# Unveiling the Strain Rate Sensitivity of G18 NiCrMo3-6 CAST Steel in Tension/Compression Asymmetry
## 4 Results and Discussion
### Determination of SRS  
After conducting the mechanical tests, the SRS parameter was calculated with the aforementioned methodology. The obtained SRS behavior is constructed as a surface plot in Figure 8, aiming to display the distinct behavior in compression/tension asymmetry, The experimental results (only the mid-valued ones) are depicted in Figure 9.  
In Figures 8 and 9, as a first observation, it could be stated that the studied material has a dominantly saturation-type plastic response. This character is more dominant for moderate strain rates. Secondly, the strain rate dependence can also be detected in both stress states which is more effective in the near-yielding zone. With the increasing plastic strain, the rate sensitivity diminishes and even turns out to be negative. In other words, G18 NiCrMo-3-6 shows a strain softening effect at moderate strain rates compared to the quasi-static condition. This fact was interpreted as an adiabatic heating-induced thermal softening effect. This finding agrees well with the previous literature data [34; 35; 36]. Since the studied material could be accepted as a high-strength metal by having a UTS nearly 1.0 GPa, there is a significant amount of inelastic energy introduced to the system during the deformation process. In our view, this is the basic reason behind the strain softening phenomena after moderate strain values (i.e., \(\varepsilon_{p}\geq\) 0.30). Since the obtainable strain values are much higher for compression tests, this strain softening is modest on the tension side. In addition, it is also worth emphasizing that there is also a slight change in yield stress for tension/compression asymmetry, as expected. The compressive yield stress is bigger than the tensile one of 4%. As a result, separate characters of SRS parameter were quantified and the analytical formulations are also provided in Figure 8. The strain softening character in high strain value at compression tests yields a negative SRS phenomena. Therefore, in high strain rate in compression, the material at the onset of plastic deformation exhibits  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & \(\mathbf{\sigma_{0}}\) (0.001 s\({}^{-1}\)) & \(\mathbf{\sigma_{0}}\) (0.1 s\({}^{-1}\)) & \(\mathbf{\sigma_{0}}\) (10 s\({}^{-1}\)) & **B** & **n** \\ \hline Compression & 815.64 (\(\pm\)1.93\%) & 862.79 (\(\pm\)1.89\%) & 882.18 (\(\pm\)2.89\%) & 368.20 & 0.2798 \\ Tension & 808.65 (\(\pm\)2.70\%) & 858.73 (\(\pm\)2.84\%) & 876.80 (\(\pm\)1.36\%) & 551.30 & 0.5075 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Extracted parameters from curve fitting operations (standard deviation values were indicated in parentheses).  
Figure 6: (a) Test results of uni-axial compression tests; (b) test results of uni-axial tensile tests.  
Figure 7: (a) Test results of uni-axial compression tests; (b) test results of uni-axial tensile tests.  
rate sensitivity unlike the moderate strain values. This type of flow stress character cannot be precisely predicted using the conventional approach of the Johnson-Cook plasticity [37] model where the flow curve is raised or lowered via a constant multiplier term.