PSI - Issue 17

C.A.R.P. Baptista et al. / Procedia Structural Integrity 17 (2019) 324–330 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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Prior to the LSPwC treatment, the specimens were lightly sanded with 400 grit abrasive paper. The treated area was a square measuring 15.0 mm on each side and with the center positioned on the theoretical crack path at 10.5 mm from the notch tip. This area was covered in a zig-zag pattern having scanning direction perperdicular to the crack line and starting from the track closest to the notch tip. Fig. 3 shows a fatigue specimen with LSPwC footprint ahead of the pre-crack. Both sides of each specimen were treated.

Fig. 3. View of fatigue specimen with LSPwC footprint.

Surface residual stresses in the x (longitudinal) and y (transverse to theoretical crack path) directions in the laser processed region were determined by X-ray diffraction using a Rigaku model Ultima IV equipment and Cr-K α radiation. Roughness Ra (ANSI) and surface hardness HR15T measurements were also performed. The fatigue crack growth (FCG) tests were conducted at room temperature in laboratory air using a MTS model 810 servo-hydraulic machine and were performed with constant load amplitude under force control. The test frequency was kept constant at 10 Hz and the loading waveform was sinusoidal. Two distinct loading regimes, defined by the load ratio R (min/max), were adopted: R = 0.2 and R = 0.5. Additional FCG tests were performed at R = 0.2 for the material conditions AR and LP50. The compliance method of crack length monitoring was used during the tests and the five-point incremental polynomial technique was employed for computing the crack growth rate. Crack closure calculations were performed using the automated linear-quadratic spline method, in which the “load vs. COD” plots are modeled by two-section least square fit curves. 3. Results and discussion The measured surface properties of the tested material conditions are shown in Table 1. It can be seen that, compared to the AR condition, the laser processing caused a substantial roughness increase and also lead to a small increment in hardness. Both overlapping conditions presented similar properties. On one hand the increase in arithmetical mean deviation of the profile, Ra, is due to the ablation phenomenon, which seems to be the same whatever of the adopted overlapping ratios. On the other hand, the increase in hardness could be associated to the increase of material defects after peening, because the time is too short for any diffusive phenomena to occur.