(The) influence of heterogeneity on shock wave propagation in brittle materials

Abstract

Brittle materials are important in armor application because they are generally lighter and have higher strength than metal. The ceramics which have high strength are used as an armor structure with the combination of metals and composites to protect a vehicle through design to match a threat. But a ceramic has brittle nature, it is vulnerable to an impact and susceptible to microstructure and flaws. So, it is necessary to strengthen ceramic structure by the exquisite design of microstructure, addition of other materials, and setting of suitable boundary condition. Ceramics, glasses and other high strength brittle materials which are utilized as an armor structure have the advantage of protecting threat at the initial stage, but their performance is abruptly decreased due to the fragmentation of material when the penetrator has progressed. It is helpful to improve material properties if we understand the shock interaction accurately in material at ballistic impact. Alumina 995 was used to study the effect of heterogeneity when a shock pulse is loaded. The dynamic response of heterogeneity in material through the planar impact experiment and the numerical analysis were studied. Some experimental methods were described in chapter 3 to observe a shock interaction and fracture behavior. The planar impact method using a gas gun and a velocity interferometer is described in detail. Another method using an exploding bridged wire (EBW) is a simple way to load a short shock impulse to a specimen without massive penetration. A shadow high speed photography was introduced in this study to take a series of photo in real time using an Edge-on Impact (EOI) technique. Some transparent material like glass, fused silica, and quartz etc. were used as specimen to transmit flash light and make shadow image. The impact experiments and numerical analysis were performed and described in chapter 4. Glass, one of amorphous material in which was applied some artificial heterogeneity was observed to identify the effect of shock wave and fracture under detonation. Simulations for experiments were carried out to compare with the experimental results. It is found that a modification of some known material data is effective to describe the precise fracture behaviors. Of the material data from Johnson-Holmquist model of soda lime glass, the spall strength is more effective parameter to decide the fracture pattern than Hugoniot elastic limit (HEL). The value of spall strength of $\sigma_{spell}$ = 0.07 GPa is the best effective value to match experimental results. The shock wave propagates similar to the longitudinal sound wave and changes its intensity by the reflection and transmission at the boundaries. It caused the meaningful fracture damage difference in material. The attenuation of shock intensity after it passes through the NBR rubber is higher than that after it transmits the stainless steel plate as an inter-layer. Also, the damage is reduced in the next layered material when the lower acoustic impedance material is used as an inter-layer. So, the effect of acoustic impedance of boundary with various material is observed. It plays an important role to attenuate shock wave, and reduce fracture damage so that is identified to be one of the important parameters in designing an armor structure. The fracture behavior of crystal structure under the impact by the EBW detonation was studied by using a fused silica and single crystal quartz. Fused silica behaves similar to amorphous glass, but quartz shows crystal axis characteristic in the shock interaction and macrocrack formation. By analyzing the stress intensity factor, the crack growth speed of z-axis at flaw is faster than that of x-axis. Some internally isolated cracks were well developed on the z-cut quartz seed crystal, but few was observed on the x-cut specimen. The energy dissipation rate of each axis was assessed from the equation of state. The transmitted stress in the z-cut quartz displays almost twofold higher than that in z-cut. It is found that the macro crack generated in material shows different angle with the wave propagation direction following impact energy. In the rear part of chapter 4, BK7 glass block was used to study the effect of the ballistic performance in real situation by the impact of a long-rod penetrator. As a bulk block is used as a target, it shows 3-D material behavior compared with 1-D effect in EOI test. Mono block and inter-layered block were used as specimen, and the correlation of a ballistic performance and physical/mechanical response were analyzed. Through this study, dynamic behaviors of material within dozens of $\mu s$ were studied according to the condition of internal flaws, boundary condition and layer design. These results will give the valuable information on understanding of dynamic impact, and precious data and concept to improve numerical model development. In addition, it is helpful to design material structure and armor system to improve survivability of the future armored system.