## 4 Concluding Remarks  
This paper illustrated the capabilities of stereo-DIC to evaluate the full-field deformation and strain maps over large areas of welded assemblies with a real case study. As a first result, DIC revealed an unexpected deviation from the nominal geometry of the part already in the pre-assembly stage, highlighting the importance of always complementing numerical analyses based on nominal geometries with experimental data [29]. A key issue in determining distortion between two geometries is the definition of their relative position. In fact, an RBM between the two reconstructed geometries occurs when the part is moved and replaced during and/or between measurements. The two plate geometries measured before and after welding can therefore be superimposed in a number of different ways, each resulting in a different distortion map. An optimal solution for this problem is removing the existing RBM between the two geometries using an optimization procedure aimed at finding the translation vector and the rotation matrix that minimizes the distance between homologous point pairs of the two configurations. This strategy is only possible when clouds of matched point pairs are available (as in DIC measurements) and not in the case of unrelated geometries, such as those obtained with CMM or optical 3D scanners [44]. In the latter case, the ICP method is most commonly used, as it can automatically cope with mismatched and differently-sized point clouds. Figure 8: Results of the test to evaluate the measurement accuracy of the stereo–DIC system used in this work. Three feeler gauges of 0.1 mm, 0.5 mm, and 1 mm thickness were attached to the front plate of the traction frame subassembly with adhesive tape and provided with a speckle pattern (**a**). Thickness plot obtained from DIC measurements (**b**). The measurement of the thickness of the 0.1 mm gauge was affected by its imperfect adhesion to the plate due to its high flexibility. Figure 7: Full-field maps of the first principal strain (**a**), second principal strain (**b**) and von Mises equivalent strain (**c**) induced by welding on the front plate of the traction frame subassembly. However, the effectiveness of the ICP is strongly dependent on the object geometry (e.g., for the geometry under analysis there are two possible RBMs at an angular position of 180\({}^{\circ}\) relative to the \(z\) axis that cause the two reconstructions overlap). In this work, the above two approaches have been used sequentially to improve the accuracy of the results. The procedure is not fully automated since it still requires the input of the set of reference points to be overlapped, which should be selected by the user among those less unaffected by deformation. In this context, the possibility of experimentally evaluating the strain maps (which are not affected by RBM) is certainly an advantage over the most commonly used 3D scanners (either mechanical or optical). One of the major limitations of the stereo-DIC technique is the need to prepare the surface to be examined with a stochastic pattern of appropriate characteristics [18]. A stencil optimized for speckle size and distribution will be used in future investigations to speed up and automate the procedure. The high-temperature paint was found to be resistant to the temperatures generated by the welding process. However, wear of some areas of the pattern was observed due to the friction of the plate with the workbench in the first stage of the welding process when the plate was placed face-down to weld the rear parts. This wear could be virtually eliminated, or at least reduced, by designing a suitable support structure with an optimized distribution of contact pins. The advantages of this measurement technique are mainly related to its full-field and non-contact nature. The sensitivity of the technique can be further increased by using cameras with higher spatial resolution and/or by increasing the magnification factor of the measurement using a multi-camera setup [45].