# A Novel Low-Cost DIC-Based Residual Stress Measurement Device
## 2 Device Design  
For developing the measurement device, existing commercial solutions, such as the RS-200 [42], MTS3000-Restan [43], VIC-3D [44] and Prism ESPI [45] and novel experimental setups [39; 46] were analysed to develop the target specifications. This highlighted several essential considerations for a combined hole-drilling DIC measurement device such as the eccentricity and the depth resolution of the hole drilled, as well as the required resolution for the DIC camera and its positional accuracy [47; 48; 49; 50; 51]. Beyond the capabilities of these devices, DIY 3D printers and milling machines were considered which have similar actuation requirements. To design the measurement device, an iterative development cycle was followed. Initial tests revealed that the device needed to incrementally drill through a sample and capture an image at every increment. It was also determined that horizontal imaging was key for achieving the stack of images where the displacements can be tracked. The influence of microscale chips from the drilling also needed to be carefully considered. The virtual and physical versions of the measurement device are shown in Figure 1.  
The measurement sequence involved moving the drill, along the same axis as the microscope, to the measurement point. After each increment, the drill returned to the home position, capturing an image, which continued until the drilling operation was complete. The actuation was achieved using stepper motors, NEMA 17, which have a maximum motor speed of 2400 RPM and a step angle of 0.9\({}^{\circ}\) as well as low vibration and audible noise. For both axes, 150 mm T8 lead screws with a 2 mm pitch were used along with linear rail guides that ensured load bearing in all directions. Furthermore, lithium grease was used to further reduce the effects of jerk and friction in the actuation. To control the motors, EasyDriver motor boards were used that allow micro-stepping to achieve an \begin{table}
\begin{tabular}{c c} \hline \hline
**Specifications** & **Details** \\ \hline Complete measurement process & The device and the associated software take the user through the complete measurement process. \\ \hline Fast measurements & Each measurement takes around 25 min with 45 min of preparation. \\ \hline Low cost per measurement & Significantly reduces the cost of measurement by requiring minimal preparation. \\ \hline Non-contact measurements & The surface of the material is minimally disturbed when making measurements e.g., via the addition of strain gauges. \\ \hline Fit for lab use & Able to make measurements for different applications (materials research, aerospace etc.). \\ \hline Precise hole positioning and incremental drilling & Able to drill actuate both axes to a resolution of 0.05 mm. \\ \hline Zero-point detection & The device can detect the zero depth of the work sample. \\ \hline Flexible Sample Size & Can accommodate multiple sample sizes via flexible clamping. \\ \hline A low level of expertise needed & The device and the associated software do not require specialised knowledge to operate. \\ \hline Evaluation process integrated & The device evaluates the results of residual stress measurements. \\ \hline Comparable to ASTM E837-E13a & The results are comparable to ASTM E837-E13a, the hole-drilling method measurement standard. \\ \hline Aimed towards the & The device costs \(\varepsilon\)380. \\ lower-midrange market & Documentation for the device is open-sourced. \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Target specification of device offered by the measurement device. actuation resolution of 0.05 mm. Beyond these considerations, any inaccuracies within the actuation system were minimised by rigid body compensation, commonly used in DIC applications [52; 53]. The drivers were connected to an Arduino UNO microcontroller, which was controlled using a USB interface with a PC.