A non-microstructural crack formation model for understanding fatigue life degradation in shot peened carbon steel under LCF loading

• Fatigue-life degradation in shot-peened (SP) carbon steel was investigated.
• Cyclic relaxation of CRS and work hardening were measured by XRD technique.
• Remarked cyclic relaxation of CRS was observed at LCF regime.
• Fatigue crack formation for specimen without SP was investigated for comparison.
• Non-microstructural crack formation model was proposed to understand degradation.

The present paper was based on the experimental investigation from the previous results that analyzed the residual stress relaxation and its fatigue behavior in shot-peened (SP) specimen during fatigue loading Kim et al. (2013) [12,10]. Here, the non-microstructural crack formation model was proposed to understand fatigue life degradation of SP carbon steel under LCF loading. When the applied stress exceeded the yield strength of the original material from the previous results, the SP specimen showed a shorter fatigue life as compared to the specimen without SP. To demonstrate the empirical model, the fatigue crack initiation and propagation in no SP specimens were observed by the plastic replication method at the LCF loading. The results showed the following: (i) remarked residual stress relaxation occurred under LCF loading which leads to the ability to have a significant influence on the degradation of fatigue life in the SP specimen; (ii) the existing microcracks on the treated surface caused earlier fatigue failure owing to crack formation, which was not influenced by microstructural effects. This presentation will discuss the non-microstructural crack formation in detail and suggest a mechanism for the fatigue life degradation.