延性金属层裂自由面速度曲线特征多尺度模拟研究

以延性金属钽为研究对象，对钽在平板撞击下的层裂行为进行了多尺度下的数值模拟研究，从微观视角对自由面速度曲线上的典型特征进行了新的解读。在宏观尺度，对比分析了光滑粒子流体动力学法（smootfied particle hydrodynamics, SPH）与Lagrange网格法以及几种本构模型的模拟结果及其适用性。通过与实验数据的对比表明，Steinberg-Cochran-Guinan本构模型在层裂模拟中与实验数据吻合较好，通过改变加载条件获得了不同应变率下的自由面速度曲线，分析了不同应变率下的自由面速度曲线中的典型特征。在微观尺度，采用分子动力学方法获得层裂区域内损伤演化情况，揭示了宏观尺度自由面速度曲线典型特征所蕴含的物理内涵。分析表明，层裂表现为材料内部微孔洞形核、长大和聚集的损伤演化过程，自由面速度曲线上的典型特征与层裂区域的损伤演化过程存在密切关联。Pullback信号是层裂区域内微孔洞形核的宏观表征；自由面速度曲线的下降幅值在一定程度上反映了微孔洞的形核条件，由此计算得到的层裂强度实际上是微孔洞的形核强度。此外，Pullback信号后的速度回跳速率反映了微损伤演化的速率。

Multi-scale simulation study on characteristics of free surface velocity curve in ductile metal spallation

The spallation characteristics of ductile tantalum metal under planar plate impact was analyzed through a multi-scale perspective. And the typical characteristics of the free-surface velocity curve on the macro-scale were interpreted from the micro-scale to reveal the physical meanings corresponding to these typical characteristics. On the macro-scale, the spallation behaviors of the ductile tantalum metal under planar-plate impact were numerically simulated through the smooth particle hydrodynamics (SPH) and Lagrange methods, and the free-surface velocity curves of the tantalum during spallation were obtained. In addition, the free-surface velocity curves obtained by the Johnson-Cook model, Steinberg-Cochran-Guinan model and Zerilli-Armstrong model were compared in the numerical simulations. Comparison with the experimental data shows that the Steinberg-Cochran-Guinan constitutive model has a better performance in the macro-level simulation. The free-surface velocity curves at different strain rates were obtained by changing the loading conditions, and the typical characteristics of the free-surface velocity curves at different strain rates were discussed. Results show that there is an exponential relationship between spall strength and strain rate, and the spall strength obtained from the simulation has a good agreement with the experimental data. On the micro-scale, the damage evolution in the spallation region was obtained by molecular dynamics simulation conducted in the LAMMPS software, and the loading strain rate was consistent with that on the macro-scale. The micro-scale simulation reveals the physical connotation of the typical characteristics of the macro-scale free-surface velocity curve. Micro-scale analysis shows that spallation is the response of damage evolution of nucleation, growth, and aggregation of voids. From the multi-scale perspective analysis, the typical characteristics on the free-surface velocity curve are closely related to the damage evolution in the spallation area: the pullback signal is a macroscopic response of the void nucleation in the spall area; the decline amplitude of the free-surface velocity curve reflects the void nucleation condition, and the spall strength reflects the nucleation strength of the voids. What’s more, the velocity rises to the first peak beyond the minima after the pullback signal reflects the rate of damage evolution. The multi-scale perspective analysis is helpful to fully understand the physical mechanism of the spallation under planar-plate impact.