Study on Macroscopic Aspects of Shock Compression Behavior of Polymer Materials

研究结果如下：首次建立了聚合物冲击波的Hugoniot测量方法。通过两种不同的实验方法实现了自由面速度或应力波剖面的连续观测以及冲击波Hugoniot参数的连续观测。我们测量了两个不同结晶度的聚乙烯样品。因此，我们发现两个样品的Hugoniot曲线在质点速度150m/S附近都有一个曲率，并且在这个区域附近的数据有一定的散布。为了检验这种散布可能是由于激波传播速度的不稳定所致，我们开发了一种新的连续观测激波传播速度的技术。首先对PMMA试件进行了对比测量，未发现明显的不稳定性。而对于聚乙烯样品，在Hugoniot曲率区域附近观察到冲击衰减特别大。高应力区，冲击速度基本不变。这一结果表明，由于应力松弛而吸收了能量。这种衰变特性被发现取决于聚乙烯样品。冲击波速度的不稳定性表明，冲击波Hugoniot不是一个简单的材料压缩曲线，不能用以冲击波速度为参数的简单冲击跳跃条件来描述。此外，我们还讨论了在150m/S粒子速度范围内发现的任何聚合物材料的Hugoniot曲线偏转的物理问题。在挠度点之前的低应力区有很大的Gruneisen参数3-4，而挠度点后Gruneisen参数减小到很小的0.1。较大的Gruneisen参数对应于分子间振动模的Gruneisen参数，该振动模被认为是由激波驱动选择性激发的。由此讨论得出激波阵面后强热不平衡的结论。

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.