# A Novel Low-Cost DIC-Based Residual Stress Measurement Device
## 1 Introduction
The principle involves the introduction of a small hole (typically of diameter 1.8 mm and about 2.0 mm deep) at the location where the residual stresses are to be measured. The method is mainly used to measure Type I residual stresses which makes it particularly useful in applications where macroscale stresses are dominant. Appropriate strain gauges cost a minimum of \(E50\) and can only be used for one measurement. The process of gluing the strain gauge can also be complex, requiring two hours or more. Digital Image Correlation (DIC) is an optical technique used to track and register changes in 2D or 3D measurements [28; 29; 30; 31]. It is a non-contact method that measures full-field displacements/strains through tracking subsets, which are a group of neighbouring pixels that identify a region of the sample. These subsets require the use of speckle patterns, which are random contrast points used to register and track each subset. Compared to strain gauges, DIC offers significant advantages as summarised in Table 1.  
Some of the most common applications of DIC include tensile strain [32] and thermal expansion coefficient measurements [33]. In recent years, DIC and similar speckle pattern methods, such as electronic speckle pattern interferometry [34] have been applied to destructive and semi-destructive residual stress measurements [35; 36; 37]. Another application of DIC includes microscale residual stress measurements, such as the Focused Ion Beam-DIC measurements [38]. A survey conducted in the UK, investigating the importance of residual stress measurements for researchers and their industrial use, shows that 50% of the participants indicated the residual stresses were of high importance and 30% ranked them as medium importance [39]. Furthermore, the study highlights the hole-drilling measurement method as the most popular measurement method. This shows a residual process device that can simplify and innovate the hole-drilling measurement process has significant potential for use in academic research and in industrial applications. A review of academic and industrial collaborators was conducted to identify the key benefits such a device must offer, summarised in Table 2. Highlights include fast measurements with minimal preparation. The preparation process for hole-drilling measurements with strain gauges can take up to 8 h [40]. For more complicated measurement methods, such as synchrotron X-Ray measurements, the preparation can take months for an experiment proposal and an appropriate beam time [41]. With the DIC hole-drilling measurement method, this can be reduced to 45 min. Another important factor is the cost. This can be significantly reduced via DIC as the only consumable required for data collection is the paint for speckle patterns (less than \(\ell 1\) per point). To achieve a DIC hole-drilling measurement device, a complete product development cycle was followed, where a complete measurement device was designed and produced, the results of which are presented here. This was followed by the validation of the DIC hole-drilling measurement technique implemented in the device. The validation procedure involved conducting a four-point bending test on a control sample, where residual stresses were locked into the material. These stresses were then theoretically and experimentally estimated using strain gauges and the DIC hole-drilling process, allowing for comparable results to industry-standard techniques.