*2.1. Mechanical Design*

When developing the drilling device, the authors were inspired by the construction of the SINT MTS 3000 Ring-Core system. However, the DIC system used in the designed device was, among other things, also used for the deformation analysis. The advantage of the strain analysis performed by the full-field optical methods is that the correction of the hole position carried out at the end of the measurement is unnecessary. If there occurs an insignificant change in the position of the drilled hole, ultimately it means that the deformation analysis will be made at a distance very close to the planned one. In the case of residual stresses determination, deviation from the analyzed location up to 1 mm does not cause a significant measurement error in the investigated stress levels. In respect of the experimental procedure, it is required to keep the essential principles valid for the commonly used hole-drilling method, e.g., regular changing of milling tools whose failure may lead to negative influenced results [28].

The designed mechanism allows motion in vertical and horizontal directions through the ball screws with pitch accuracy T5 (±0.023/300 mm). The ball screws are driven by B&R servomotors 8LVA23.B1015D100-0 controlled by servo driver 8EI4 × 5MWD10.0600-1. The servomotors have a nominal speed of 1500 rpm, a maximum speed of 6600 rpm, and a nominal torque of 1.33 Nm. The servomotors are connected to the ball screw by a flexible coupling. They use EnDat encoders with resolution 18/16 bits single turn/bits multi-turn, respectively. The pitch screw is 5 mm per round. The designed device uses a spindle with power of 850 W for drilling controlled by a frequency converter. It is possible to attach the single- or stereo-camera digital image correlation system to the lateral static part of the device, i.e., the camera will only move in the horizontal direction or to the block the milling tool is attached to. In such a case, the camera moves in two mutually perpendicular directions, i.e., horizontal and vertical.

#### *2.2. Control System Design*

The system is controlled by PLC 4PPC70.0702-20B with an implemented touch screen (power panel) for a HMI (human–machine interface). The flow of the information is shown in Figure 2. There are two working modes: manual and automatic. The manual mode allows the user to control the motion in both axes either by constant velocity motion or positioning the axis by setting the position value. This mode is used for the adjustment of the milling cutter with respect to the surface of the analyzed specimen. On the other hand, the automatic mode performs strictly predefined cycles of the drilling process defined by the ASTM E837-13a standard. In automatic mode, there are several parameters such as

number of cycles, steps of drilling, required positions in both axes, delay after each cycle, velocity of drilling and the velocity of motion in both axes, which need to be set.

**Figure 2.** Information flow of control system.

The algorithm of automatic mode is as follows (Algorithm 1).
