Study On Experimental Techniques For Precise Measurements Of EOS In Metallic Materials

Equation of state data at extremely high pressure is required for the analysis of many problems pertaining to physics, geophysics, astrophysics, etc. An important equation of state measurement is the velocity of shock waves generated in a sample by impact from a high velocity disk projectile. Such experiments are vitally important in physics for establishing to high precision the behavior of materials at high pressure, including materials used by researchers as standards. To conduct these experiments there must be two basic and vital conditions-the precise measurements of shock arrival time and to determine the true shock transit time at the sample.The present paper describes an novel experimental technique for precise measurements of EOS in metallic materials with a 25-mm two-stage light-gas gun. In order to accurately measure shock arrival time, the exact spring shorting electrical pins have been used as shock wave detectors. A fast response multichannel electrical recording system has been developed and constructed. The system is a significant improvement over other similar diagnostic system. As a result the shock arrival time detection system has a time resolution of 0.1ns and uncertainty less than 0.5ns for the measurement of each given signal. This subnanosecond time resolution allows us to observe and measure the tile and concave or convex distortion of the shock front and the impactor. The tilt and distortion have been measured over the velocity range of 2-7 km/s with Cu, Ta, Pt impactor. Consequently, we find a common quadratic equation to describe prevalent distortion mode, and the typical fit standard deviation is less than Ins.On the order hand, the statistical analysis method of the experimental data and the pins arrangement is also the fundament and the key for the shock velocity measurements. In order to accurately determine the true shock transit time, a analysis method called 'three-dimension shock front shift reconstruct method-TDSFSRM' has been developed, so that the shock transit time can be found by all pins data impenetrate to a best-fit surface. Popularly, the procedure of TDSFSRM is more general since it does not require the pin locations to lie on a circle or in any predetermined pattern. A new rearrangement called 'cartwheel' has been designated which eliminates the critical reliance on a single center pin. The rearrangement is more sensitive and efficient for measure the curvature of the shock front, is the most robust to pin loss, has the least uncertainty in transit time than the conventional method. An optimization method has been also discussed for designs of the pin rearrangement and thetargets. The Monte-Carlo method has been used to determine the error from TDSFSRM as B type uncertainty of measurement of transit time.A series of experiments have been conducted for measure Hugoniot curve of Cu Ta and Pt in the shock pressure range 30-600GPa. The experimental results show that the shock velocity were measured to about 1% uncertainty. The accuracy of the Hugoniot data of the three metals and of the fits qualifies these metals as equation-of-state standards for shock-wave experiments.