Piyush Jagtap

Evaluating shock absorption behavior of small-sized systems under programmable electric field

A simple ball-drop impact tester is developed for studying the dynamic response of hierarchical, complex, small-sized systems and materials. The developed algorithm and set-up have provisions for applying programmable potential difference along the height of a test specimen during an impact loading; this enables us to conduct experiments on various materials and smart structures whose mechanical behavior is sensitive to electric field. The software-hardware system allows not only acquisition of dynamic force-time data at very fast sampling rate (up to 2 × 10(6) samples/s), but also application of a pre-set potential difference (up to ±10 V) across a test specimen for a duration determined by feedback from the force-time data. We illustrate the functioning of the set-up by studying the effect of electric field on the energy absorption capability of carbon nanotube foams of 5 × 5 × 1.2 mm(3) size under impact conditions.

Critical Evaluation of Relative Importance of Stress and Stress Gradient in Whisker Growth in Sn Coatings

The role of stress state and stress gradient in whisker growth in Sn coatings electrodeposited on brass is examined. The bulk stress in Sn coatings was measured using a laser-optics-based curvature setup, whereas glancing angle x-ray diffraction was employed to quantify the stress near the surface; this also allowed studying the role of the out-of-plane stress gradient in whisker growth. Both bulk stress and near-surface stress in the Sn coating evolved with time, wherein both were compressive immediately after the deposition, and thereafter while the bulk stress monotonically became more compressive and subsequently saturated with aging at room temperature, the stress near the surface of the Sn coating continually became more tensile with aging. These opposing evolutionary behaviors of bulk and near-surface stresses readily reveals establishment of a negative out-of-plane stress gradient, which is required for the spontaneous growth of whiskers. The importance of the out-of-plane stress gradient was also validated by externally imposing widely different stress states and stress gradients in Sn coatings using a 3-point bending apparatus. Additional whisker growth occurred in the coatings subjected to external tensile stress; however, this was accompanied by a higher negative out-of-plane stress gradient. The results conclusively demonstrate the important role of the negative out-of-plane stress gradient on whisker growth, as compared to only sign and magnitude of stress.

Effect of electric field on creep and stress-relaxation behavior of carbon nanotube forests

Carbon nanotube forests (CNTFs) are porous ensembles of vertically aligned carbon nanotubes, exhibiting excellent reversible compressibility and electric field tunable stress-strain response. Here, we report the effects of electric field on the time dependent mechanical behavior, namely creep and stress-relaxation, of CNTFs. Creep and stress-relaxation experiments were conducted under constant compressive stress and constant compressive strain, respectively, wherein variation of the strain and the stress, respectively, as functions of time were measured. Creep strain-time data of CNTFs showed a primary creep regime followed by a steady-state creep regime. The creep rate was substantially retarded upon application of electric field. The steady-state strain rate showed a power-law dependence on the stress; however, the stress exponent reduced when an electric field was applied. On other hand, electric field enhanced stress-relaxation in CNTFs, leading to a lower value of stress at a given time. However, the effect of electric field on the stress-relaxation reduced with compressive strain. Based on the Garofalo model of creep, a unified model for explaining the overall time dependent mechanical behavior of CNTFs and the observed experimental results was developed.

Macro and micro-texture study for understanding whisker growth in Sn coatings

The current work is aimed at analyzing the effects of crystallographic texture, both macro and microtexture, on whisker growth. The macro-texture of the Sn coatings deposited on brass substrate was systematically studied by varying the process parameters used for electro-deposition. The combination of process parameters, such as deposition temperature and current density, and the resulting macro-texture most prone to whisker growth were identified by monitoring the whisker growth in Sn coatings. The electron back-scatter diffraction (EBSD) technique was then used to identify the grains where whiskers actually grow. Both macro- and micro-texture of Sn coatings revealed that coatings with pre-dominant low Miller index grain orientations, such as (100), were highly susceptible for whisker growth. Whisker propensity decreased as the texture transitioned from low to high Miller index grain orientation. The microtexture mapping of the Sn coatings using EBSD technique confirmed that whisker grows from low Miller index grains with (100) or near (100) orientations, surrounded by grains of similar orientation followed by grains with high index plane grains, such as (211), (210) or (321). This particular double ring configuration along with local stress field can be used to predict the locations for whisker growth.