## 1 Introduction
Compared to the aforementioned contact methods, DIC has three clear advantages: (i) it is a full-field technique, i.e., it provides a regular and dense data set over the entire surface of the object under investigation, (ii) it is capable of time-resolved measurements at the camera frame rate, and (iii) it is not sensitive to rigid body motion, thus allowing for an accurate and unbiased measurement of shape distortion. DIC has been applied to both in situ and post-veld measurements. In particular, the DIC method has been shown to be capable of providing full-field information on deformation and strain fields near [21; 22; 23; 24; 25], far [26], and on the backside [27; 28; 29] of welds. However, most papers in the literature analyze welding-induced distortion on laboratory specimens of simple geometries (mainly butt-welded plates [13; 22; 23; 27; 29; 30; 31; 32; 33]) and give little to no information on the hardware and software parameters of the DIC test, e.g., [31; 32]. Indeed, experimental tests are mostly reported to complement theoretical and numerical analyses. Only a few papers report on the assessment of the accuracy of the displacement and strain measurements of the DIC system used, e.g., [22; 23; 34], prior to using experimental data for validation purposes. This paper reports and discusses in detail an experimental stereo-DIC test setup to address a specific industrial problem. In particular, we aimed to improve the estimation of the welding-induced distortion of a S355 JR- EN10025-2:2019 steel plate of a MIG-welded bogie train bolster subassembly (Figure 1). Minimizing the shape deviation of the front face of this plate (highlighted in Figure 1) from its nominal design is an absolute prerequisite for the successive, correct assembly of the traction frame to the train body through a threaded flange. The flatness of the front plate is currently measured against a reference plane on 100% of the products at a limited number of points (<5) using a CMM with a depth measurement resolution of 10\({}^{-2}\) mm. A 2 mm machining allowance is prescribed to bring the flatness tolerance to \(\pm\)0.2 mm through post-weld machining. This work investigates the possibility of replacing the CMM measurement with a full-field DIC measurement. Indeed, the ability of DIC to provide accurate full-field data over the entire front surface of the plate could be advantageous in terms of saving time in post-weld inspection and reducing errors in the finishing process. Due to the complex 3D geometry of the traction frame and the need to repeatedly reposition the assembly during the welding process, we opted for an off-line measurement by testing the front plate before and after welding in the cooled state. We show how the experimental data obtained with off-line DIC measurements allow for the evaluation of the full-surface deformation and strain maps induced on the front plate by the welding process. Particular attention has been paid to verifying that the resolution and accuracy of the measurement, as a result of the DIC hardware and software settings, were appropriate for the specific application [35]. The paper is structured as follows. Section 2 describes the DIC method, the experimental setup, and the measurement procedure. Section 3 presents the results of the experimental tests and discusses the possible sources of errors associated with DIC measurements of weld distortion. Section 4 summarizes the main findings of the study, compares the method with the current CMM inspection process, and outlines directions for future research.