# A Novel Low-Cost DIC-Based Residual Stress Measurement Device
## 3 Experimental Procedure
### Strain Gauge Calibration Measurements  
For the strain gauge calibration, a 4-point bending method was selected as it provided a uniform stress distribution between the two central loading points where peak stresses could be observed, and the strain gauges could be applied. Aluminium 6082 was selected to be the sample material as it offers ductility, ease of machining and precise knowledge of mechanical properties.  
To ensure the sample did not buckle, the material length was chosen to be longer than the experiment setup, and checks were performed via through Euler's Critical Load and Bending Calculations. Two Micro-Measurements EA-06-062 RE-120 general-purpose residual stress-strain gauge rosettes were positioned diagonally within the peak stresses induced by the bending. For the surface preparation and gauge installation, Measurements Group Bulletin B-129 was followed. The test sample with the rollers of the four-point bending setup is represented in Figure 5.  
As for testing equipment, a 50 kN Instron 3369 was used. A CNC machine was preferred and then used to drill multiple holes incrementally, quickly and accurately. To measure the strains, a data logger was used to arm the strain gauges and record the measurements.  
With the preparations completed, a preliminary 4-point bending test was performed using a same-sized spare sample at a load rate of 5 kN/min up to a total force of 7 kN (corresponding to a moment of 156 Nm). This force, in combination with the specimen dimensions, can be used in combination with Equation (1) to determine that this corresponds to a residual stress of 85 and \(-\)85 MPa at points T and C, respectively. It should be noted that Equation (1) is the standard theoretical estimate for residual stress generated  
Figure 4: Probing functionality for accurate zero-point detection.  
in a 4-point bend test using beam theory. This framework is based on the approximation of negligible deformation of the specimen. Therefore, in order to ensure this estimate remained valid, relatively low magnitude stresses were generated (<100 MPa). It should be noted that the use of low magnitude stresses was also useful to quantify the sensitivity of the method being developed. The estimated values of stress facilitated the validation of the calculations of the force required for plastic deformation and was followed by repeating the same procedure with the armed sample.  
The second stage of the experiment, the hole drilling, involved clamping the sample to the CNC using machine vices with improvised supports. The holes were drilled using a 3-axis drilling program, where the machine coordinates were set to zero at each location manually using a wide-field eyepiece. The machine RPM was 3500, with a feed rate of 0.1 metres per minute at incremental depth of 0.05 mm. Before any measurement, the gauges were zeroed, and the strain was recorded as the holes were being drilled. The process of quantifying the experimental residual stresses is as described in Appendix A.  
By applying the Integral Method with Tikhonov Regularisation as detailed in Appendix A, the Cartesian residual stresses at the strain gauges were estimated as a function of depth. The results were as expected, where the stresses in Y and XY directions were measured to be minimal, with the X Stresses shifting from tensile to compressive at points T and C, respectively, as shown in Figure 6.