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Session: Poster pitches day 1
Session starts: Monday 26 June, 09:50
Presentation starts: 09:50

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Cauê Carvalho (Engineering School of Lorena, University of São Paulo (EEL/USP))
João Paulo Pascon (Engineering School of Lorena, University of São Paulo (EEL/USP))
Milton Sergio Lima (Photonics Division, Institute for Advanced Studies (IEAv/DCTA))
Carlos Antonio Baptista (Engineering School of Lorena, University of São Paulo (EEL/USP))

Abstract:
Regarding fatigue life extension, techniques such as shot peening, cold expansion and laser shock peening are well-known in the aircraft industry as residual-stress-based approaches. Compared with the latter, laser heating is a less expensive technique, because it can be applied with continuous wave laser equipment; and has been successful in reducing fatigue crack propagation rates in laboratory specimens. Although this effect is usually related to the original residual stress field, it is known that cyclic loading and crack growth can cause relaxation and redistribution of residual stresses. In this work, M(T) specimens made of 2.0 mm thick AA2198-T851 alloy sheets with L-T and T-L crack orientations were treated with a fiber laser (power 200 W, displacement speed 1 mm/s) to produce two heating lines ahead of each of the crack fronts, on one of each specimens’ face. The specimens with and without laser treatment were tested under constant-amplitude loading at a ratio of R = 0. Electrical resistance strain gages bonded along the crack path right next to the first heating line on the treated specimens and in a similar position on the non-treated ones were employed to register the deformation behaviour ahead of the approaching crack tip. A numerical model for the stress-strain state ahead of the fatigue crack considering an elastoplastic material with strain hardening behaviour was developed. From the symmetry condition, half of the specimen was discretized with plane quadrilateral finite elements of linear order. The constitutive model included plane-strain conditions, linear elastic response, the anisotropic Gurson-Tvergaard-Needleman (GTN) yield criterion coupled with damage, associative plastic flow rule and nonlinear isotropic hardening Swift model. The mesh refinement was concentrated around the crack-tip and strain gauge regions. Numerical simulations were successful in describing the measured strain behaviour in the as-received specimens. The experimental results showed significant fatigue crack growth retardation experienced by the laser-treated specimens; this effect was more pronounced in the L-T orientation. Thereafter, the model was calibrated with imposed average compressive stresses in order to reproduce the experimental deformation behaviour ahead of the crack tip, qualitatively showing how crack extension affects the residual stress field.