## 3 Materials and Methods  
### Tensile Specim for Elastic Properties  
In principle, the proposed methodology is valid for determining the residual stresses on a specimen of any material provided that the constitutive relation remains within the linearity during the relaxation, i.e., when the hole is drilled. In this study, the material investigated is polylactic acid (PLA), which is a well-known and very common material in 3D printing, especially when combined with the fused deposition modelling (FDM) process.  
A tensile test was initially performed to characterize the material, i.e., to find both the elastic properties (Young's modulus and Poisson's ratio) and the maximum tensile stress. As is evident in Figure 11a, the tensile specimen was 3D printed according to the plate type with a flat section, as proposed in the ASTM E8 tensile test standard [44]. However, the cross-section had to be quite large in order to allow the application of both the extensometer and additional strain gages. The Young's modulus was determined by following the approach proposed by the dedicated ASTM 111 standard [45] and considering the extensometer measurement. Figure 11b shows the tensile curve up to large strains, where most of the section was fractured, as this fracture happened gradually. The very initial part of the curve was obviously of more interest for the present application. As is evident in Figure 11c, the first part of the curve was a ramp with a constant slope at least up to a certain limit, which was approximately at 20 MPa. After that stress, the curve shows a progressive curvature and the maximum stress of 35 MPa is reached. After selecting a section of the curve below the proportional limit, the Young's modulus of the printed material was obtained. This value was considered in the subsequent analysis instead of data from the literature, as this parameter is somewhat affected by the direction of the printing texture, the size of the filament, and other details of the printing process.  
Figure 11: (**a**) Tensile test specimen in PLA with both extensometer and strain gages. (**b**) Tensile curve up to almost complete fracture, obtained with the extensometer measurement. (**c**) First part of the tensile curve, showing the Young’s modulus determination and the proportional limit.  
As discussed before, the numerical procedure assumes a linear isotropic material, which requires the Young's modulus and the Poisson's ratio. Despite small variations of the latter, the material parameters produce little effect in terms of the back-calculated stresses; thus, an experimental determination of the Poisson's ratio was pursued. Two strain gage grids were aligned with the longitudinal direction and the transversal directions. These two grids were obtained with a strain gage rosette, in which only the two orthogonal strain gages were wired and measured (see Figure 12a). In principle, the longitudinal strain gage and the extensometer should provide identical measurements. As shown in Figure 12b, these two values were indeed quite similar, especially within the first strain range from 0 to 500 \(\mu\varepsilon\). After that, some small bending effects or any other source of misalignment could cause deviation between the two measures. The ratio between the transversal (absolute value) and the longitudinal strain slopes was calculated for the initial small strains, obtaining the Poisson's ratio. As shown in Figures 11 and 12, the two obtained material parameters were \(E=2580\) MPa and \(\
u=0.353\).