Laser shock peening in aircraft applications and its effect on fatigue life of 7075-T7351 aluminium alloy
Järvinen, Heini (2019)
2019
Materiaalitekniikan DI-ohjelma - Degree Programme in Materials Science and Engineering
Tekniikan ja luonnontieteiden tiedekunta - Faculty of Engineering and Natural Sciences
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Hyväksymispäivämäärä
2019-09-04
Julkaisun pysyvä osoite on
https://urn.fi/URN:NBN:fi:tuni-201908202961
https://urn.fi/URN:NBN:fi:tuni-201908202961
Tiivistelmä
Fatigue and other failure mechanisms, such as corrosion and stress corrosion cracking, are severe problems in metallic aircraft components. Many surface treatment methods exist to enhance the service life of metallic components. One is laser shock peening (LSP), which is a new and promising method to improve the mechanical materials performance and extend the service life of metallic components. In laser shock peening process, a short length laser pulse is focused onto material surface which generates a high pressure that plastically deforms the material and induces deep (over 1.5 mm on average) compressive residual stresses into material. This greatly improves the fatigue resistance of material. Laser shock peening has already been used to treat many aircraft components, such as turbine blades, fastener holes and wing attachment lugs.
In this study, 7075-T7351 aluminium alloy samples treated with laser shock peening and shot peening were fatigue tested until failure and the results were compared. Before the fatigue tests, surface roughness and residual stresses were measured from sample surface. Furthermore, the depth of residual stresses was measured. After fatigue failure, the fracture surfaces were analysed with scanning electron microscope. In addition, laser shock peened specimens were polished, and the same measurements were conducted to study more the effect of surface roughness.
The results showed that laser shock peened specimens had lower fatigue life than shot peened specimens due to higher surface roughness and lower compressive residual stresses on the surface. The shot peened specimens, on the other hand, presented notable compressive residual stress on their surface which delayed the fatigue crack initiation and growth on the surface. The higher surface roughness of LSP samples was caused by absence of absorbent protective coating during processing which melted and roughened the surface. Low or zero compressive residual stresses on the surface were most probably caused by softening effect due to surface melting and by high surface roughness. The surface melting caused relaxation of the residual stresses on the surface. Compressive residual stresses were found also deeper in the material, but they were not able to compensate the harmful effects of high surface roughness, which reduced the fatigue life compared to shot peened specimens. However, the fatigue life of LSP samples was slightly higher than with the untreated specimens due to the deeper stresses. In addition, the depth of the compressive residual stresses of LSP samples was not higher than the depth of shot peened samples which was suggested to result from unsuccessful predesigning of residual stresses and stress distribution in the specimens.
In this study, 7075-T7351 aluminium alloy samples treated with laser shock peening and shot peening were fatigue tested until failure and the results were compared. Before the fatigue tests, surface roughness and residual stresses were measured from sample surface. Furthermore, the depth of residual stresses was measured. After fatigue failure, the fracture surfaces were analysed with scanning electron microscope. In addition, laser shock peened specimens were polished, and the same measurements were conducted to study more the effect of surface roughness.
The results showed that laser shock peened specimens had lower fatigue life than shot peened specimens due to higher surface roughness and lower compressive residual stresses on the surface. The shot peened specimens, on the other hand, presented notable compressive residual stress on their surface which delayed the fatigue crack initiation and growth on the surface. The higher surface roughness of LSP samples was caused by absence of absorbent protective coating during processing which melted and roughened the surface. Low or zero compressive residual stresses on the surface were most probably caused by softening effect due to surface melting and by high surface roughness. The surface melting caused relaxation of the residual stresses on the surface. Compressive residual stresses were found also deeper in the material, but they were not able to compensate the harmful effects of high surface roughness, which reduced the fatigue life compared to shot peened specimens. However, the fatigue life of LSP samples was slightly higher than with the untreated specimens due to the deeper stresses. In addition, the depth of the compressive residual stresses of LSP samples was not higher than the depth of shot peened samples which was suggested to result from unsuccessful predesigning of residual stresses and stress distribution in the specimens.