## 2 DIC System and Experimental Measurements  
Digital Image Correlation is a well-established non-contact technique capable of providing a high-spatially dense set of data on shape, displacement and deformation of an object surface provided with a natural or synthetic stochastic pattern [18]. In particular, DIC allows the 3D position of a given set of material points to be tracked through a sequence of deformation states by matching homologous point pairs in the corresponding images. To perform 3D surface measurements, two cameras are used to acquire the images of the region of interest (ROI) from two angled views. The spatial matching between the reference image (e.g., right view) and the current image (left view) is performed by finding the point pairs in the two images that have the most similar grey-scale distribution within a square subset of pixels [18]. After proper calibration of the stereo-camera system, the reconstruction of a given 3D point on the object surface is then obtained by stereo triangulation from its matched image points [36]. Figure 1: 3D models of the traction frame alone (bottom image with the front plate highlighted) and assembled with the moving bolster (top image). During the last decades, the DIC method has proven its extreme versatility of application in terms of size, materials, geometries, type/amount of deformation, and experimental conditions [19]. The main strength point of DIC lies in its full-field capabilities, and thus the ability to extract a highly dense set of data over the entire object surface portion captured by the cameras. In contrast to other optical methods (e.g., laser-interferometry [37]), the measurement sensitivity of the DIC method depends strongly on the spatial resolution of the images [38], the geometric parameters of the optical set-up (e.g., stereo angle between the cameras, working distance), and the optical settings of the camera lenses (mainly focal length and aperture). Since the result of a DIC measurement is highly dependent on the experimental conditions and on the image data analysis settings [35], it is good practice to report the selected parameters and to assess the metrological performance of the DIC setup used for the experiments. The experimental tests of this study were carried out in a laboratory environment (Figure 2) on the frontal plate of the traction frame (Figure 1) before and after welding the part to the final assembly (Figure 2) in the PROMEC workshop. The plate's overall dimensions are 588 mm long, 308 mm wide, and 18 mm thick (the side wings of the plate have a reduced thickness of 12 mm). Prior to testing, a stochastic speckle pattern was applied to the plate surface using a silicone scrubbing pad and a high temperature (up to 800\({}^{\circ}\)) resistant black paint. The stereo-DIC setup used in this work (Figure 2) features two scientific-grade cameras (Falcon 4M30, 2352 \(\times\) 1728 pixels CMOS 8-bit sensor, Teledyne DALSA, Waterloo, ON, Canada) equipped with 28-105 mm Nikkor zoom lenses, Nikon Corporation, Tokyo, Japan and two LED panels (Yongnuo YN600L, 3200-5500 K, 4800 lumen, Yongnuo, Tsuen Wan, China). The stereo-DIC system was calibrated according to a widely-used procedure involving a closed-form solution followed by a non-linear optimization routine for the refinement of intrinsic and extrinsic parameters [39]. An even, 4 th order polynomial function was sufficient to correct for the low radial distortion of the aspherical Nikkor lenses. The two pairs of images acquired before and after welding the front plate to the rest of the traction frame are shown in Figure 3a-d. The WD, stereo angle, and lens settings were adjusted in order to sample the surface of interest with maximum spatial resolution [38].