Intense research efforts have been made to understand the variation of strength with strain rate, while the mechanisms governing the strain-rate dependence of hardness have been rarely studied. In this work, we conducted a comprehensive investigation on strength, hardness and failure mechanism of AISI 4340 steels with 4 different microstructural states in a wide range of loading rate. We found strain-rate hardening behavior in all 4 steels, while the degree of hardening, characterized by the normalized dynamic strength (NDS) or the normalized dynamic hardness (NDH), depends on not only materials but also loading modes. With the increase of material strength, the NDS significantly increases but the NDH decreases slightly. The inconsistency between the variations of NDS and NDH is because the hardness is governed by not only yield strength but strain-hardening ability. Although the yield strength increases as rising strain rate, the strain-hardening ability reduces obviously, particularly for high-strength materials due to shear softening under dynamic loading conditions, which also causes a transition from homogeneous deformation to shear fracture. The variations of NDS and NDH could reflect the trends for resisting not only the ductile perforation (strength limited) but also the shear plugging failure (toughness limited), allowing to propose a criterion for locating the optimal strength-toughness combination against dynamic perforation, which was further verified by the ballistic penetration testing results. Moreover, similar behaviors of NDS and NDH of various other alloys were also found, which may suggest the extensive applicability of the current approach.

为了了解强度随应变率的变化，已经进行了深入的研究，而很少研究控制硬度的应变率依赖性的机理。在这项工作中，我们对AISI 4340钢在4种不同的组织状态下在大的加载速率下的强度，硬度和破坏机理进行了全面的研究。我们发现所有4种钢的应变速率硬化行为，而以归一化动态强度（NDS）或归一化动态硬度（NDH）为特征的硬化程度不仅取决于材料，而且取决于加载方式。随着材料强度的增加，NDS显着增加，但NDH略有下降。NDS和NDH变化之间的不一致是因为硬度不仅取决于屈服强度，还取决于应变硬化能力。尽管屈服强度随应变率的升高而增加，但应变硬化能力却明显降低，特别是对于高强度材料，由于在动态载荷条件下的剪切软化，这也导致了从均质变形到剪切断裂的转变。NDS和NDH的变化不仅可以反映抵抗延性穿孔（强度受限）的趋势，而且还可以反映剪切堵塞失败（韧性受限）的趋势，从而可以为确定动态穿孔的最佳强度-韧性组合提供标准。弹道穿透测试结果进一步证实了这一点。而且，