Effect of laser heating on mechanical properties, residual stresses and retardation of fatigue crack growth in AA2024

Abstract

Local laser heating treatment using a defocussed laser beam was applied to the surface of 2‐mm‐thick AA2024‐T3 sheets. Two different treatment designs—namely, lines and circles—as well as the positioning and number of treatments were investigated regarding their potential to retard the propagation of through‐thickness fatigue cracks. The highest fatigue crack growth life extension of 285% was achieved by the application of four laser heating lines or two circles on each specimen side. The induced compressive residual stress field through the LH process is primarily responsible for an improvement in fatigue crack growth life. An emphasis was placed on investigating the effect of the treatment on the possible reduction of tensile and fatigue strength (S‐N life). If only one line was applied transverse to the loading direction or only one circle was used, the reduction of fatigue strength was comparable to the reduction of fatigue strength resulting from the stress concentration introduced due to the presence of a rivet hole.

On the prediction of fatigue crack growth based on weight functions in residual stress fields induced by laser shock peening and laser heating

“…In recent times, the application of LH for the retardation of fatigue crack growth (FCG) in aircraft aluminum alloys has been extensively studied. [6][7][8] The process involves heating the specimen surface with a defocused laser beam. In this process, a continuous-wave laser with an output power up to several kW can be used.…”

“…In case of thin sheet material, tensile as well as compressive average residual stresses through the thickness can be generated. 8 Besides the advantages of a simple, flexible, and fast process application on the specimen surface, some challenges have to be mentioned. The LH lines should not be placed in areas susceptible to fatigue damage, where the local material weakening due to the possible remelting of specimen surface and the presence of tensile residual stresses is unfavorable.…”

“…The LH lines should not be placed in areas susceptible to fatigue damage, where the local material weakening due to the possible remelting of specimen surface and the presence of tensile residual stresses is unfavorable. 7,8 Moreover, LH sequences, for example, lines or circles have to be placed in such a way that the propagation direction of the possible initiated crack will intersect the area of compressive residual stresses and an initiated crack is located at an optimal distance to the laser heated area, in order to benefit from the retardation effect of the compressive residual stresses placed in front of the laser heated area. 6,8 In contrast to LH, the LSP process utilizes a pulsed laser that allows the generation of compressive residual stresses in the area where the LSP treatment is applied.…”

“…7,8 Moreover, LH sequences, for example, lines or circles have to be placed in such a way that the propagation direction of the possible initiated crack will intersect the area of compressive residual stresses and an initiated crack is located at an optimal distance to the laser heated area, in order to benefit from the retardation effect of the compressive residual stresses placed in front of the laser heated area. 6,8 In contrast to LH, the LSP process utilizes a pulsed laser that allows the generation of compressive residual stresses in the area where the LSP treatment is applied. 9,10 In case of the LSP process, material surface is usually covered with a flowing water layer.…”

“…16 Two possible application scenarios for the LH or LSP as residual stress engineering techniques for thin aircraft structures are considerable: design and repair. 8,11 Reliable prediction of the influence of induced residual stress fields on FCG, that is, fatigue crack retardation and acceleration, is mandatory to account for residual stresses in a damage tolerant design as well as to apply residual stress engineering techniques in repair processes of structural components, in which cracks of small size are identified. The residual stress engineering treatments can be applied to arrest cracks and retard FCG.…”