PSI - Issue 19

Lloyd Hackel et al. / Procedia Structural Integrity 19 (2019) 452–462 Valentin LOURY--MALHERBE/ Structural Integrity Procedia 00 (2019) 000–000

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3. Laser peening of wrought In718 : Laser peening work began with evaluation of residual stress generated and retained in wrought In718 where at first a sample was only laser peened as a baseline and then a second sample laser peened and thermally exposed at 760oC. Figure 1 shows residual stress measurements made on wrought In718 that was laser peened using a fixed irradiance of 10 GW/cm2 and 18 ns pulse duration and 2 layers of laser peening. The samples were 50 mm × 50 mm × 12 mm blocks peened on one of the large faces. Peening was done using square spots 3.0 mm on a side and overlaps of approximately 3% of spot width. The spot pattern of the second peening layer was offset from the previous pattern by one half spot-width in each direction. Following the peening, the residual stress retained in the block was measured using both a slitting crack-compliance technique [4] and x-ray diffraction (XRD) [5] with electro-polish etching for making depth measurements. The slitting technique gives excellent results of stress vs. depth through nearly the entire thickness of the coupon but is known to be inaccurate near the surface. The XRD has best accuracy at and near the surface. Initial peening was done using 2 layers of an aluminum tape ablative layer that insulates the surface being peened from the high temperature of the laser generated plasma. Stress values were measured using both slitting and XRD residual stress measurements.

Figure 1. Residual stress measurements of laser peening of wrought In718. Measurements made of the same sample by slitting (crack compliance) and x-ray diffraction (XRD). Slitting over reports stress near the surface. XRD is limited to about 1 mm depth, is more accurate there and results converge near 0.5 mm (0.02 inch) depth.

The triangle identified line of Figure 1 gives the stress result vs. depth which we interpret as follows: It is known that slitting can overestimate the surface stress so it is expected that the stress near and at the surface would be less as measured by the XRD. Although not confirmed it was that the yield stress of this variant of In718 was about 700 MPa (100 ksi). Considering that the surface layer would slightly. strain, a residual stress from the peening of -560 MPa (- 80 ksi) is reasonably expected. More importantly, the laser peening is seen to penetrate to a depth of about 1.9 mm (0.075 inches). At this depth of plastic deformation by the laser peening, the sample has experienced some strain of the plastically deformed area as well as retained residual stress. This strain results in a bending of the sample generating a stress release that can be calculated from the geometric bending [6], [7]. The significance is that the degree of plastic deformation generated by the peening is dependent on material and peening parameters but that the actual compressive stress intensity and depth are also critically functions of the geometry of the component being