Nilanjan Mitra

Seismic Design of Reinforced Concrete Beam-Column Joints with Headed Bars

Section 12.6 provisions of ACI 318-08 detail the development of headed and mechanically anchored deformed bars for the first time in the Code series. Prior to this, Joint ACI-ASCE Committee 352 published design recommendations for headed reinforcement used in reinforced concrete beam-column joints (ACI 352R-02). However, both ACI 318-08 and 352R-02 are based on quite limited experimental research. Given this concern, these ACI standards and recommendations were evaluated using an extensive database encompassing most available test data for reinforced concrete beam-column joints with headed bars subjected to reversed cyclic loading. The primary objectives of this study are to document the experimental investigations in a uniform format; provide a detailed review for the test data; and, finally, propose design guidelines to supplement ACI 352R-02 and 318-08 on the subject of headed bars anchored in beam-column joints.

Experimental Testing to Determine Concrete Fracture Energy Using Simple Laboratory Test Setup

Insight into reinforced concrete structural behavior can be provided and experimental investigation can be complemented by nonlinear finite element analysis. Specification of material parameters, including post-peak tensile response curve shape, fracture energy, and concrete tensile strength is typically required in a concrete structure finite element models development. A number of different tests for fracture energy and post-cracking response determination has resulted from previous research. A very stiff, closed-loop test machine typically needs to be used in these methods so load application can be performed under displacement control since extremely brittle response is exhibited by test specimens. A recommended fracture energy test method was employed in a recently computed University of Washington study, with an open-loop testing machine and test specimens modified to include counterweights. Test-generated post-cracking response data and fracture energy data fall within the typically observed recommended test range. Additionally, the laboratory-observed load-displacement response was reproduced, with acceptable accuracy through using these data for concrete constitutive model calibration for nonlinear finite element analyses.

Non-contact near-field underwater explosion induced shock-wave loading of submerged rigid structures: Nonlinear compressibility effects in fluid structure interaction

A novel theory has been developed for underwater explosion induced shock wave loading considering nonlinear compressible medium both front and back of the free standing rigid plate. Both analytical formulation and numerical simulations have been performed in this manuscript for different type of shock loading profiles, different plate masses as well as different backing conditions of the plate. The results obtained have been asymptotically validated against existing theories in acoustic range. Impulse transmission, maximum momentum transfer, and cavitation inception have been determined for different cases and have been shown to be dependent on the fluid structure interaction parameter as well as the backing condition of the plate. Impulse design transmission curves have also been developed for plates of intermediate mass for different shock intensities and profiles thereby removing the need to perform numerical simulations. The major conclusions of the paper are that nonlinear compressibility and varying...

Shock induced phase transition of water: Molecular dynamics investigation

Molecular dynamics simulations were carried out using numerous force potentials to investigate the shock induced phenomenon of pure bulk liquid water. Partial phase transition was observed at single shock velocity of 4.0 km/s without requirement of any external nucleators. Change in thermodynamic variables along with radial distribution function plots and spectral analysis revealed for the first time in the literature, within the context of molecular dynamic simulations, the thermodynamic pathway leading to formation of ice VII from liquid water on shock loading. The study also revealed information for the first time in the literature about the statistical time-frame after passage of shock in which ice VII formation can be observed and variations in degree of crystallinity of the sample over the entire simulation time of 100 ns.

On shock response of nano-void closed/open cell copper material: Non-equilibrium molecular dynamic simulations

Non-equilibrium molecular-dynamic simulations were carried out on model three-dimensional nano-void copper material with different idealised pore structure and porosity to highlight differences in response behaviour between them when subjected to various piston velocities simulating planar shock loading of different intensities. This article demonstrates and explains from a mechanistic perspective the differences in response observed with respect to Hugoniot elastic limits, dislocation line and jet formation, void collapse mechanism and hot spot generation, specific volume, partial recrystallisation and temperature evolution in void collapsed regions, shock and particle velocity curves.

On core compressibility of sandwich composite panels subjected to intense underwater shock loads

Novel analytical models have been proposed in this study which extends current available fluid-structure interaction (FSI) theories for explosion induced shock loading on monolithic and laminated composite plates to sandwich composite panels, featuring core compression. The proposed models have been asymptotically validated against other FSI existing theories in low pressure range. A qualitative comparative analysis of the proposed models has been made with other existing FSI theories from the viewpoint of energy conservation. Core compression as predicted by the proposed models can be utilized for more economical, robust design of blast resistant sandwich composite structures.

Shock compression of polyvinyl chloride

This study presents shock compression simulation of atactic polyvinyl chloride (PVC) using ab-initio and classical molecular dynamics. The manuscript also identifies the limits of applicability of classical molecular dynamics based shock compression simulation for PVC. The mechanism of bond dissociation under shock loading and its progression is demonstrated in this manuscript using the density functional theory based molecular dynamics simulations. The rate of dissociation of different bonds at different shock velocities is also presented in this manuscript.

A metastable phase of shocked bulk single crystal copper: an atomistic simulation study

Structural phase transformation in bulk single crystal Cu in different orientation under shock loading of different intensities has been investigated in this article. Atomistic simulations, such as, classical molecular dynamics using embedded atom method (EAM) interatomic potential and ab-initio based molecular dynamics simulations, have been carried out to demonstrate FCC-to-BCT phase transformation under shock loading of 〈100〉 oriented bulk single crystal copper. Simulated x-ray diffraction patterns have been utilized to confirm the structural phase transformation before shock-induced melting in Cu(100).

Mixed-mode fracture of sandwich composites: Performance improvement with multiwalled carbon nanotube sonicated resin

An experimental investigation on sandwich composite materials composed of glass-fiber face sheet and polyvinyl-chloride foam core has been carried out. The research demonstrates improvement in mixed-mode delamination fracture toughness values of samples under mixed-mode bending condition. The improvement is recorded with addition of a certain percentage by weight of multiwalled carbon nanotubes in comparison to conventional samples. An easy and cost-effective methodology of multiwalled carbon nanotube insertion through sonication of epoxy resin followed by mixing with hardener and vacuum resin infusion technology for manufacturing of sandwich composites has been utilized in this study. The study also identifies the optimum weight percentage of multiwalled carbon nanotube addition in the resin system for maximum performance gain in mixed-mode fracture toughness. The results of observations in this study have been supported by field emission scanning electron microscope studies as well as high-resolution transmission electron microscope analysis.

Failure Initiation of Reinforced-Concrete Beam-Column Connections - Binomial Logistic Regression Based Probabilistic Model

The inelastic mechanisms leading to failure (characterized by strength degradation) initiation in a weak-beam-strong-column reinforced concrete beam-column (RCBC) frame connection subjected to seismic loading are either non-ductile joint failure prior to beam yielding or a ductile mechanism of beam yielding followed by joint failure. Using an extensive assembled database of interior and exterior beam-column joints, binomial logistic regression models are developed to provide a probabilistic estimate of qualitative inelastic mechanisms based on quantitative design parameters obtained from specimen material and geometry. The relative importance of these quantitative parameters is also quantified in this research. The models provide opportunity to an engineer designing new connection to modify geometrical and/or material parameters to obtain ductile mode of response; an engineer retrofitting damaged connection to obtain an understanding of factors influencing initiation of strength degradation and thereby develop suitable retrofit measures for similar connections at eminent risk of failure.