Study on Macroscopic Aspects of Shock Compression Behavior of Polymer Materials

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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.