Strain Rate Dependency of Dynamic Flow Stress of Steel Iron at Very High Strain Rates

已知体心立方金属中流动应力的应变速率敏感性在约1×10^3/s以上的高应变速率范围内大大增加。采用改进的霍普金森压杆系统对低碳铸铁进行了高应变率试验，应变率范围主要为4×10^3 ~ 5×10^4/sec。首先，为了评估瞬时应变速率和应变速率历史对铸铁在非常高的应变速率下的动态流变应力的影响，在应变速率从大约10000到20000/秒的范围内进行应变速率变化试验。结果表明，瞬时应变速率起主导作用。此外，为了评估高应变速率下动态流变应力的应变速率依赖性，使用Aono等人获得的使用纯铁变形激活能E（τ）的应力-应变速率曲线。准静态应变速率区的流变应力是由位错热激活机制控制的。在应变速率为1/sec ~ 2×10^4/sec的范围内，流变应力受形成扭折对的热激活过程控制，但在较高应变速率下，第二种速率控制机制似乎占主导地位。

Strain rate sensitivity of a flow stress in BCC metals is known to increase greatly in the high strain rate ranges above about 1×10^3/s. High strain rate tests are performed for low carbon iron using a modified Hopkinson pressure bar system in a strain rate range mainly from about 4×10^3 to 5×10^4/sec. At first, in order to evaluate the effects of the instantaneous strain rate and strain rate history on the dynamic flow stress of the iron at very high strain rates, the strain rate change tests are made in the strain rate range from about 10000 to 20000/sec. The results indicate that the instantaneous strain rate plays a dominant role. Furthermore, to evaluate the strain rate dependency of the dynamic flow stress at high strain rates, the stress-strain rate curve using activation energy E(τ) for deformation of pure iron obtained by Aono et al. is used. The flow stress at quasi-static strain rate region is controlled by the mechanism of a thermal activation of dislocations over short range barriers. The flow stress at strain rates ranging from about 1/sec to 2×10^4/sec is controlled by the thermally activated process of formation of kink pairs, however at higher strain rates a second rate controlling mechanism appears to be predominant.