Research

Ramesh Lab Research

The group focuses on fundamental studies of deformation and failure (including injury) across a variety of length scales and timescales, with applications in mechanical engineering, materials science, planetary science, and biomedicine. What ties us together is a way of thinking: an interest in understanding mechanisms through experiment, theory and computation, with a particular appreciation of the value of rigorous mechanics, and an emphasis on transient phenomena. The major research projects currently underway in the group are listed below.

1. Anisotropy and rate sensitivity of textured AZ31B Mg alloy under multiaxial dynamic loading (Zhao, Kannan).
   Mg and its alloys develop strong textures after processing, thus leading to pronounced anisotropy in the mechanical response. The material properties are also found to vary significantly with strain rate and applied stress state. We seek to understand the anisotropy and rate dependence of an AZ31B Mg alloy with strong rolling texture using several experimental techniques including a servohydraulic MTS machine(10-4-10-2 s-1), conventional Kolsky bars(10+3 s-1), desktop Kolsky bars(10+4 s-1) and pressure-shear plate impact(10+5 s-1). The ultrahigh strain rate plate impact experiments allow us to interrogate the stress state effects on the material properties by applying high shearing rates with superimposed nearly hydrostatic pressures. We are also developing multi-mechanism constitutive models to better understand the contribution of the microscale deformation mechanisms, particularly deformation twinning, to the macroscopic material response. We hope these experimental and modelling efforts can unveil the relationships between deformation mechanisms and the anisotropic rate-dependent plasticity of this material.
2. The dynamics of deformation twinning in magnesium and magnesium alloys at high rates of loading (Kannan, Zhao).
   An important part of developing predictive capabilities for the plastic deformation of magnesium at high rates involves understanding the physics of specific deformation mechanisms. We seek to understand the nature of nucleation and growth of deformation twins in magnesium using fundamental experiments and modeling. Using ultra-high-speed imaging techniques coupled with high rate impact experiments, we study in-situ the nucleation and growth of twins in single crystals of magnesium. Future models will be aimed at a physical understanding of the observed nucleation and growth dynamics and extend this understanding to polycrystalline systems. This in turn will be a step towards a larger goal of predicting the strength of magnesium and its alloys based on physically robust descriptions of its underlying mechanisms.
3. Development and validation of multi-mechanism constitutive models for magnesium and magnesium alloys (Zhao, Braroo).
4. Critical mechanisms developed during hypervelocity impact on metals and ceramics (Simpson).
5. Laser shock experiments to study the deformation and failure of protection materials (Mallick).
   Ballistic experiments to understand shock loading of protection materials tend to be expensive, difficult, and dangerous to perform. We are developing a laser-driven micro-flyer facility to allow high strain rate (10+7 s-1) material characterization in an easy and quickly repeatable manner. Launches are driven by a 2.5J pulsed laser to propel flyers of thicknesses in the hundreds of microns to kilometers-per-second velocities. To understand the failure process, we use high-speed imaging (1M frames/sec) and fiber optics coupled laser interferometry (PDV).
6. Very high-rate multiaxial loading of granular materials (Sun).
7. Experimental determination of dynamic multiaxial effects on amorphization (Leonard).
8. Mechanism-based constitutive models for the dynamic deformation of geomaterials (Zeng).
   Research on the dynamic deformation of geomaterials can help us gain insight into the interactions of such materials with an impactor, as well as other geological phenomena, such as shearing during earthquakes, and crater formation. Mechanisms such as microcracking, pore collapse and granular flow contribute significantly to the final failure process of geomaterials. We are developing efficient mechanism-based constitutive models to describe the dynamic behaviors of geomaterials, with the focus on capturing these mechanisms through micromechanics models. The interaction between multiple mechanisms can be accounted for with self-consistent schemes. Drawing on mathematical representations of specimen damage based on experimental data, the response of a representative volume element (RVE) can be homogenized based on the concept of continuum damage mechanics.  Our models will be implemented in existing simulation frameworks and adopted to simulate the behaviors of geological structures subjected to impact loading.
9. Mechanism-based models for the dynamic deformation and failure of ceramics (Zeng).
   Dynamic deformation and failure processes of ceramics are linked to the evolution of pre-existing defects such as microcracks. In this work (based on the earlier work of Tonge et al.), we develop micromechanics-based damage models to describe the behaviors of ceramics under impact loading. The statistical distribution of initial microcracks is included. A wing crack model is adopted to describe the propagation of microcracks under compression, and the interaction between multiple microcracks is addressed with a self-consistent scheme. A damage tensor is used to represent the three-dimensional anisotropic damage. As the microcracks propagate and coalesce, beyond a certain threshold of damage, a granular flow model is included to describe the subsequent behaviors. The combination of this material model with a numerical technique such as material point method (MPM) can allow us to perform detailed simulations of impact events.
10. In situ visualization of the dynamic deformation and failure of brittle solids (Leong).
    High rate loading of brittle solids induces vastly different responses than quasi-static loading as a result of the activation of rate-sensitive deformation and failure mechanisms. A phase contrast X-ray imaging (PCI)-based method is being developed to identify what the dominant mechanisms are during dynamic loading. The highly penetrative depth of X-rays enables direct visualization in the bulk of the material at micron resolution. Combining this with the use of a brilliant and coherent synchrotron x-ray source, material response can be captured in real-time. We hope to identify and quantify the mechanisms so as to help develop models for the dynamic failure of brittle solids.
11. Mechanisms in ultra-high-molecular-weight polyethylene composites under dynamic loading (Parker).
12. Thermal fragmentation and the development of regolith on asteroids (El Mir).
    Virtually every object in the solar system is covered with a layer of loose, uncosolidated rocks known as “regolith”. Several mechanisms, from impact fragmentation to space weathering, contribute to the evolution of the regolith’s size-frequency distribution. Typical small (sub-km) asteroids have a rotational period of a few hours. The cyclic heating and cooling of surface rocks from these short day/night cycles lead to a spatiotemporally varying stress state in surface rocks. Cracks can then grow due to Thermal Fatigue, and a large number of cycles can lead to regolith generation and evolution by Thermal Fragmentation. In this work, we examine the extent to which thermal fatigue can weaken and fracture rocks to form regolith. We develop numerical techniques that allow us to efficiently investigate the breakdown of centimeter to meter-sized asteroidal rocks over the course of several thousands of years, as a result of a large number of periodic thermal cycles. We also explore the time-to-fracture as a function of a rock’s size, heterogeneous composition, and the asteroid’s orbital dynamics through a dimensional analysis of the major variables.
13. Hypervelocity impacts in the solar system and the disruption of spinning asteroids (El Mir).
    Asteroids in the solar system have witnessed a long history of impact events that evolved their shapes and internal structure.
    In this work, we look to understand the collisional evolution of the size frequency distribution of asteroids in the solar system with implication for understanding cratering rates on planets, and the ages of planetary surfaces. We perform numerical simulations of hypervelocity asteroid impacts through a hybrid approach consisting of a multi-mechanisms material model implemented in a Material Point Method (MPM) code for capturing the early stages of the impact (first few tens of seconds), followed by a consistent hand-off to an N-body gravity code (pkdgrav) for the late-stage ejection and gravitational reaccumulation of the fragments. We consider in particular the case of spinning target asteroids and explore the resulting change in angular momentum following impact. This approach will help constrain the evolution and structure of the asteroid belts and get a glimpse at the likely nature of the interior structure of asteroids as function of size, composition, and rotational periods – which have direct implications on asteroid hazard mitigation and mining.
14. The influence of vasculature on the onset of impact-induced traumatic brain injury (Madouh).
15. The mechanical behavior of glial cells and implications for models of traumatic brain injury (Dagro).
    The microstructure of human brain tissue is extremely compact, containing virtually no “stromal space” and a mix of neuronal, glial, and vascular cells in all anatomical regions.  Given the complexity of the microstructure, we aim to further our understanding of the mechanical response of cerebral cellular constituents in order to simplify mechanical models at the mesoscale (~10⁰-10² microns). In order to do this, we have developed a system which utilizes optical trapping to probe the mechanical properties of glial cells grown in a 3D scaffold environment at strain rates of 1-5 s^-1.  Glial cells are conventionally characterized as the softer cells which surround and support neurons (as an “isotropic matrix”). We seek to better understand their relative contribution to the mechanical response at the mesoscale. We hope these experiments can help answer this question: how do macroscale brain injury events translate to microscale damage in the cellular networks which comprise the brain?
16. Subject-specific simulations of brain deformations under mild angular acceleration and injurious loading in humans (Lu).
    Physics-based computational head models of the brain can predict brain deformations under mild angular accelerations of the head. The current study focuses on the application of 3D computational brain models for head rotation and head extension. This study is aimed to validate high-fidelity subject-specific 3D head models using the Material Point Method (MPM) to simulate realistic shearing deformations experienced by the brain. Subject-specific 3D head models were constructed from magnetic resonance (MR) images with a uniform spatial resolution of 1x1x1 cubic mm and diffusion tensor imaging scans of the brain. MPM simulation results are compared with measurement of brain deformations obtained from head axial and sagittal rotations inside the MRI scanner. The established approaches will be applied to live human subjects to conduct statistical analysis and to improve biofidelic models of the brain applicable to different loading conditions. Continuing to better understand how the geometry of the brain and its surrounding anatomy affects the biomechanical response to different accelerations will be invaluable in studies of TBI.
17. In situ visualization in a mouse model for traumatic brain injury (Rosen).
18. Multiscale Modeling for Traumatic Brain Injury (Bradfield).
    This project focuses on Traumatic Axonal Injury (TAI) across different length scales. Despite TAI occurring at the cellular level (order of nm-µm), the majority of recent computational modeling efforts solely examine loading at the tissue level (mm-cm). We seek to elucidate the mechanisms behind TAI by developing a multiscale modeling approach to investigate (1) how complex macroscale tissue loads are transferred to axons at the microscale and (2) how different microscale parameters influence axonal injury.
19. Dynamic experimental characterization of indirect traumatic optic neuropathy (Ling).