Experimental Mechanics

The current rate of technological development and interdisciplinary research has resulted in a significant increase of our ability to probe nature. In the context of mechanics and materials, improvements in solid state physics and optics have allowed us to probe the structures and properties of materials at time-scales down to the femtosecond. Many of these techniques are still areas of active scientific research by themselves.
The focus of my work is in understanding material response and the mechanisms responsible at high stresses, short time-scales and large multi-physical fields. The nature of these problems require active development of novel experimental techniques, more specifically to bridge the gap between length and time-scales in measurements. My pursuit in experimental mechanics is towards this direction using ideas from optics and physics.

In-situ high strain rate experiments

The kolsky bar, based on impact-driven elastic wave propagation in cylindrical bars, is a standard experimental technique used to probe the high strain rate uniaxial behavior of materials. Beginning with pioneering studies by Herbert Kolsky in 1949, the technique has undergone significant improvements and still stands as the most preferred method to measure high strain rate material behavior. Some of my studies on the dynamics of deformation twinning in single crystal magnesium involved combining a high-speed microscope with a conventional compression kolsky bar. Expanding beyond the limitations of conventional kolsky bar measurements, we were able to collect information about the spatial evolution of deformation twins at temporal resolutions down to hundereds of nanoseconds.



The conventional compression kolsky bar also suffers from inherent limitations in maximum strain-rates achievable. These limitations can be overcome by miniaturizing the experimental setup such as to acheive larger stresses in much shorter times. However, conventional means of measuring the propagating waves at these small dimensions are no longer possible. Some of my early work as a graduate student with Prof. K. T. Ramesh was in developing methods to improve spatial and temporal resolutions of propagating waves in a miniature kolsky bar setup. Using optical interferometry-based techniques (normal displacement interferometry or NDI), we were able to make high resolution measurements of material strength at much higher impact speeds than conventional kolsky bars.



Another significant limitation of kolsky bar measurements is an inability to probe multi-axial stress states. Recent data from literature have shown that the strength of conventional metals like copper depend on the hydrostatic pressure that the sample is subjected to. This challenges our conventional wisdom of the plastic deformation of metals being driven purely by deviatoric (or shear-dominant) stress states. The most common technique used to study these complex stress states is called the plate impact experiment. This is an area of future interest.

Electro-mechanical experiments

In the Mechanics and Materials laboratory, we study the thermo-electro-mechanics of ferrolectric materials across length and time scales. To study these problems experimentally, we use a custom-built experimental setup named Broadband Electromechanical Spectroscopy (BES). The BES is a combination of a high voltage ferroelectrics switching experiment and a broadband viscoelastic spectroscopy experiment pioneered by the group of Prof. Roderic Lakes in the late 1980's. The objective is to study polarization switching kinetics, mechanical actuation strains and viscoelastic stiffness and damping over a frequency spectrum spanning 12 orders of magnitude in electro-active material systems.

Collaborators

Prof. K. T. Ramesh , Department of Mechanical Engineering and Hopkins Extreme Materials Institute, Johns Hopkins University; Prof. D. M. Kochmann , Department of Mechanical and Process Engineering, ETH Zürich

Relevant Publications