Studies on fatigue life enhancement of pre-fatigued spring steel specimens using laser shock peening

• Laser peening significantly extended fatigue life of pre-fatigued spring steel.
• Increase in fatigue life of laser peened specimens was more than 15 times.
• Black PVC tape is an effective coating for laser peening of ground surfaces.
• Repeat peening repaired local surface melted regions on laser peened surface.
• Technique is effective for life extension of in-service automobile parts.

SAE 9260 spring steel specimens after enduring 50% of their mean fatigue life were subjected to laser shock peening using an in-house developed 2.5 J/7 ns pulsed Neodymium-doped Yttrium Aluminum Garnet (Nd:YAG) laser for studying their fatigue life enhancement. In the investigated range of process parameters, laser shock peening resulted in the extension of fatigue life of these partly fatigue damaged specimens by more than 15 times. Contributing factors for the enhanced fatigue life of laser peened specimens are: about 400 μm thick compressed surface layer with magnitude of surface stress in the range of −600 to −700 MPa, about 20% increase in surface hardness and unaltered surface finish. For laser peening of ground steel surface, an adhesive-backed black polyvinyl chloride (PVC) tape has been found to be a superior sacrificial coating than conventionally used black paint. The effect of repeated laser peening treatment was studied to repair locally surface melted regions and the treatment has been found to be effective in re-establishing desired compressive stress pattern on the erstwhile tensile-stressed surface.