# Unveiling the Strain Rate Sensitivity of G18 NiCrMo3-6 CAST Steel in Tension/Compression Asymmetry
## 1 Introduction  
The casting process--even though it is probably the most ancient manufacturing process--is still needed and/or provides a powerful alternative in the production of parts which have complex geometries, especially with large dimensions. With proper precautions, large dimensional variations in the cross-sections or large internal cavities do not lead to compelling challenges as in the case of other alternative manufacturing processes. In this regard, cast steel materials are mostly good candidates for the under-body system components of combat vehicles such as suspension arms, steering knuckles, etc., which often have large dimensions and complex geometries. When the demanding design requirements of the defense industry are taken into account, those aforementioned components should also have significant mechanical properties and durability. Therefore, high-strength structural steel castings could find a reasonable area of application in combat vehicle designs. In that sense, the G18 NiCrMo3-6 material (conforming to EN 10340:2007) with an ultimate tensile strength (UTS) of nearly 1.0 GPa comes into view for certain engineering designs of under-body systems. While having superior mechanical properties, this material has some drawbacks at the same time. A priori, G18 NiCrMo3-6 is a cast steel; thus, it should be treated with a cast iron plasticity approach since it absolutely has a certain amount of porosity. This fact, in turn, eliminates the possibility of modelling this material with the \(J_{2}\) plasticity theory where the hydrostatic stress does not contribute to yielding and hence there is not any volume change in the plastic range (i.e., the strain tensor is deviatoric by definition). In other words, the hydrostatic stress-dependent yield locus should be constructed with a proper flow rule to model the material behavior of G18 NiCrMo3-6. Secondly, this type of cast steel may possess tension/compression asymmetry, i.e., the yield stress and strain hardening behavior differs with respect to tension/compression stress states. This point clearly has to be considered by keeping in mind the fact that some under-bodycomponents encounter moment loads in several directions which means that for a specific spatial material point both tensile and compressive stress states may occur interchangeably. In addition, owing to the harsh service-life conditions, under-body components may experience road loads with a wide strain rate spectrum mostly caused by off-road scenarios. (The experimental data, derived from extensive durability tests, indicate a prevalent strain rate regime of 0.001 to 10 (s\({}^{-1}\)). This finding has shaped the design of our characterization tests.) As in the case of strain hardening, strain rate dependency could also have an asymmetric character which dictates the creation of hydrostatic stress- and rate-dependent constitutive material models that also account for tension/compression asymmetry when aiming to predict the real-life mechanical response of the designed components with high precision. Those aforementioned technical challenges have been extensively investigated by numerous researchers. Perhaps the first attempt to construct a hydrostatic stress-dependent yield function was performed by [PERSON] and [PERSON] [1]. The Drucker-Prager model is still being used at present; however, it predicts a zero resistance in tension since it was proposed for soil mechanics. Then, [PERSON] in his pioneering work proposed a hydrostatic stress-dependent yield locus definition which has still been working as a solid basis [2]. Apart from being hydrostatically stress-dependent, the [PERSON] model was also created in a damage-coupled manner which means that the damage parameter also contributes to the plastic potential which makes it quite appropriate for porous materials [3]. In the specific subfield of cast iron plasticity, tension/compression asymmetric material modeling was also investigated to an intense extent.