# Unveiling the Strain Rate Sensitivity of G18 NiCrMo3-6 CAST Steel in Tension/Compression Asymmetry
## 5 Conclusions  
In this study, the SRS of the G18 NiCrMo3-6 cast steel material was experimentally obtained including tension/compression asymmetry. For that purpose, a specific set of mechanical tests was carried out at distinct strain rate conditions. By means of this experimental effort, power-law-based strain rate-dependent flow behavior was determined and the necessary material parameters were extracted and shared with readers. To better capture SRS, the \(m\) parameter was formulated as strain, strain rate, and asymmetry-dependent fashion. In particular, two surface plots of the SRS parameter were provided with their analytical expressions regarding tension and compression stress states which is sufficient to construct an advanced visco-elasto-plastic material model for the studied material. Finally, the aforementioned material model parameters were embedded to a pressure- and rate-dependent constitutive model by means of a UMAT file. Moreover, the proposed Cocks'89-based model was verified 336 with the experimental data. The following outputs could be extracted from the current contribution:  
Figure 12: Comparison of FEA and experiment for tensile tests. Figure 13: Comparison of FEA and experiment for compression tests. * The G18 NiCrMo3-6 material possess an asymmetric yielding character, in other words, it yields stress in compression at a rate 4% bigger than the tensile one. * Indeed, the strain hardening and SRS character is also quite asymmetric for the studied material. This fact was quantified with the help of power-law-based flow stress formulation and the determination of the m parameter. From a general point-of-view, SRS is more dominant in the tensile direction; however, it has a decreasing tendency with increasing strain as in the compressive stress states. In the view of the authors', the shared material data within this study provide an important resource for the upcoming research activities on G18 NiCrMo3-6 about which the existing literature data are extremely limited. * Unlike the tensile test in a medium-strain rate regime, strain softening phenomena were observed in compression tests which were interpreted as the effect of adiabatic heating. Owing to this finding on the compression side, the material exhibits a rate sensitivity character up to a certain strain value but then the strain softening effect contributes to the plastic response. This fact can be handled through defining SRS parameters as a function of strain as in the proposed constitutive model. On the contrary, a conventional Johnson-Cook type formalism cannot catch up with these phenomena where the effect of rate contribution is formulated through a constant multiplier term. * The created constitutive model (UMAT file) runs without any problem which was formulated on a finite strain basis and uses implicit time integration scheme. This UMAT file could easily serve in inspecting the effect of material parameters (like the initial void volume fraction, etc.) on the macro-mechanical performance of any design which is made up of G18 NiCrMo3-6. However, it is also noteworthy that in the proposed model, the void growth is just linked to the volumetric strains, meaning that any void coalescence effect is not included in the formulation. Indeed, this fact may also give rise to precision loss especially in a low-stress triaxiality regime. * In our view, the present material model and the verified parameter set significantly enhance the technical knowledge level related to the G18 NiCrMo3-6 material and would serve as a solid basis for the upcoming research studies. Our efforts will focus on creating proper coupling between the estimated void volume fraction and any appropriate damage rule to improve the proposed model which would account for both damage, void coalescence, and localization phenomena. Furthermore, plasticity-induced heating would also be studied to estimate the strain softening behavior via proper thermo-coupled plasticity formulations. Conceptualization, B.C. and O.A. ; methodology, B.C. and O.A. ; software, B.C.