Steel

Abstract

Many of recent papers on fatigue crack propagation have pointed out the importance of applied stress as factor determining crack propagation rate. However, the relation between plastic deformation ahead of crack tip and propagation rate has been observed by few investigators.In this study, 0.01% C steel specimens were initially cold-rolled to 15 reduction percent and the slip bands zone ahead of carck tip during fatigue was investigated by means of an optical microscope. The mechanism of fatigue crack propagation in this material is discussed on the basis of the interrelation found among the three parameters; the propagation rate, the slip bands zone size ahead of crack tip and the applied stress amplitude. Discussion was also made about the dormant or delayed period seen in the crack propagation. The results are summarized as follows:(1) The fatigue crack propagation process of 15% cold-rolled specimens has three stages, that is, the rapid propagation stage (Stage 1), the constant growth rate stage of long period (Stage 2) and the final stage of faster growth yielding complete fracture (Stage 3).(2) The crack propagation rate Rc in Stage 2 has no relation with the stress concentration factor of notched specimens and is expressed in terms of nominal stress amplitude σ as follows, Rc=5.3×10-12σ5.6(3) The relation between Rc and the slip bands zone size ξ0 ahead of crack tip is established asRc=2.1×10-4ξ01.8Therefore, Rc is approximately proportional to plastic energy ahead of crack tip.(4) The dormant or delayed period in crack growth is caused in the forms as (i) grain boundary blocking, (ii) crack branching, (iii) grain boundary propagation and (iv) unfavorable predeformation for crack propagation. The crack propagation blocked by grain boundary is found more often under lower stress amplitude, while the higher stress amplitude fatigue shows crack propagation delayed by crack branching.