# Plastic Behavior of Laser-Deposited Inconel 718 Superalloy at High Strain Rate and Temperature
## 4 Experimental Results and Data Analysis  
As mentioned above, the engineering strain was evaluated starting from the displacement time history profile obtained from the digital image analysis of the recorded videos. For each loading condition, the kinematic quantities (e.g., displacements and velocities at the two sides of the specimen) were compared with those obtained from the standard signal processing for the Hopkinson setup. For the latter, an algorithm for the correction of the wave dispersion was applied, and the wave signals were properly synchronized based on the strain-gauge positions along the bars. No further operation was needed: thanks to the water-cooled supports, the heated portions of the bars were very small, and no effects were induced on the wave propagation [24]. The comparison is reported in Figure 7a in terms of velocity profiles at the two sides of the specimen obtained for a high strain rate test performed at 1000 \({}^{\circ}\)C. The curve indicated with v\({}_{1}\) is the velocity measured on the input side, whereas the curve indicated with v\({}_{2}\) is the velocity measured on the output side of the specimen. The solid lines represent the measurements performed starting from the strain-gauge signals, while the markers represent the measurements performed starting from the video analysis. Of course, the time resolution of the video was lower: the interval between the images was 10 \(\upmu\)s (100,000 fps), whereas the strain-gauge signals were acquired at 2.5 MHz. In any case, it was possible to conclude that the time resolution of the video was sufficient to well appreciate the quantities' evolution. The evaluation of the kinematic quantities from videos has the advantage of not being affected by problems related to wave dispersion and of not requiring corrections in case bar heating modifies wave propagation. The velocity profiles obtained with the two methodologies differed considerably only in the phase in which the specimen breaks: the springback of the material produces a fast movement of the two portions of the sample in the direction of the bars with a consequent oscillation phase. Figure 7b shows the check of the equilibrium condition for the same test. This operation was performed for all the tests by comparing the forces on the input (referred to as F\({}_{1}\)) and output (referred to as F\({}_{2}\)) sides of the specimen. In this case, the force signals were evaluated with standard wave equations [25]. A further consideration was about the high quality of the signals: the curves reported in the diagrams were not filtered, and the noise to signal ratio was very low. The tensile response at various temperatures obtained in high strain rate tests on DA and HT specimens are reported in Figure 8. As previously mentioned, specimens in the aD condition were not tested because interest in such a condition is negligible, stemming from its poor mechanical properties. In the diagrams in Figure 8, the results are shown in terms of engineering stress versus engineering strain. For each loading condition, only one repetition is shown for the sake of clarity. As expected from the results of the microstructure analyses, the HT specimens had a higher strength. In any case, for both sets of specimens, the results showed that the mechanical strength of the material was reduced by increasing the temperature. For temperatures up to 750 \({}^{\circ}\)C the reduction was limited, and the material behavior was characterized by low strain-hardening. In the temperature range of 800-900 \({}^{\circ}\)C, there was a significant drop in mechanical strength, while at 900-1000 \({}^{\circ}\)C the further reduction was limited. Figure 6: Scheme of the temperature control systems adopted in dynamic high temperature tests (**left**); comparison between heating obtained on pure tungsten and Inconel 718 cylindrical dog-bone specimens with the same dimension by imposing the same heating power (**right**).