Study on Macroscopic Aspects of Shock Compression Behavior of Polymer Materials

我们的研究结果如下：首次建立了聚合物激波的Hugoniot测量方法。通过两种不同的实验方法，实现了自由表面速度或应力波剖面的连续观测以及激波Hugoniot参数的连续观测。我们测量了两种不同结晶度的聚乙烯样品。因此，我们发现两个试样的Hugoniot曲线在粒子速度为150m/s的点周围都有曲率，并且在该区域周围数据有一定的散射。为了检验散射可能是由于激波传播速度的不稳定造成的，我们开发了一种连续观测激波传播速度的新技术。PMMA试样首先测量比较，显示没有明显的不稳定。而对于聚乙烯试样，在Hugoniot曲率区域附近观察到特别大的激波衰减。高应力区，冲击速度几乎不变。这个结果表明由于应力松弛而吸收能量。发现这种衰减特性取决于聚乙烯试样。激波速度的非定常性表明，激波Hugoniot不是简单的材料压缩曲线，不能用以激波速度为参数的简单激波跳变条件来描述。此外，我们还讨论了在150m /s粒子速度范围内，任何高分子材料的Hugoniot曲线的偏转的物理性质。挠曲点前应力较低区域的Gruneisen参数很大，为3-4，挠曲点后减小到很小的0.1。较大的Gruneisen参数对应于分子间振动模态，被认为是由激波驱动选择性激发的。由此得出激波前缘后的强热不平衡。

Results of our study are as follows :We have first established Hugoniot measurement method for polymer shock waves. Continuous observation of free surface velocity or that of stress wave profile together with the shock Hugoniot parameters are realized by two different experimental methods. We measured two different polyethylene specimens with different crystallinity. Consequently, we found that Hugoniot curve for both specimens have a curvature around a point of the particle velocity of 150m/s, and a some scatter of the data around this region.To examine that the scatter might be due to the unsteadiness of the shock propagation velocity, we have then developed a novel technique of continuous observation of shock propagation velocity. PMMA specimen was first measured for comparison, which showed no appreciable unsteadiness. While for polyethylene specimens, shock decay was observed especially large in the vicinity of the curvature region of the Hugoniot. Higher stress region, shock velocity seemed almost constant. This result indicates an absorption of energy due to stress relaxation. This decay characteristics is found to depend on the polyethylene specimen. Unsteadiness of shock velocity suggests that shock Hugoniot is not regarded as a simple material compression curve, which could not be described by simple shock jump condition with shock velocity as a parameter. Furthermore, we have discussed the physics of the deflection of Hugoniot curve found for any polymer materials again in the region of 150 m/s particle velocity. Very large Gruneisen parameter of 3-4 was suggested at the lower stress region before the deflection point, while it decreases to a very small value of 0.1 after the deflection. Large Gruneisen parameter corresponds to that of intermolecular vibration modes, which is supposed to be excited selectively by the shock drive. Strong thermal nonequilibrium after shock front is then concluded from this discussion.