# A Novel Low-Cost DIC-Based Residual Stress Measurement Device
## 3 Experimental Procedure
### DIC-Based Measurements
The strain estimates resulting from this analysis were better than \(10^{-6}\), demonstrating that this was likely to be the baseline sensitivity for the device/software. In the second case, sequential images were collected as the sample was incrementally rastered in the X direction with a step size of 50 \(\upmu\)m to assess the impact of bulk shift. The resulting strain estimates were similarly on the order of \(10^{-6}\), demonstrating that the software was capable of correcting for bulk drift without outputting artificial strain estimates. Stage accuracy was also tested during this process by repeatedly imaging a nominally identical location before and after a stage movement (plus homing). This process revealed that alignment could be reliably achieved to within 15 \(\upmu\)m. This is significantly less than the 10's of microns shift typically induced in the sample during milling and therefore it was deemed to be acceptable for subsequent analysis. Image manipulation was next used to assess the precision of the DIC measurements by artificially inducing an apparent strain and drift in a speckle pattern image. This was performed by using the software ImageJ to implement a digital zoom of the image from 90% to 110% in increments of 1%. Drift was simulated by incrementally translating the image by 20 pixels in both vertical and horizontal directions, in a step of 2 pixels. It was found that the estimates of strain from the digital zoom were correct within \(10^{-7}\) and that the digital drift was fully accommodated (strain values of 0). There was therefore confidence \begin{table}
\begin{tabular}{c c c} \hline \hline
**Category** & **Parameter** & **Selected Value** \\ \hline \multirow{4}{*}{Experimental Setup} & Image Resolution and Frame Rate & 2.7 k @ 30 FPS \\ \cline{2-3}  & Speckle Pattern Density & 3–7 Pixels, with a target of 50\% \\ \cline{2-3}  & Image Format & 8 bit, TIFF \\ \cline{2-3}  & Illumination used & Ring light, perpendicular to the sample \\ \hline \multirow{2}{*}{Pre-processing} & Images Averaged & 25 images each increment \\ \cline{2-3}  & Rigid Body Compensation & Yes, the first image reference \\ \hline \multirow{4}{*}{DIC Settings} & Image Region Analysed & 3 \(\times\) 3 mm section surrounding the drilled hole \\ \cline{2-3}  & Correlation Subset Radius & 48 pixels, circular section \\ \cline{2-3}  & Subset Spacing & 2 pixels \\ \cline{2-3}  & DIC Calculation Algorithm & Inverse compositional method [63] \\ \hline \multirow{4}{*}{Residual Stress Estimation} & Calibration Factors & Adjusted for the DIC method and hole size \\ \cline{2-3}  & Hole Diameter & 2 mm \\ \cline{1-1} \cline{2-3}  & Youngs Modulus & 70 GPa \\ \cline{1-1} \cline{2-3}  & Poisson Ratio & 0.33 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Key parameters used in the experiment. that the correlation system was calibrated effectively and that the estimates of strain were likely to be reliable. It should be noted that the hole-drilling methodology is based on the assumption that the hole drilled into the sample is cylindrical in form. Whilst it is clear that the exact shape of the milled hole is highly sample dependent, in particular on material, clamping and, therefore, geometry, it will also be dependent upon the design of the milling rig. To gain an indication of the performance of the use of the rig on this sample, the roundness of the holes on the surface were assessed during the milling process. It was found that this measure decreased from 1 at the start of milling to values of 0.9991 and 0.9987 at the final depth for the T and C locations, respectively. This indicated that although the hole did become elliptical this transition was minor, suggesting that this sample was likely well mounted and that the rigidity of the rig was sufficient to prevent significant deviations when milling this aluminum alloy. To achieve comparable results, hole-drilling measurements were evaluated by mimicking strain-gauge measurements, allowing for more direct implementation of the integral method described in ASTM E837-13a.