Fatigue crack growth studies on wrought and laser-peened 304 stainless steels

Abstract

Fatigue is categorised as progressive structural damage within materials caused by cyclic loading and it is a common issue for machine components. Laser Shock Peening (LSP) is a surface enhancement method which delays crack growth by improving mechanical properties by the means of generating deep compressive residual stresses as a result of multiple laser impacts. However, not much fatigue crack growth studies have been conducted especially on laser-peened 304 Stainless Steel.

In this project, the effect of fatigue crack growth rate in 304 Stainless Steel was investigated by performing fatigue tests through varying load ratios. Experimental data indicated that higher load ratios result in faster crack propagation rate when compared at a specific ∆K. Additionally, fatigue tests were compared between compact specimens under different surface conditions - Unpeened, LSP without ablative layer, and LSP with ablative layer. Interestingly, results from a-N curve and da/dN vs ∆K curve revealed that LSP had no significant effect in increasing fatigue life and delaying crack propagation.

The effects of LSP were insignificant due to a few reasons. Firstly, experiment conducted at high loading parameters might increase the effectiveness of crack propagation thus, disallowing crack closure from compressive residual stresses to take effect. Secondly, microstructural analysis showed that martensitic strain-induced transformation may be formed throughout repeated loading. As the yield strength of the material is improved due to compressive residual stress state within martensite, high loading conditions might eradicate this benefit as the material is known to be brittle after phase transformation. Lastly, the thickness of the specimen might compromise the effects of compressive residual stresses due to insufficient depth of plastic strain from LSP. As a result of stress balance, tensile stresses residing at the middle section of the specimen may encourage crack growth.

Hence for future work, it is recommended to conduct experiments at lower loading conditions with thinner specimens to observe the effects of LSP more prominently. Additionally, it is advised to use non-phase transformation materials for simpler and straightforward analysis.