Role of Laser Shock peening on Fatigue Deformation of HSLA Steel under Extreme Stress Conditions

Abstract

Engineering and structural components are frequently subjected to symmetrical or asymmetrical cyclic loading under a wide range of service conditions. In the case of symmetrical cyclic loading, the strain development is consistent since many such components are specifically designed for specific applications. However, loading asymmetry can develop as a result of seismic events, accidental extreme stress excursions, or adjustments to the operational environment. Asymmetric stress is considered more detrimental to the structural components and leads to cyclic damage known as ratcheting fatigue. Ratcheting induces plastic strain on a structure that can cause a significant reduction in fatigue life. Since fatigue is mainly considered to occur from the surface, different surface treatments, including flame hardening, shot peening, ultrasonic shot peening, laser shock peening (LSP), etc., are widely used to protect components subjected to symmetric cyclic loading and comparatively less explored for the asymmetric loading condition. In the current investigation, it was aimed to study the effect of LSP on the surface properties and ratcheting behavior of ASTM A 588 Grade D High Strength Low Alloy (HSLA) steel. For this, three sets of mean stress and stress amplitude were chosen to conduct uniaxial ratcheting experiments at ambient temperature. The tests were performed on both untreated and laser-treated specimens. The salient findings indicate that laser shock peening greatly enhances surface mechanical characteristics with minimum degradation of the surface roughness. LSP produced compressive residual stress (CRS) of -500 MPa on the treated surface and the compressive zone was extended up to a depth of ~500 μm. Ratcheting strain accumulation was significantly decreased, which improved fatigue lives – about twice as compared to that in untreated specimens. The compressive residual stresses that were induced and the marginal grain refinement that occurred during the LSP improved the fatigue lives of the specimens.