Electrical Resistivity Measurement Of Shock Damaged K9 Glass

The dynamic damage and fracture of brittle materials under shock compression is a basic scientific subject and has an important application in engineering. For the brittle damage and fracture under shock compression, a great many factors make it being complex and difficult, such as shock pressure, strain rate, physical and mechanical properties of the sample itself, etc. In the past decades, the study of brittle damage and fracture was mainly focused on the mechanical characterizations, including stress, strain and acoustic impedance. Compared to the static loading, the experimental methods on the dynamic damage and fracture of brittle materials were relatively fewer. In this thesis, we proposed a new experimental method to measure the electrical resistivity on a shock damaged brittle material, and to understand the electrical property change during the brittle damage and fracture process.For K9 glass, the electrical resistivity has been measured under the plate impact experiment. The main research and results are as follows:(1). A series of technical improvements have been made for the traditional Weir's differential method, including:(a) the thicknesses of the sample and the flyer were increased; (b) a trigger pin and a piezoelectric pin were added to detecte the time of flyer impacting at the sample and the shock wave arriving at the sample rear surface, respectively; (c) the voltage of measurement was enhanced, and raised up to 1500V; (d) a checking and calibration system before shock was added. The improved circuit and target assembly was more capable to observe the electrical resistivity variation with loading time under the low shock compression.(2). The electrical resistivity of K9 glass has been measured at different loading stresses below Hugoniot elastic limit. Experimental results indicate that the electrical conductivity of K9 glass increases as the shock wave propagating in it. When the shock stress is below 1.0GPa, K9 glass may consider as the insulator. While the shock stress increases to about 2.7GPa, the resistivity begins to decrease slowly to the order of 103Qm and decreases to another order of magnitude as the shock stress up to 6.0-7.7GPa. Based on the literature, it was believed that the resistivity of the insulated materials could have a pronounced reduction only under the high pressure of shock compression (much higher than the Hugoniot elastic limit of materials) owing to the effect of shock compression. Experimental results of this thesis might throw a new light on understanding of the dynamic damage of brittle materials under shock wave loading.(3). Experiments were designed carefully to analyse the physical mechanism for the electrical resistivity reduction of K9 glass. Firstly, the shock compression effect was excluded by the resistivity measurement of both the condenser charge method and the differential circuit method. Secondly, the dynamic cracks expanding during the shock wave propagation was confirmed by the multipoint DPS technique. The speed of crack expanding coincided with the experimental results of Hu's work on K9 glass and Xia's supershear rupture transition. Thirdly, the measured voltage signals have been analyzed in detail. It is noted that the electrical resistivity of K9 glass could further reduce when the failure wave arriving at the electrode top, and a characteristic inflexion point may appeare in the recorded voltage signal. Confirmation of the inflexion point, not only demonstrats the effect of the failure wave on electrical resistivity reduction, but also suggests a new method for observation of the failure wave.(4). Combining the charged particle emission phenomenon during the brittle fracture with the models of compressed atoms, bonds breaking and positive hole pair (PHP), this thesis has proposed that the newly formed dynamic cracks in K9 glass are conductive. The observed charging features on the electrical resistivity of K9 glass has been explained well by the dynamic conductive cracks. In addition, the physical analysis of conductive cracks under shock compression has been used to discuss the electromechanical couping breakdown phenomenon, and the breakdown behavior and physical characteristics of fuctional material under shock loading has been analysed also.