# A Novel Low-Cost DIC-Based Residual Stress Measurement Device
## 4 Results and Discussion  
Figure 9 compares the Cartesian residual stresses calculated for the strain gauge and the DIC measurements as a function of depth at points T and C. Compared to the strain gauge measurements, the DIC results show similar trends, where the strains in all directions align with reference measurements. While both holes for strain gauge and DIC measurements were drilled starting from the surface of the sample to a hole depth of 1 mm, significant errors were observed at the initial measurement depths, which can be seen in the DIC XY strain measurement at point T. This was initially unexpected, as the initial strain relief at the surface was expected to be minor. However, upon further investigation it was revealed that this artifact was associated with the 'bedding in' of the drill bit and the associated lateral forces as the hole was established. Despite making use of rigid-body compensation and imaging-averaging error, the sensitive nature of DIC means that even very small sub-pixel offsets can lead to blur in the averaged images leading to inaccurate subset tracking. However, as more strain is relieved, the residual stresses are more accurately tracked and measured. Therefore, only measurement points between 0.2 mm and 1 mm were analysed. Another factor which needed to be accounted for in the precision of the DIC measurements was the eccentricity of the hole. Measurement of the shape of the final milled shape provided quantitative estimates of this value of +/ 0.05 mm for both points T and C. The influence of this on the resulting stress/strain estimates has been the focus of previous research [65, 66], which revealed that this value corresponds to a nominal increase in the relative error of 5%. Accordingly, the confidence limits of the DIC results were modified to account for this effect as shown in Figure 10.  
Although the strain gauge measurements for both holes aligned with the theoretical estimations, deviation in the measurements could be observed as the gauges were incrementally drilled. As is the case with most metals, the single crystal of the aluminium 6082 alloy used in this study is anisotropic. However, given that the nominal grain size for this material is expected to be between 1 and 10 \(\upmu\)m, there will be between 10\({}^{6}\) and 10\({}^{9}\) grains within the gauge volume. It is possible that some preferred grain orientation was induced by the manufacturing process, however this was expected to be minimal. Therefore, it was believed that the grain orientation was close to random and that an isotropic approximation for the macroscale behaviour was a good approximation. These errors may be due to experimental errors, such as drill wear, heat and vibrations induced by the drilling, or minor eccentricity and concentricity errors [67, 68, 69]. Figure 8: Positioning of the virtual extensometers for calculating X, Y and XY strains. Figure 10: Comparison of the average theoretical, strain gauge and DIC estimations of residual stress at points C and T. The error bars represent the 95% confidence values of the stress. Figure 9: Comparison of DIC and strain-gauge measurements as a function of hole depth at (**a**) point T and (**b**) point C. The error bars represent the 95% confidence intervals of each point. At point T, both strain gauge and DIC measurements were observed to align, with the highest measurement deviation observed in X strains, with a mean measurement error of 30 MPa. This can be due to the positioning of virtual extensometers compared to the strain gauges, where small positional deviations can lead to notable measurement offsets. At point C, a similar error was seen in the X strain as well, with a mean measurement error being 73 MPa. These errors were expected due to the semi-destructive nature of the hole-drilling method, which prevents measurements to be conducted at the same position, resulting in an underestimation in the estimated stresses.