Member Spotlight – Alison Trachet, PhD candidate
The science of ceramics
The familiar adage “in the blink of an eye” indicates something that happens very quickly. Just how long does it take to blink? Researchers have measured a blink to be approximately 100 to 400 milliseconds long (1/10 to 1/4 of a second) , which seems like no time at all. However, an event such as a car crash occurs in 2 ms (about 100 times faster than you can blink), and the impact of a projectile on a target occurs on the order of microseconds (about 100,000 times faster than you can blink) . Materials can respond differently depending on how rapidly they are loaded; picture how different the damage will be if a car lightly taps the bumper of another car in a parking lot versus a high-speed collision. It is critical to evaluate the performance of materials in high-strain-rate or dynamic (rapid) loading environments.
My research focuses on the performance of personnel ceramic armor, the thick plates that are inserted into bullet-proof vests. Historically, body armor has been manufactured from a variety of materials including bronze, iron, and leather, but advanced ceramics such as silicon carbide (SiC) and boron carbide (B4C) are used today. Density (weight), hardness, and strength are important parameters to consider when designing armor and components for space shuttles and satellites. Compared to metals, ceramics are lightweight (2-3 times lighter than steel), harder, and stronger. Hardness indicates how easily something is scratched, and compressive strength represents the weight a material can withstand before breaking. SiC and B4C are among the hardest known materials, second only to diamond and boron nitride, and their compressive strengths are approximately 100 times greater than concrete and 5 times greater than steel. Thus, these ceramics are ideal for use in armor and many other applications.
Many methods exist for producing these types of ceramics, but a common production method is pressureless sintering, akin to the kiln-firing of clay. The material is heated at high temperatures for long periods of time to produce fusing between adjacent particles. Two examples of pressureless-sintered silicon carbide are shown in the images. These images show how the materials look on a very small scale. Accounting for the difference in scale, the image on the left has much smaller particles or grains than the one on the right. Just as DNA determines our eye color, the microstructure of a material determines its properties, including strength and hardness. The ceramic in the left image is stronger than the one in the right image due to its fine-grained microstructure.
Once the ceramic has been manufactured, it must undergo a series of tests to evaluate its performance. Traditional mechanical test methods involve testing the material at quasistatic rates (think of it as a snail’s pace – something so slow it seems like it is not moving). However, impact is a rapid process involving high pressures and short time spans. Materials do not behave the same under static and dynamic processes, yet most analytical models rely upon quasistatic values to predict performance. To refine these models and consider how material behaves over a range of rates and loads, we need to test them at high strain rates. The split-Hopkinson pressure bar (SHPB), shown in the schematic, consists of striker, incident, and transmission bars. The sample is sandwiched between the incident and transmission bar. Using a gas gun, the striker is fired at the incident bar, leading to the incident and transmission bars smashing together and breaking the sample. The entire process is captured with a high-speed camera (see video), allowing us to analyze how the material breaks.
Though my research is geared toward armor applications, ceramics also experience high-strain-rate loading during processing (e.g., cutting or drilling) and while in service (e.g., surfaces grinding together or scratching). The data obtained from high-strain-rate tests is critical for developing accurate models and ultimately improving material performance. So, the next time you hear “in the blink of an eye”, remember that a blink takes longer than a day in the world of high-strain-rate mechanics.
More about Alison
Alison Trachet is a Ph.D. candidate in the Materials Science and Engineering (MSE) department at the University of Florida (UF). She is conducting research on the mechanical properties of advanced ceramics under Dr. Ghatu Subhash, the Knox T. Millsaps professor in the Mechanical and Aerospace Engineering (MAE) department at UF. Alison has undergraduate and master’s degrees in Civil Engineering from UF and the University of Texas at Austin, respectively. During her employment at a forensic engineering firm in Orlando, FL, she discovered her passion for understanding materials at the microstructural level and how the microstructure relates to bulk material properties. Alison has recently been awarded a Fulbright fellowship for 2015-2016, and she will be traveling to Salzburg, Austria to conduct research on Roman-era ceramics at the University of Salzburg.
- Ramot, D., “Average duration of a single eye blink”, http://bionumbers.hms.harvard.edu/bionumber.aspx?s=y&id=100706&ver=0, 2008, Accessed March 25, 2015.
- Meyers, M. Dynamic Behavior of Materials. Wiley-Interscience, New York, NY, 1994.
- Pittari, J., Subhash, G., Trachet, A., Zheng, J., Halls, V. and Karandikar, P. (2014), The Rate-dependent Response of Pressureless-sintered and Reaction-bonded Silicon Carbide-based Ceramics. International Journal of Applied Ceramic Technology. doi: 10.1111/ijac.12332.