## 1 Introduction  
The hole-drilling method (HDM) is a well known technique with a strong tradition, which allows to evaluate the residual stresses at the surface and within a certain initial depth, usually on the order of 1 mm [1; 2; 3]. The HDM is usually implemented with a strain gage rosette that measures the _relaxed_ (or _relieved_) strains. HDM with strain gages can be performed either as a through-hole (such as for thin sheet metal specimens), or more often as an incremental blind hole. Incremental hole-drilling requires that the strain gage readings are collected and saved for each step of the milling tool, and then converted into a certain distribution of stresses. These methodologies (through-hole and incremental blind hole) are covered by the ASTM E837 standard [4; 5], which provides a reference for the implementation of the method, including best practices and calculation instructions to obtain the calibration coefficients. The implementation of this measurement technique can, however, require attention to certain details that are disregarded by the standard, such as the plasticity effect [6; 7; 8], possible small thickness of the plate in which the residual stress needs to be measured [9], the possibility of a chamfer at the bottom of the hole [10], and imperfect local planarity of the specimen, such as when the residual stress measurement is performed at the external surface of a cylindrical shaft [11]. Another source of uncertainty is the hole's eccentricity; however, this can be corrected provided that the eccentricity itself is accurately known [12; 13; 14; 15]. Several analyses have been published in past years on different HDM errors and categorizing the different sources of error [16; 17; 18; 19]. A general description of the ill-posed HDM inverse problem was recently provided by [PERSON] et al. [20]. In light of this ill-posedness property, regularization approaches and techniques such as in the recent ASTM E837 standard (2020 and 2013 editions) [4; 5] are discussed, which are based on the Tikhonov regularization [21]. For example, this approach is followed and implemented in several studies by [PERSON] et al. [22; 23; 24]. However, other approaches are possible, such as simple polynomial, spline filtering, or other techniques such as Gaussian Process Regression [25]. However, the role of the regularization must be balanced in terms of the degrees of freedom introduced, which depends both on the quality of the measured data and the assumptions imposed in reaching the solution [20]. Regularization approaches can be tested on a known solution, which is very seldom available when operating with residual stresses. Of course, independent measures can be carried out and comparisons provided; however, even in this way, sources of incompatibility can arise between the solutions under comparison. For this reason, an interesting approach is to use a bending bench for validation, in which a well-known and simple stress distribution is imposed, and then retrieved, to test the quality of the inversion and its regularization [26; 27; 28]. The HDM can alternatively be implemented by recording and analysing the displacements near the drilled hole instead of reading the strains at specific locations, as with the strain gage rosette. This is basically done by combining the hole-drilling with one of two techniques: electronic speckle pattern interferometry (ESPI) [29; 30; 31] or the digital image correlation (DIC) [31; 32; 33; 34; 35; 36]. Recently, dedicated equipments have been proposed to implement the HDM in combination with the DIC technique [37; 38], confirming the promising opportunity of this approach. The DIC method can, in fact, be extensively and efficiently applied to different monitoring methods, especially of components produced by any additive manufacturing technique [39].