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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Higher energy laser peening, such as used in this work, employs laser energy output in the range of 20 J/pulse. The high energy enables the use of a relatively large spot size (1 cm2) while keeping the irradiance at the required high power density needed to plastically yield the material. This can be contrasted to low-energy laser peening of 1 J/pulse or less and the correspondingly smaller (0.05 cm2) footprint needed to keep the irradiance sufficiently high, in the 3 to 10 GW/cm2 range [20,21]. The large spot size enables the shock wave to deeply penetrate as a planar wave before geometrically rarefying. Deep levels of plastic deformation generate deep compressive stress critically important to enhancing fatigue strength and preventing intergranular stress corrosion cracking (CISCC). As an example of the deep compressive stress generated by laser peening we show in Figure 2 peening done in 316L stainless steel using high energy laser peening. Residual stress measurements were made using a slitting technique [22]. The peening was done using one of our high energy laser systems and slitting and analysis by DeWald [19]. In this peening evaluation, a 30 mm thick block of normally annealed 316L stainless steel was peened with parameters of 8 GW/cm2 irradiance, 21 ns pulse duration and three layers of peening using a layer of adhesive-backed aluminum tape as the ablative layer. Post laser peening Dewald adhered strain gauges to the block and electron discharge machine (EDM) cut across the top surface and systematically through a center cross-section of the block, measuring strain release as a function of cut depth. Using the block geometry, the elastic modulus and Poisson’s ratio for the material and inputting the measured strain release, he computed residual stress as a function of depth in the block as shown by the dark diamonds of Figure 2. His uncorrected data shows a zero crossing of approximately 4 mm depth. However due to its finite 30 mm thickness and unconfined edges, the block responded by straining (bending) in addition to retaining residual stress. The degree of bending is determined from a simple bending moment calculation associated with the linear portion of the right hand side of the uncorrected data. Extrapolating the thickness of the block to near infinity where the stiffness becomes correspondingly large enables calculation of the depth of plastic deformation generated by the laser peening and thus the eigenstress. This eigenstress is represented in the figure by the open diamonds. From this data it can be concluded that the plastic deformation of the laser peening penetrated to approximately 11 mm. This deep penetration is the realm enabled by the high- energy laser peening. This result shows, that even in a 30 mm thick sample, the depth of stress generated by peening is the resultant of the plastic response generated by the process and the response of the component geometry equilibrating stress and strain. We have built a finite element analysis (FEA) code and use our library of measured values of plastic deformation of specific materials generated by laser peening to accurately predict stresses and strains of laser peening on a node-by-node basis in specific material and component geometries. Results for the 316L material in canister geometry will be shown in a section of this report to follow. Deep compressive stress is key to prevent pitting from evolving into CISCC initiation and crack propagation: It has been commonly observed that a close relationship exists between pitting and stress corrosion cracking in steels [23]. Pitting often grows under the surface of a component and notoriously tends to trigger failures by fatigue or stress corrosion cracking. A pitting-potential model introduced in 1976 by Jose R. Galvele has had a major influence on the development of corrosion science [24]. The electro-chemistry of reactions, such as chloride ions in austenitic stainless steels, drives the pitting corrosion [25-26]. The chlorine ion has been identified as a species which attacks or breaks down a protective film leading to localized dissolution. Although the understanding of the science of pitting is continuing to evolve and mature, it is clear that its evolution into chlorine induced stress corrosion cracking (CISCC) is enhanced by and even requires the presence of tensile stress. Surface treatments such as shot peening, hammer peening, ultrasonic peening and laser peening generate compressive stresses at various depths into a surface and can retard the initiation and evolution of CISCC [27]. However, as illustrated in Figure 3, in any peened component tensile stress will appear below the compressive layer as the component responds to the plastic deformation of the peening. Thus the depth of compressive stress generated by the various treatments combined with component geometry is a critically important consideration. Pitting that penetrates beneath the compressive layer can easily evolve into CISCC as discussed in Woldemedhim et al. [6] and in the above section “Laser peening and residual stress”. As an example, we illustrate in Figure 3 that the pit depth can penetrate deeper than shallow peening applications such as shot peening and ultrasonic cavitation peening. When growing beyond the shallow peening depth, a pit reaches intense tensile stress and can rapidly initiates stress corrosion cracking and corrosion-fatigue cracking. In contrast, the deep, 5 mm to 10 mm deep plastic response generated by the