# A Novel Low-Cost DIC-Based Residual Stress Measurement Device
## 4 Results and Discussion
### Device Performance Critique  
This paper aimed to develop a low-cost, novel measurement device that combines the DIC and the hole-drilling method. The device was successfully implemented, and offered significant timesaving, where the preparation time was reduced from 2 h per point to 45 min per measurement. It should be however noted that the length of the experiments remains comparable; both measurement methods take around 25 min for a 1 mm deep hole. The measurements validated the performance of the device as well, where a hole-drilling resolution of 0.05 mm and full comparability to the ASTM E837-13a were achieved. A key challenge that needs to be overcome when performing the hole-drilling measurement technique is to ensure that the position of the milling increment is well aligned to the starting point. There are four key methods that can be used to improve the reliability of this process:  
1. _Ensure the sample is held securely._ In this study, this was achieved by designing a range of sample holders specifically tailored to standard shapes/component designs. However, this is something that the user needs to keep in mind when performing this type of analysis, meaning that subsequent gripping methods may be required for non-standard geometries. 2. _Maximize the rigidity of the rig._ Flexibility in the rig may lead to deformation of the rig/assembly during the milling/actuation process. Therefore, the design was specifically tailored and subsequently optimized in order to reduce deflection during these processes. 3. _Minimize the mass of the actuated components._ Reducing the mass of the milling section of the rig reduces the force required by the motors in order to perform movements. Mass refinement was used extensively in the design of these sections of the device to ensure that the rig can be realigned to the highest precision possible. _Make use of reliable references and a repeatable actuation system_. An extensive design procedure was implemented to maximize this aspect of the design including the selection/use of lead screws, couplings, guides and suitable motors, as well as lubrication grease. The resulting system was tested to quantify the alignment offset associated with the design, which was found to be at worst approximately 15 \(\upmu\)m. This magnitude offset is on the same order of magnitude as errors in tool shape or the flexing of the drill bit and therefore this was deemed to be acceptable. Another significant benefit was the cost, where the cost of the experiment was limited to the running costs, such as the cost of the paints and the drill bit. This eliminated the need to buy and install specialised equipment, such as strain gauges, data loggers and terminal connectors. The cost of the device is also estimated to be significantly lower than the equivalent strain gauge measurement devices, such as the RS-200, which has a price of \(\epsilon\)6200 [70]. The device has a production cost of \(\epsilon\)350, which is kept minimal for the mass adoption of the device, particularly in developing countries where the cost of residual stress equipment and the associated measurements can be prohibitive. All the assembly drawings, manual, g-code and calibration coefficients required have been provided for free, in an open-source format in the Supplementary Materials of this paper. The'self-construct' nature of the device also overcomes the need for a license or installation cost from a specialist, albeit at the cost of the time/effort required to produce and assemble the device. Other major benefits include the need for little expertise to operate the measurement device and open-source documentation, allowing researchers worldwide to benefit. One additional factor which should be highlighted is that the software used to control the device (Grbl) and perform the DIC (NCorr) are free, open-source packages, meaning that there is no cost associated with this aspect of the device.