PSI - Issue 19

Lloyd Hackel et al. / Procedia Structural Integrity 19 (2019) 346–361 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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Figure 1. Laser peening plastically compresses material normal to a surface generating a transverse compressive stress field and a degree of component strain. Specifics depend on component material and geometry.

Unlike processes like shot peening and deep cold rolling, laser peening creates a flat impact profile normal to the surface and is thus known for creating very small degrees of cold work, typically 3% to 5%. Typically hardness and yield strength of the treated material remains nearly unchanged. Shot peening requires multiple impacts, estimated for example at 13 impacts for 100% coverage, and due to the spherical (Hertzian) nature of the impacts, the shot generates transverse as well as normal forces and plastic deformation. This working of the surface increases hardness and generates cold work. In contrast, laser peening with square or rectangular beam spots precisely placed one next to another, generates 100% coverage in only one impact per beam spot. The impact is totally normal to the surface thus generating little hardening or cold work. Additionally, the large footprint of the laser, typically 3 mm to 10 mm on a side and the strong nature of the shock result in a very deep (up to 10 mm) plastic deformation of the material before the inward propagating shock drops below the yielding limit of the material. Prevey et al. provide a detailed study showing the low cold work of laser peening in comparison to cold work generated by shot and gravity peening [17]. Their conclusions are inferred from angular dispersion measured in x-ray diffraction. The intense shock over a relatively large area (10 mm2 to 100 mm2) used in laser peening generates large dislocations deep into materials, dislocations that help resist crack initiation and growth thereby supporting enhancement in fatigue strength and lifetime of treated components. The process creates compressive stresses that resist the advance of cracks thereby providing superior resistance to stress corrosion cracking in susceptible materials. By selectively inducing compressive stress into areas of components susceptible to high tensile loads, the laser peening enables higher levels of tensile fatigue loading before the fatigue limit is reached. Corrosion-fatigue tests of laser peened samples have been shown to exhibit superior performance [18].

Figure 2. Residual stress measured in 316L stainless steel that was laser peened at 8 GW/cm2 irradiance, 21 ns pulse duration and 3 layers of peening with aluminum tape ablative layer. The solid markers represent the measured residual stress determined by the Slitting Method. The linear dashed line is a fit to the slope of the bending stress profile resulting from the bending of the panel. The open markers represent the summation of measured stress and resultant bending stress giving the equivalent (eigenstress) stress that would have resulted from an infinitely thick panel. This represents the actual depth of plastic response to the laser peening. [19]