Fatigue Effects of Laser Shock Peening Minimally Detectable Partial-through Thickness Surface Cracks

David L. Eisensmith

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Laser Shock Peening (LSP) has evolved as a viable alternative to other surface treatments (shot peening, burnishing, etc.) which induce beneficial residual stress into structural components. Fatigue life improvements have been recognized by the aerospace industry. Aerospace engineers are inherently risk adverse due to component failures having potentially catastrophic consequences. With fatigue causing an estimated 55 of aircraft structural component failure, fatigue life benefits of LSP cannot be neglected. Aircraft service life extensions compound fatigue issue, especially for non-economically feasible replace components. In-service components may have surface flaws below current inspection limits. Aerospace engineers are concerned with LSP application over an existing crack causing unintended detrimental consequences. To address this concern, two differing types of LSP (circular and square spot shapes) were applied over a partial-through thickness surface fatigue crack of 0.25" (6.35 mm), in 7075-T651 aluminum. The crack length is the minimal detection limit of surface scanning eddy current. Specimens were fatigued under constant amplitude cyclic loading. Baseline specimens fatigue limit was 49, 048 cycles. Peened specimens survived to run-out exhibiting no crack growth when examined with an optical microscope, utilizing marker banding techniques. The result is more than 2000 increase in fatigue life.