MSE Seminar: Todd Hufnagel, Johns Hopkins University
Kimball Hall B11
Life and death of materials in the fast lane: Deformation and fracture under dynamic loading The mechanical behavior of materials is commonly studied in the laboratory under slow loading rates. But behavior under high-rate loading is important in many situations - automobile crashes and ballistic penetration of armor being two examples. Because macroscopic phenomena such as plastic deformation and fracture result from underlying time-dependent microscopic processes, it should come as no surprise that the behavior of materials under dynamic loading can be quite different from that observed at lower rates. But studying these processes in detail is complicated by the short time scales, which range from a few milliseconds to less than a microsecond. Synchrotron x-ray radiation has characteristics that make it ideally suited for providing new insights into the microscopic processes that govern deformation and fracture of materials under dynamic loading. In this talk we will focus on two classes of time-resolved experiments: X-ray diffraction and x-ray phase contrast imaging. Examples of x-ray diffraction will focus on plastic deformation of metals, including deformation twinning in magnesium and transformation-induced plasticity in steels. Turning to imaging, we will describe how phase-contrast imaging exploits the spatial coherence of synchrotron radiation to provide enhanced sensitivity to cracks and other internal defects. Examples here will include crack nucleation and propagation in chemically-strengthened glass, and deformation of granular solids under impact loading. Throughout the talk we will emphasize how quantitative observations of structural evolution relate to other data collected in the experiments (such as the stress-strain history) and how these can be used to inform computational models of the behavior of materials under dynamic loading.