Research

                                      Whiting School of Engineering, Johns Hopkins University                       

Flaw-dominated dynamic brittle failure under biaxial tension

Effect of surface flaws on dynamic brittle failure

  

RESEARCH PROJECTS

   (A) Deformation failure and damage mechanisms in FCC materials

  • Aluminum has been an attractive material for structural applications because of its high strength to weight ratio. Engineers are trying to understand (all the way to the atomistic detail) on how these materials behave under loads. We come up with analytical and computational models to simulate that behavior, which may then be passed on to engineers and metallurgists.
  • A class of materials that has very fine grain sizes (< 100 nm) are called nanocrystalline (NC) materials. I am particularly interested nanocrystalline metals and nanoceramics. These materials have strengths higher than their coarse grained counterparts. New deformation and failure mechanisms are observed, e.g. extended dislocations and twins haven't been observed in coarse grained aluminum. however, these mechanisms were observed in NC Al under high rate shear experiments.
  • This calls for an updated understanding of deformation mechanisms in these materials, and new analytical models for their use in engineering design and analysis.
  • Molecular Dynamics simulations are being carried to investigate the energy landscape for slip and twinning.  Analytical expressions are derived which will be applicable for partial dislocations based on the non-singular elastic dislocation theory for use in models of discrete twinning dynamics in nanocrystalline f.c.c. materials.

   (B) Kinetics of moving twin boundaries in f.c.c. metals

  • Twin boundary motion is simulated using molecular dynamics. IN collaboration with Dr. Tim Wright, an analytical model is being put together for dynamics of twin boundary motion for application to fcc and hcp materials.

   (C) Modeling damage and failure in aluminum nitride

  • Brittle failure of ceramics under impact is controlled by micro-scale flaws (e.g. pores, inclusions, weak interfaces), and their interactions.
  • Micro-scale flaws in these materials (e.g. Aluminum nitride, and Boron Carbide) are being modeled explicitly within finite element simulations, to understand the macro-scale failure behavour.
  • The brittle failure simulations are regularized by incorporating one or more physical length scales.

 

RESEARCH AS A GRADUATE STUDENT

  1. Modeling compressive behavior in a closed-cell polymer foam.

    MPM simulation of closed-cell polymer foam under confined compression

    2. X-ray radiography of polymer foam in confined compression

    • PMI foam is used as cores in the sandwich structures, particularly by the aerospace industry.
    • We carried in-situ compression experiments under X-Ray tomographic system for direct comparison of the microstructure evolution.
    • Nanoindentation experiments were used to characterize the viscoelastic properties of the cell-wall material.
    • Material Point Method was used to establish and model the compressive behavior, and results were compared with experiments.
    • Simulations will be useful in designing optimum microstructures for improved fuctionality.
    • Collaborators: Dr. Jay Hanan (Oklahoma State Univ.), Dr. Hrishikesh Bale (now at Lawrence Berkeley National Lab.), Dr. Hongbing Lu
  2. Multiscale simulations of dynamic problems in solid mechanics using Material Point Method

    Mode-II crack propagating at super-shear speeds

    • Developed a 3D version of the Material Point Method for solving dynamic problems in solid mechanics.
    • Implemented plasticity and viscoelasticity material models.
    • Implemented cohesive zone model for dynamic fracture
    • Implemented friction and improved contact capabilities.
    • Coupled with Molecular dynamics technique for modeling multiscale failure.
    • Support: Air Force Office of Scientific Research
    • Collaborations: Dr. Ranga Komanduri (Oklahoma State University), Dr. Samit Roy (Univ. of Alabama), Dr. Bo Wang, and Dr. Hongbing Lu
  3. Modeling compressive behavior of silica aerogels for microstructure-property correlation.
    • A novel light-weight nanostructured material with astonishing, thermal, acoustic and strengths to weight properties, with numerous applications for the industries in the transportation, and energy sectors.
    • Support: National Science Foundation
    • Collaboration: Dr. Nicholas Leventis, and Dr. Hongbing Lu
  4. Characterization of mechanical properties of Eardrum using nanoindentation.
    • The overall goal of the project was to develop an improved understanding of the human middle-ear hearing system.
    • An experimental technique was developed to characterize the in-plane and out-of-plane viscoelastic properties of the tympanic membrane using nanoindentation.
    • Support: National Institute of Health, Hough Ear Institute, and National Science Foundation.
    • Collaborators: Dr. Rong Gan (Univ. of Oklahoma), Dr. Hongbing Lu (now at Univ. of Texas), Dr. Chenkai Dai (now at School of Medicine, Johns Hopkins University), and Dr. Huang Gang.