UC San Diego

Nowadays polymer has become one of the most used engineering materials. Polymers have pervaded our life to such an extent that we cannot avoid using them on any given day. Polymers have special characteristics and offer some advantages over the traditional materials such as metals, ceramics, glasses, etc. Such advantages are, for example, corrosion resistance, low density, high ductility and toughness. Their mechanical properties can be easily modified and improved when combined with other materials to form polymeric composites. In this research, polyurea is studied due to its promising advantages for blast-and-shock protection and acoustic applications. Polyurea-based composites are also created in order to improve properties of polyurea and expand its usages. Fabrication procedures for polyurea, phenolics-microballoon filled polyurea, glass-microballoon embedded polyurea and milled-glass reinforced polyurea are discussed. In order to fully understand their mechanical behaviors which depend strongly on frequency (or time), temperature, and pressure, various characterization techniques spanning from quasi-static to high frequency dynamic ranges are conducted on these materials. Some new techniques; for example, ultrasonic wave measurement under low and high pressures and acoustic ball impact test, are developed for specific test conditions that cannot be accessed by other traditional testing techniques. The ultrasonic wave measurement under low and high pressures allows us to measure wave speed and attenuation of acoustic wave in viscoelastic materials in the pressure range of 0 to 1 GPa. The acoustic ball impact test allows us to measure the wave speed and attenuation in kHz frequency range and the temperature range of -50 to 50 C, which is not practical for ultrasonic wave measurement using regular acoustic transducers. To reduce the time for experimental characterization, mathematical models based on micromechnics are also created in order to accurately estimate mechanical properties of polymeric composites. The models can be used for both elastic and viscoelastic composites with various shapes of inclusions. Moreover, experiment-based constitutive model for polymeric materials, which is implementable by finite element software, is presented. Lastly, a design of a novel periodic layered composite is demonstrated. The designed composite can be potentially used for acoustic sensing, transmitting, and silencing applications.