Tezeswi Tadepalli

Energy Dissipation and the High-Strain Rate Dynamic Response of Vertically Aligned Carbon Nanotube Ensembles Grown on Silicon Wafer Substrate

The dynamic mechanical behavior and high-strain rate response characteristics of a functionally graded material (FGM) system consisting of vertically aligned carbon nanotube ensembles grown on silicon wafer substrate (VACNT-Si) are presented. Flexural rigidity (storage modulus) and loss factor (damping) were measured with a dynamic mechanical analyzer in an oscillatory three-point bending mode. It was found that the functionally graded VACNT-Si exhibited significantly higher damping without sacrificing flexural rigidity. A Split-Hopkinson pressure bar (SHPB) was used for determining the system response under high-strain rate compressive loading. Combination of a soft and flexible VACNT forest layer over the hard silicon substrate presented novel challenges for SHPB testing. It was observed that VACNT-Si specimens showed a large increase in the specific energy absorption over a pure Si wafer.

Surface Fractal Analysis for Estimating the Fracture Energy Absorption of Nanoparticle Reinforced Composites

In this study, the fractal dimensions of failure surfaces of vinyl ester based nanocomposites are estimated using two classical methods, Vertical Section Method (VSM) and Slit Island Method (SIM), based on the processing of 3D digital microscopic images. Self-affine fractal geometry has been observed in the experimentally obtained failure surfaces of graphite platelet reinforced nanocomposites subjected to quasi-static uniaxial tensile and low velocity punch-shear loading. Fracture energy and fracture toughness are estimated analytically from the surface fractal dimensionality. Sensitivity studies show an exponential dependency of fracture energy and fracture toughness on the fractal dimensionality. Contribution of fracture energy to the total energy absorption of these nanoparticle reinforced composites is demonstrated. For the graphite platelet reinforced nanocomposites investigated, surface fractal analysis has depicted the probable ductile or brittle fracture propagation mechanism, depending upon the rate of loading.

Numerical and experimental blast response characterization of sandwich composite structural panels

Experimental blast response and quasi-static material property data were obtained for E-glass and carbon face skin sandwich composite panels with balsa, polyvinyl chloride foam, and TYCOR® cores. The pressure versus impulse (P–I) curve methodology enabled the generation of a database of performance envelopes for these sandwich composite panel configurations under different blast loading scenarios. The strength versus deformation properties of various undamaged sandwich composite panels are established numerically and idealized for single-degree-of-freedom modeling. Results show good correspondence between model predictions and experimental results for performance evaluation of the various sandwich composite structural panel configurations that were investigated.

Subsystem Instability Conditions in Steel Frames Associated with Extreme Lateral Loading

The present study presents a methodology for evaluating performance of steel moment resisting frames subject to extreme lateral loads such as earthquake, blast, tornado, and surge. Current analysis and design procedures specialized to individual hazards are viewed in a broader context in which multiple hazards are considered together. The premise is that the multi-hazard view will more effectively identify trade-offs in design choices that meet a specific hazard only and will lead to a better appreciation of both safety and economy especially in regions not dominated by a single hazard. This paper focuses on the design of a steel moment resisting building frame using AISC 360-05 composite beam flexural strength and beam-column interaction equations. As an example of the multihazard approach, beam and column sections are designed to satisfy a seismic load case and associated load combinations provided in the International Building Code 2006 and by reference ASCE 7-05. The frame is then analyzed under demands from an accidental blast pressure load case and load combination developed according to TM 5-1300 guidelines. Conclusions regarding potential instability are established using AISC beam-column interaction ratios applicable to the seismically resistant columns accounting for moments estimated using finite element models of gravity response of the frame system to a post-blast state of the damaged system. Observations are offered on how the blast analysis might affect the design process if brought in at a pre-construction rather than a retrofit stage.