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CP620, Shock Compression of Condensed Matter - 2001 edited by M. D. Furnish, N. N. Thadhani, and Y. Horie © 2002 American Institute of Physics 0-7354-0068-7/02/$ 19.00 ON THE SHOCK RESPONSE OF POLYCHLOROPRENE J.C.F. Millett, N.K. Bourne, G.T. Gray III* and G. Cooper Royal Military College of Science, Cranfield University, Shrivenham, Swindon, SN6 8LA, UK. *Los Alamos National Laboratory, MS-G755, Los Alamos, NM 87545, USA. Abstract. It is increasingly import to understand the high-strain rate response during impact of polymeric materials due to their widespread use in automotive and aerospace applications. There are three broad classes of polymer materials; elastomers, thermoplastics and thermosets. It is one of the first of these three classes that is investigated in this work. Research has been conducted to determine the Hugoniot of polychloroprene which was investigated using plate-impact experiments measuring the stress at varying positions whilst simultaneously probing the shock velocity in the material and the stress-particle velocity state. The shock-particle velocity data allows deduction of the constants c0 and S for the material whilst the other measurements made allow plotting of the measured stresses with the hydrodynamic curve to view the offset of the Hugoniot in a single series of experiments. For perfectly elastic/plastic materials, the shear strength of the material is given by the offset of the Hugoniot and hydrostat. In this material it is found to be either constant or to reduce slightly with increasing shock stress. The shear strength of polychloroprene is explored further using embedded lateral stress gauges. From these measurements, the shear strength has been determined as a direct experimental variable, and it's variation with shock stress deduced. INTRODUCTION investigated for use as a transparent material used as a window in interferometric experiments (1), there is relatively few experimental investigations on polymers. Knowledge of the response of polychloroprene to mechanical stimuli is also scarce, with high-rate data even more so. Some data for polychloroprene does exist for specific applications such as seismic events (2) and for use as underwater rubbers (3). Interest has also been expressed in the effect upon the mechanical properties of polychloroprene by chemical attack from hydrocarbons (4, 5). Basic shock properties of polychloroprene above 7 GPa have been assembled (6) and recently, polychloroprene has been proposed as a backing material for manganin stress gauges (7), where it was suggested that its close match in impedance with the gauge backingmaterial would result in less imposed noise on the gauge signal. In response to this lack of data, the Hugoniot of polychloroprene has recently been reported at low stress levels (8). It was observed in that work that whilst the Hugoniot measured in The response of polymeric materials to shock loading has excited increased interest in recent years. In particular, they are of significance as components of composite materials, both in inert systems such as automotive, aerospace, and armour applications, and in energetic compositions, where they are used as the binder phase in polymerbonded explosives. In both, a fundamental requirement to understand the shock behaviour of the individual polymeric constituents is necessary if the response of the whole system is to be physically modelled using a micromechanical approach. Polymers fall into three broad categories; thermoplastics such as polymethymethacrylate (PMMA), thermosets such as epoxy resins and elastomers such as polychloroprene. It is the latter material, which is the subject of this paper. However, the understanding of the response of polymers to shock loading is not extensive. With the exception of PMMA, which has been 131 behaviour at high pressures for these materials may give an indication of the mechanisms operating. stress-particle velocity space lay on an extension of the data of Marsh (6), the shock velocity-particle velocity data was very different, indicating a change in behaviour at ca. 6 GPa. Whilst no definitive explanation was given, it was noted that similar observations were made by Champion (9) who investigated the shock response of polytetrafluroethylene (PTFE). Here it was suggested that this was due to changes in the rotation of the CF2 units, from 180° every 13 units to 180° every 9. It was thus suggested that similar processes might explain the differences between the data collected (8) and that of Marsh (6). Finally, when the shock velocity data (which was used to determine the hydrodynamic curve) was plotted alongside the Hugoniot, the points lay within experimental error of the measured stresses. However, it was also observed that the hydrodynamic curve calculated from measured parameters, was slightly above the measured stresses. If genuine, this would suggest that the shear strength of polychloroprene (that is the offset of the Hugoniot from the hydrostat) decreases with increasing shock stress. The observation of the complex response of polymeric materials drives the experimentalist to determine techniques that complement and check upon measurements made using specific tools. The aims of this work are thus two fold. Firstly, to develop a novel arrangement by which the shock velocity and the induced stress are measured using a pair of Lagrangian measurement stations within the target. Secondly, having deduced the parameters necessary for the evaluation of the hydrodynamic curve, to check the deduced values of shear strength with those determined by the direct measurement of the lateral and longitudinal stresses during impact. In doing so, it must be realised that there are several assumptions involved in this process. The first is that the shock velocity is indeed constant and that two stations are sufficient to determine this speed and work is in hand to address this issue. The second is that the assumptions of the simple theory are correct for these materials. The strengthening of polymers with increasing shock stress requires some mechanism analogous to the storage of dislocations in metals to explain the hardening or direct affect of the shock on elastic response. The determination of the shear modulus and its EXPERIMENTAL Plate impact experiments were performed on a 5 m long, 50 mm bore single stage gas gun. Manganin stress gauges (MicroMeasurements type J2M-SS-380SF-025) were introduced to 11 mm thick samples of polychloroprene in such an orientation that renders them sensitive to the lateral component of stress. The gauge position was 4 mm from the impact surface. Voltage-time data was reduced to stress-time using the methods of Rosenberg and Partom (10), using a modified analysis that does not require prior knowledge (11) of the impact stress. Specimens were aligned to better than a milliradian using an adjustable stage, and velocity measured to within 0.1 % by the shorting of sequentially mounted pairs of pins. Impact stresses in the range 1.2 to 3.7 GPa were achieved by firing flat and parallel flyer plates of PMMA, dural (aluminium alloy 6082-T6) and copper at velocities between 347 to 778 ms'1. Specimen configurations showing gauge placement are presented in Fig. 1. Flyer, Plate Stress Gauge FIGURE 1. Specimen configuration and gauge placement. MATERIALS DATA The properties of the polychloroprene used in this investigation were density (p0) 1.42 g cm"3, longitudinal sound speed (CL) 1.23 mm us"1, tensile strength 5.0 GPa and elongation 250 %. 132 Additionally, the shock parameters of this material (CQ and 5) were determined to be 1.4 mm jas"1 and 4.0 respectively. 4- / (1) Where P is the hydrostatic pressure. In Fig. 3, a typical lateral stress gauge trace from the experiments conducted is shown. The impact conditions were a 10 mm dural flyer at 538 m s"1, inducing a longitudinal stress of 1.04 GPa. The resultant lateral stress is ca. 0.7 GPa. The trace rises to a plateau and shows a gentle rise and then a decrease thereafter. This is in contrast to other elastomers such as estane and Kel-F where the pulse top is flatter. On the other hand it does not show the falling pulse found for the thermoplastic PMMA. RESULTS AND DISCUSSION In Fig. 2, the shock Hugoniot of polychloroprene is presented using the data from this work and reference (8). 0.8 I 0.6 o< 0.4 0 0.2 0.4 0.6 0.8 1 Particle Velocity (mm jus' ) 0.2 I FIGURE 2. Shock Hugoniot of polychloroprene. The curve is a hydrodynamic curve calculated from the CQ and S terms measured in the experiments described thus o^=poU&up, with C/s=l.4+4.0 iip. 0 0 1 Time (jus) It will be noted that the calculated pressures derived from the measured shock velocities are slightly higher than their measured stress counterparts at 2 and 4 GPa. Although this may be in part related to experimental error, it is also possible that it is a genuine reflection of the materials response to shock-loading. Note that it would have been usual to include a strength term, (i.e. the HEL) in calculating the pressure from the shock velocity which makes the fact that it is higher more interesting. It is felt that the data points should, however, lie close to the hydrodynamic curve since the quoted quasi-static uniaxial stress strength, at 5.0 MPa, is very low, and at the uniaxial-strain state stress levels investigated here, will have minimal effect upon the final result. If the effect is real, it would indicate a reduction in shear strength (T), as can be seen from the well-known relation, FIGURE 3. Typical lateral gauge trace in polychloroprene. The lateral stresses deduced from this, and the other shots were used, in combination with knowledge of the longitudinal stresses, to calculate the shear strengths, using the well-known formula 2r = ax-<7y. (2) The results are presented in Fig. 4. The calculated points from these experiments are shown with errors largely determined from the uncertainty in the lateral stress noted. There is an initial increase in the strength with stress up to ca. 1 GPa but from this stress onwards the strength is effectively constant with increasing impact stress, at an average level of ca. 0.33 GPa. This would be the case if the HEL of the material were 1 GPa but this is high given the measured quasi-static strength. 133 Thus the interpretation of the rising portion of the plot remains to be determined. simple rationalisations of these phenomena cannot explain these mechanical behaviours suggesting that more complex consideration of the shock response of polymers is necessary for a complete understanding. 0.4 0.3 CONCLUSIONS 1.0.2 Lateral stress measurements have been made in the elastomer, polychloroprene. From this, the shear strength was calculated, and shown to be effectively constant over the impact stress range studied. The results here agree with previous measurements of the Hugoniot, where differences in the measured stress values and those calculated from the shock velocity also indicated a near constant shear strength. 0.1 0 # 1 2 3 Longitudinal Stress (GPa) 4 FIGURE 4. Shear strength versus impact stress in polychloroprene. The straight line is a simple fit to indicate trends. REFERENCES 1. Barker, L.M. and Hollenbach, R.E. J. Appl Phys. 41 (1970) 4208-4226. 2. Buland, P., Dalbera, J. and Lafolie, R., in 10th European conference on Earthquake Engineering, Sum, Editor. 1995, Balkema: Rotterdam, p. 2029-2032. 3. Pillau, V.B. and Das, J.N. Plastics, Rubbers and Composites Processing and Applications 18 (1992) 155-160. 4. Celina, M., Wise, J., Ottensen, D.K., Gillen, K.T. and Clough, R.L. Polymer degradation and Stability 68 (2000) 171-184. 5. Unnikrishnan, G. and Thomas, S. Polymer 39 (1998) 19333938. 6. Marsh, S.P., LASL Shock Hugoniot data. 1980, Los Angeles: University of California Press. 7. Marom, H., Sherman, D. and Rosenberg, Z., in Shock Compression of Condensed Matter 1999, M.D. Furnish, L.C. Chhabildas, and R.S. Hixson, Editors. 2000, American Institute of Physics: Melville, New York. p. 597-600. 8. Millett, J.C.F. and Bourne, N.K. JAppL Phys. (2001) In press. 9. Champion, A.R. /. Appl. Phys. 42 (1971) 5546-5550. 10. Rosenberg, Z. and Partom., Y. J. Appl. Phys. 58 (1985) 3072-3076. 11. Millett, J.C.F., Bourne, N.K. and Rosenberg, Z. J. Phys. D. Applied Physics 29 (1996) 2466-2472. 12. Millett, J.C.F. and Bourne, N.K. J. Appl. Phys. 88 (2000) 7037-7040. 13. Batkov, Y.V., Novikov, S.A. and Fishman, N.D., in Shock Compression of Condensed Matter 1995, S.C. Schmidt and W.C. Tao, Editors. 1996, American Institute of Physics: Woodbury, New York. p. 577-580. 14. Bourne, N.K., Millett, J.C.F., Barnes, N., and Belcher, I. The deviatoric response of an epoxy resin to onedimensional shock loading, in These proceedings. (2001) 15. Bourne, N.K., Millett, J.C.F., Gray III, G.T. and Mort, P. On the strength behaviour of Kel-F-800 and Estane polymers, in These proceedings. (2001) 16. Barnes, N., Bourne, N.K., Millett, J.C.F. The shock Hugoniot of an epoxy resin. These proceedings. (2001) Similar measurements have been made in other polymers such as PMMA, both by ourselves (12) and Bat'kov et al. (13), an epoxy resin (14), Kel-F800™ and estane (15). However, in contrast to polychloroprene, all those materials display a rapidly increasing shear strength with increasing impact stress. An increase in the shear modulus (with pressure) could result in an increasing shear strength with impact stress. The fact that such behaviour does not occur in polychloroprene would suggest that in this particular material, this may not the case or higher longitudinal stresses are required to see the effect. Other measurement of the Hugoniot of an epoxy resin (16), where both stress and shock velocity measurements were made, showed that the calculated stresses from the shock velocity were significantly lower than the directly measured values. From equation 1, this suggests that the shear strength is increasing with increasing shock stress. This has been shown experimentally in another paper (14) where shear stresses were measured. Thus differences between the calculated and measured Hugoniot in stress-particle velocity space can be used to suggest the trends in shear strength at different stress levels. However, the trend seen here, where the hydrodynamic curve plotted from measured values of shock velocity lies above the stress points determined simultaneously, lies at odds with the lateral stress measurements showing a positive shear strength. It appears that 134