K.T. Ramesh

K.T. Ramesh
Johns Hopkins University | JHU · Whiting School of Engineering

PhD

About

367
Publications
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14,802
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Publications

Publications (367)
Article
Computational models of the human head are promising tools for estimating the impact-induced response of the brain, and thus play an important role in the prediction of traumatic brain injury. The basic constituents of these models (i.e., model geometry, material properties, and boundary conditions) are often associated with significant uncertainty...
Preprint
Computational models of the human head are promising tools for the study and prediction of traumatic brain injuries (TBIs). Most available head models are developed using inputs (i.e., head geometry, material properties, and boundary conditions) derived from ex-vivo experiments on cadavers or animals and employ linear viscoelasticity (LVE)-based co...
Preprint
Computational models of the human head are promising tools for estimating the impact-induced response of brain, and thus play an important role in the prediction of traumatic brain injury. Modern biofidelic head model simulations are associated with very high computational cost, and high-dimensional inputs and outputs, which limits the applicabilit...
Article
Twinning is a prominent deformation mode that accommodates plasticity in many materials. This study elucidates the role of deformation rate on the atomic-scale mechanisms that govern twin boundary migration. Examination of Mg single crystals deformed under quasi-static compression was compared with crystals deformed via plate impact. Evidence of tw...
Article
Advances in brain imaging and computational methods have facilitated the creation of subject-specific computational brain models that aid researchers in investigating brain trauma using simulated impacts. The emergence of magnetic resonance elastography (MRE) as a non-invasive mechanical neuroimaging tool has enabled in vivo estimation of material...
Article
Full-text available
Magnesium (Mg) and its alloys have been the subject of intensive scientific research and development in the communities of materials science and engineering, mechanical engineering and manufacturing. Considering their light weight and high specific strength, current and potential applications include the aerospace industry, automobiles, and vehicle...
Article
Magnesium alloys are an attractive material system for protection applications due to their high specific strength and stiffness, but exhibit low ductility in these applications. The potential to address this shortcoming through materials-based-design has motivated the Center for Materials in Extreme Dynamic Environments (CMEDE) to focus on improvi...
Article
We present a simplified constitutive model, based on the dominant deformation mechanisms, to capture the mechanical behavior of magnesium. This approach takes into account 1) the accumulation and annihilation of 〈a〉 and 〈c+a〉 dislocations, 2) the volume fraction of extension twins, and 3) two material domains - the parent region and the twinned reg...
Article
We describe models for the behavior of hot‐pressed boron carbide that is subjected to extreme dynamic environments such as ballistic impact. We first identify the deformation and failure mechanisms that are observed in boron carbide under such conditions, and then review physics‐based models for each of these mechanisms and the integration of these...
Article
Out‐of‐plane anisotropy in the mechanical response of boron carbide was studied by performing nanoindentation experiments on four specific crystallographic orientations of single crystals i.e., ( , ( , , and ( . For each orientation of the single crystals, in‐plane variations of indentation modulus and hardness were also studied by monitoring the r...
Article
Computational models of the brain and its biomechanical response to skull accelerations are important tools for understanding and predicting traumatic brain injuries (TBIs). However, most models have been developed using experimental data collected on animal models and cadaveric specimens, both of which differ from the living human brain. Here we d...
Article
Understanding the particle-scale dynamics of granular materials during rapid compaction and flow is of fundamental importance for manufacturing, planetary science, geology, and defense applications. Time-resolved 2D radiography and static 3D x-ray tomography are powerful in situ tools for studying particle-scale dynamics but provide detail only in...
Article
A finite deformation mechanism-based thermodynamically consistent constitutive framework is presented for describing the dynamic behaviors of brittle materials under impact loading. The framework is developed based upon a multiplicative decomposition of the deformation gradient in terms of multiple mechanisms, including recoverable elasticity, crac...
Article
Full-text available
Suhas Eswarappa Prameela, K. T. Ramesh and Tim Weihs highlight how students and postdocs can develop their competencies and skills during research collaboration.
Article
Full-text available
The mechanical response of geomaterials to extreme dynamic conditions is essential to understanding planetary impacts, other geological and seismic events, and underground explosions. In this study, we present a multimechanism constitutive model to describe the dynamic response of polycrystalline quartz (e.g., nonporous quartzite), and implement it...
Article
Both engineered and natural materials may be subjected to extremes of elevated pressure, temperature, and strain rate due to shock loading; examples include ballistic impact of projectiles on armor, planetary impacts, and high-speed machining operations. Experimental techniques for ascertaining the macroscopic response of materials to shock loading...
Article
Full-text available
We examine the statistical distribution of critical nucleation pressures necessary to dynamically grow voids during the spall failure of an AZ31B magnesium alloy. The approach uses laser-driven micro-flyers to generate spall over times of the order of tens of nanoseconds, allowing us to focus on void nucleation processes rather than void coalescenc...
Article
Dynamic point impact loading is a primary cause of fracture of glass screens on mobile devices. An improved understanding of crack initiation and evolution under high-speed indentation could contribute to the development of materials with better performance, but experimental observations are challenging due to the short timescales and limited depth...
Article
Full-text available
We have investigated the deformation and failure mechanisms of a rolled AZ31B magnesium alloy under both quasi-static (\(10^{-3}\,{\text {to}}\,10^{-2}\,{\text{s}}^{-1}\)) and dynamic (\(10^{3}\,{\text{s}}^{-1}\)) compressive loading. The observed anisotropy in the plastic response originates from the different in-plane and out-of-plane deformation...
Presentation
The need to counter weapons of mass destruction with reduced collateral damage in urban environments has necessitated the development of countermeasures containing pre-formed, dense reactive materials. A crucial first step in understanding the impact and combustion performance of high-density reactive materials compacts is to characterize the effec...
Article
Full-text available
Boron carbide is an excellent protection material but it is also very brittle, and granular flow, triggered by progressive fracture during impact, greatly influences its ballistic performance. Transmission electron microscopy observations of samples recovered from controlled pressure-shear plate impact experiments have elucidated the extent to whic...
Article
Full-text available
Plastic flow is studied as a function of strain rate for uniaxial compression along the principal directions of an AZ31B magnesium alloy processed by equal channel angular extrusion across 8 decades of strain rate. The effect of strain rate on flow stress is found to be a function of loading orientation. Work hardening rates are also found to vary...
Article
Full-text available
Laser-driven micro-flyer plates exhibit high planarity within the first 500 µm of travel, but deform into curved impactors due to loss of flight-driving plasma at the edges of the plate and interaction of the plate with the atmosphere. This time-of-flight based tunability of impactor geometry offers adjustable loading conditions in stress space. He...
Article
We examine the influence of shear anisotropy of brain tissue on the potential for mild traumatic brain injury. First we develop a new constitutive description for the white matter in the brain that can capture the anisotropic behavior of the white matter in both tension and shear. The material parameters for the models are determined using a set of...
Article
Full-text available
Measurements of the mechanical response of biological cells are critical for understanding injury and disease, for developing diagnostic tools, and for computational models in mechanobiology. Although it is well known that cells are sensitive to the topography of their microenvironment, the current paradigm in mechanical testing of adherent cells i...
Article
Although x-ray tomography is commonly used to characterize the three-dimensional structure of materials, sometimes this is impractical due either to limited time for data collection (such as in rapidly-evolving systems) or the need to limit the radiation exposure of the sample. In such situations, it is desirable to extract as much information as p...
Article
Full-text available
An increase in arterial pressure within the cerebral vasculature appears to coincide with ischemia and dysfunction of the neurovascular unit in some cases of traumatic brain injury. In this study, we examine a new mechanism of brain tissue damage that results from excessive cerebral arteriole pressurization. We begin by considering the morphologica...
Article
Regolith generation by thermal fatigue has been identified as a dominant mechanism for the breakdown of small (cm-sized) rocks on certain airless bodies. Simple numerical models for thermal fatigue seemed to indicate that this breakdown occurs faster in the larger decimeter-sized rocks, which is in contrast to the predictions of disruption models t...
Article
Full-text available
We describe a laser-launched micro-flyer apparatus designed for spall strength measurement. The launcher uses a single pulse from a pulsed laser that is stretched in time to nominally 20 nanoseconds using an optical ring cavity, while inexpensive multi-lens arrays are used to spatially homogenize the beam. The velocimetry technique that we develope...
Conference Paper
The Hopkins Extreme Materials Institute (HEMI) recently installed a hypervelocity impact facility (HyFIRE) including a two-stage light gas gun at Johns Hopkins University in Baltimore, MD. The HyFIRE launcher has a launch tube bore diameter of 7.62 mm and is designed to attain launch velocities up to 7 km/s. The enclosed ballistic range and termina...
Conference Paper
Full-text available
Dynamic fragmentation through high-rate impact generates large numbers of fragments with various shapes and sizes. The fragmentation failure mode is an important part of the protection capacity of advanced ceramics which typically feature high strength and low density but fail in brittle modes. The penetration resistance of these brittle materials...
Article
Full-text available
The mechanical properties of biological materials are commonly found through the application of Hertzian theory to force-displacement data obtained through micro-indentation techniques. Due to their soft nature, biological specimens are often subjected to large indentations, resulting in a nonlinear deformation behavior that can no longer be accura...
Article
An understanding of the dynamic failure of damaged ceramics is important in protection applications, where the interaction of the projectile with cracked material is a contributing factor in the overall system performance. In this paper, we investigate the effects of pre-existing internal cracks on the quasi-static and dynamic compressive behavior...
Article
We employ an advanced 3D computational model of the head with high anatomical fidelity, together with measured tissue properties, to assess the consequences of dynamic loading to the head in two distinct modes: head rotation and head extension. We use a subject-specific computational head model, using the material point method, built from T1 magnet...
Article
Full-text available
Brittle and quasibrittle materials such as ceramics and geomaterials fail through dynamic crack propagation during impact events. Simulations of such events are important in a number of applications. This paper compares the effectiveness of the embedded finite element method (EFEM) and the extended finite element method (XFEM) in modeling dynamic c...
Article
We present a hybrid approach for simulating hypervelocity impacts onto asteroids. The overall system response is separated into two stages based on their different characteristic timescales. First, the short-timescale fragmentation phase is simulated using a modified version of the Tonge–Ramesh material model implemented in a Material Point Method...
Article
Full-text available
Pressure-shear plate impact experiments generate normal and transverse particle velocities during high strain rate deformations. Traditionally, freespace lenscoupled tabletop laser interferometry techniques are used together with diffraction gratings to interrogate the evolving velocity vector at the back face of the target plate. Recently, fiberop...
Article
We demonstrate the use of X-ray phase contrast imaging with sub-microsecond temporal resolution to obtain quantitative visualization of dynamic fracture processes in brittle solids. We examine an amorphous solid (fused silica), a ceramic single crystal (single-crystal quartz), and a polycrystalline ceramic (boron carbide), in the form of single-edg...
Article
A physics-based finite strain crystal plasticity constitutive model for body-centered-cubic (BCC) single crystals is developed to capture the strong temperature, rate, and orientation dependence of mechanical behavior. The key features of the model include twinning-anti-twinning asymmetry of shearing resistance, a yield criterion that incorporates...
Article
This paper examines the anisotropic and rate-dependent mechanical behavior of a rolled AZ31B magnesium alloy under multiaxial loading over a wide range of strain rates ranging from 10⁻⁴ to 10⁵s⁻¹. It is shown that stress state, strain rate as well as loading orientation collectively influence the plastic flow. The rate dependence of the plastic flo...
Article
We report in situ time-resolved measurements of the dynamic evolution of the volume fraction of extension twins in polycrystalline pure magnesium and in the AZ31B magnesium alloy, using synchrotron x-ray diffraction during compressive loading at high strain rates. The dynamic evolution of the twinning volume fraction leads to a dynamic evolution of...
Article
In-situ ultra high speed optical imaging with 200 ns temporal resolution is used to gain insight into the evolution of twinning in single crystal magnesium under dynamic loading. Under compression along the a-axis, nucleation of twins is observed to occur on two conjugate {101¯2} twin planes. Twin nucleation appears to be stress-driven with the fir...
Article
Deformation twinning is an important deformation mechanism in a variety of materials, including metals and ceramics. This deformation mechanism is particularly important in low-symmetry hexagonal close-packed (hcp) metals such as Magnesium (Mg), Zirconium (Zr) and Titanium (Ti). Extension twins in Mg, Zr and Ti can accommodate considerable plastic...
Article
Full-text available
Based on transmission electron microscopy results from pure Mg single crystal examined in the current work, and Mg alloys and other hexagonal metals in literature, a characteristic dislocation substructure inside 101¯2 twins is identified. Abundant non-basal [c] and ⟨c + a⟩ perfect dislocations, as well as basal I1 stacking faults with widths on th...
Article
Full-text available
Traumatic brain injury such as that developed as a consequence of blast is a complex injury with a broad range of symptoms and disabilities. Computational models of brain biomechanics hold promise for illuminating the mechanics of traumatic brain injury and for developing preventive devices. However, reliable material parameters are needed for mode...
Article
Full-text available
Protection materials are continuously facing higher velocity threats; characterizing their response to these threats is necessary to improve performance. We have previously examined failure of boron carbide and magnesium systems at strain rates of up to 10⁵ per second [12,13]. To study dynamic deformation at higher strain rates closer to the hyperv...
Article
We examine the strengthening mechanisms within a high-strength aluminum alloy with the objective of providing guidelines for increased strength. First, we measure the mechanical behavior of the age-hardenable Al–Cu–Mg–Ag alloy known as Al 2139 in the T8 condition, and observe strengths of 500 MPa at quasistatic strain rates and average strengths of...
Article
We present an analytic closed form solution for anisotropic change in compliance due to the spatial evolution of planar wing-cracks in a material subjected to largely compressive loading. A fully three-dimensional anisotropic compliance tensor is defined and evaluated considering the wing-crack mechanism, using a mixed-approach based on kinematic a...
Article
Normal plate impact experiments were conducted to study the spall behavior of AMX602 magnesium alloy fabricated via Spinning Water Atomization Process (SWAP), which was followed by cold compaction and hot extrusion. Incipient spall damage in the specimens was measured at different shock conditions using 51 mm and 105 mm diameter bore gas guns. The...
Article
Mechanics lies at the heart of many of the underpinnings of modern technological civilization: materials, infrastructure, transportation, health and security. The mechanics of dynamic failure processes also has a major bearing on the potential catastrophes that threaten civilization, including airbursts and major asteroid impacts. Recent events (su...
Article
Full-text available
Boron carbide has a wide range of solubility, but the effects of stoichiometry on its microstructure and mechanical response are not well understood. In this study, detailed microstructural characterization was carried out on three hot-pressed B-rich boron carbide samples. Lattice parameter measurements from XRD identified the compositions to be B4...
Article
A minor error was present in one equation in the original paper, “Multi-scale defect interactions in high-rate brittle material failure. Part I: Model formulation and application to AlON” and in the corresponding computational implementation. The correction is provided here. None of the primary conclusions of the original paper are affected.
Article
Single crystals of pure Mg were deformed along the crystallographic axis under normal plate impact loading of microsecond duration. Post-test electron microscopy analyses of the recovered specimens revealed the presence of extension twins. Twins were observed to grow to significant sizes and an estimate of the twin tip velocity was obtained. Second...
Chapter
Pure Mg single crystals were deformed at room temperature along two orientations in sequence, in order to activate a specific dislocation slip mode followed by \( \left\{ {10\bar{1}2} \right\} \) twinning. The defects in both the matrix and twin crystals were analyzed with a transmission electron microscope (TEM) . This study reveals the collective...
Article
Diffuse axonal injury (DAI) is a debilitating consequence of traumatic brain injury (TBI) attributed to abnormal stretching of axons caused by blunt head trauma or acceleration of the head. We developed an anatomically accurate, subject-specific, three-dimensional (3D) computational model of the human brain, and used it to study the dynamic deforma...
Article
Electrical response from dynamic compression tests on x-cut single crystal α-quartz specimens at strain rates of 10(E)3 1/s reveal nonsimilar stress-charge behavior during damage evolution. Specifically, when quartz is undergoing extensive and irreversible dynamic brittle fracture under a compressive stress impulse of up to 1.8 GPa, the effective p...
Article
The distributions of size and chemical composition in the regolith on airless bodies provides clues to the evolution of the solar system. Recently, the regolith on asteroid (25143) Itokawa, visited by the JAXA Hayabusa spacecraft, was observed to contain millimeter to centimeter sized particles. Itokawa boulders commonly display well-rounded profil...
Article
In this paper, the impact-induced fragmentation of a commercially available hot-pressed boron carbide is explored. Fragmentation has been noted previously by many authors to be important in the impact performance of advanced ceramics, and so this paper seeks to provide some of the first near-complete and detailed measurements of individual fragment...
Article
Full-text available
IntroductionA new data science initiative in materials research has been launched at The Johns Hopkins University within the Materials in Extreme Dynamic Environments (MEDE) Collaborative Research Alliance (CRA). Our first goal is to build a solution that facilitates seamless data sharing among MEDE scientists. We expect to shorten the design and d...
Article
Experimental studies have identified an anomalous grain size dependence associated with the critical tensile pressure that a metal may sustain before catastrophic failure by cavitation processes. Here we derive the first quantitative theory (and its associated closed-form solution) capable of explaining this phenomena. The theory agrees well with e...
Article
Time-resolved normal plate impact experiments and spall recovery experiments were conducted to study the spall behavior of AZ31B-4E magnesium alloy processed via Equal-Channel Angular Extrusion (ECAE). The spall strength and incipient spall damage in the specimens were measured at different shock stresses using 51 mm and 105 mm bore gas guns. The H...
Article
Full-text available
The Acoustic Emission of deformation twinning in Magnesium is investigated in this article. Single crystal testing with combined full field deformation measurements, as well as polycrystalline testing inside the scanning electron microscope with simultaneous monitoring of texture evolution and twin nucleation were compared to testing at the laborat...
Article
Effects of microstructure and material properties on the mechanical behavior of hot-pressed boron carbide are presented. The microstructure and intrinsic microstructural inhomogeneities have been characterized using scanning electron microscopy characterization techniques (SEM/EDS/EBSD). In situ mechanical characterizations of the boron carbide mic...
Article
Dynamic brittle fragmentation is typically described using analytical and computational approaches for tensile stress-states. However, most fragmentation applications (e.g., impact, blast) involve very large initial compressive stresses and deformations. In this study, the compressive fragmentation of brittle materials is investigated experimentall...
Article
Within this two part series we develop a new material model for ceramic protection materials to provide an interface between microstructural parameters and bulk continuum behavior to provide guidance for materials design activities. Part I of this series focuses on the model formulation that captures the strength variability and strain rate sensiti...
Article
Our traditional view of void nucleation is associated with interface debonding at second-phase particles. However, under extreme dynamic loading conditions second-phase particles may not necessarily be the dominant source of void nucleation sites. A few key experimental observations of laser spall surfaces support this assertion. Here, we describe...
Article
We developed a dynamic inflation experiment to measure the elastodynamic behavior of soft materials. In the experiments, a shock tube was used to apply dynamic pressurization to thin polydimethylsiloxane specimens. Two high-speed cameras were used to image the deforming specimen and three-dimensional digital image correlation was used to determine...
Article
This study investigates the effects of the processing-induced defect population on the dynamic compressive strength and failure of a hot-pressed boron carbide. Quantitative microscopic analysis was used to determine the distributions of three types of processing-induced inhomogeneities: aluminum nitride, small graphitic particles and pores, and lar...