# Plastic Behavior of Laser-Deposited Inconel 718 Superalloy at High Strain Rate and Temperature
## 3 Experimental Procedure
### High Temperature and High Strain Rate Setup  
The high strain rate tests were performed on a Hopkinson bar setup in the direct configuration. A scheme of the setup developed at DYNLab is reported in Figure 2, and the complete description was reported in [22; 23]. Below, the main characteristics are summarized.  
The direct tensile condition was generated by a hollow striker bar, which was launched by compressed air against the anvil placed at one end of the input bar. The incident stress level depends on the pressure of the compressed air, which was adjusted as a function of the testing temperature to maintain as stable a nominal strain rate as possible. For the used configuration, the time duration of the incident wave was about 600 \(\upmu\)s. The strain in the bars was recorded by means of semiconductor strain gauges, for which the nonlinear relationship was calibrated. The alignment of the system and the avoidance of internal and initial clearance were guaranteed by imposing a static preload on the entire system with a hydraulic jack. The preload was measured using a piezoelectric cell.  
The Hopkinson testing system was coupled with a high-speed camera to record the video of each test and obtain the sequence of images needed to directly measure the deformation on the sample. For the testing campaign, a PHOTRON SA5 high-speed camera was used at 100,000 frames per second and at a resolution of 512 \(\times\) 128 pixels. The high level of light required was obtained using a pulsed light system able to release sufficiently constant light for about 1 ms, which was longer than the entire test duration. The starting of the camera was imposed by the strain-gauge signal on the input bar, which was used as a trigger. A delay was added for the activation of the lightening system to synchronize the constant level of light with the arrival of the incident wave on the specimen. The sequence of images obtained for a high strain rate test at 850 \({}^{\circ}\)C is reported in Figure 3. The same methodology of analysis previously explained for quasi-static tests was also applied for high strain rate tests. Each high frame rate video was digitally elaborated to obtain information about the kinematic quantities directly from the specimen (more details are given in the next section).  
Figure 2: Scheme of the Hopkinson bar setup developed at DYNLab at Politecnico di Torino (Italy), details of the adopted measuring systems, and example of an infrared image.  
In accordance with the results obtained from compression tests as performed in [20] on the same materials, the range of temperatures investigated was between room temperature and 1000 \({}^{\circ}\)C. The specimens were heated using an induction system (CEIA PowerCube 32/900) with a maximum power of 2 kW, which was scaled depending on the temperature condition to reach in each test. The solenoid was made by an open coil; this solution was adopted to have complete optical access to the specimen. The coil could be moved along the longitudinal direction of the specimens to be centered with the gauge length. To avoid problems of heating up of the experimental setup, water-cooled supports were placed near the bar/specimen interfaces (see Figure 2). Each specimen was held to the target temperature for 5 min before the test, as performed in [20]. The tests were performed in atmospheric air because no problems of oxidation were experienced even at very high temperatures for the investigated materials, as clearly visible from the images in Figure 3. To avoid any electromagnetic disturbance on the strain-gauge signals, the heating system was switched off by a trigger signal (i.e., the strain-gauge signal on the input bar). The time interval between this moment and the test start was about 250 \(\upmu\)s, which was not enough to induce a temperature decrease on the specimen.