## 1 Introduction
[23], thus establishing a methodology suitable for analysing heterogeneous strain fields using DIC. From the mechanics of material point of view, there is a requirement to investigate the strains/stresses as near to the stress concentrators as possible. The larger the facet, the larger the deviation of the reconstructed shape from the real one (e.g., when evaluating the strains around the hole). An investigation on how to properly select the facet size and the corresponding smoothing level to obtain optimum strain/stress analysis results was carried out and described in [24]. In experiments realized by DIC, the illumination variation is unavoidable, leading to intensity changes. As all the correlation algorithms are based on comparing the image intensity between reference and deformed images, in an ideal case, the intensity values should be unchanged. That is why two main approaches were used to reduce the influence of illumination variation. In the first one, researchers developed various algorithms to compress the illumination variation from the entire image. [PERSON] et al. [25] provided a modified Newton-Raphson algorithm based on the variables-based sum of squared differences function, allowing obtaining more accurate results of strain analysis than the standard sum of squared differences function. [PERSON] et al. [26] developed a modified monochromatic-light-illuminated active DIC system allowing laboratory and non-laboratory testing with reliable and accurate results in case the ambient light has been seriously changed. The computational algorithms were developed to decrease the inaccuracy of the DIC due to light reflection [27] and substantial lighting variation [28]. Recently, the second approach has been developed to improve the accuracy of the results in the case of local illumination variation, where the adjusted speckle pattern is used. The adjustment relates to the saturated pixel's surroundings or the speckle's edges [29; 30]. [PERSON] et al. [31] proposed a simple and efficient gray intensity adjustment strategy, in which feature matching and standard DIC method are utilized to estimate the intensities of saturated pixels. In order to speed up the measurement process and evaluate the strains at multiple locations, the combination of the drilling method and the digital image correlation is now being used more frequently. Determining residual stresses using the drilling method is based on sensing the strain relieved after a hole or groove has been formed in the analysed structure. Since both the hole (blind/through) and the groove can be easily refilled, such intervention does not affect the subsequent use of the structure. A standard [32] was developed for the use of a hole-drilling method, the fundamentals of which were established by [PERSON] and [PERSON] [33], with their findings complemented by experimental and numerical modeling by [PERSON] [34; 35; 36; 37]. Since the principle of DIC is to capture and correlate images of analysed structure regions at each measurement step, its application with commercially produced drilling equipment is limited. The main reason for this is the overlapping view of the analysed object by the body of the drilling device, which allows either only imaging the object from a greater distance (leading to a reduction in image resolution) or the design and implementation of custom drilling/sensing equipment for the quantification of residual stresses using DIC [38; 39]. A unique drilling/sensing device was designed and constructed at the authors' workplace, enabling the application of a single-camera (2D) or stereo-camera (3D) correlation system Q-400 (Dantec Dynamics GmbH, Ulm, Germany). Unlike almost all custom drilling/sensing equipment, this device is unique in that it allows horizontal and vertical movement of not only the milling tool but also the DIC system, and thus images can be acquired with the desired quality for different specimen shapes and thicknesses from a suitable distance [40].