EDUCATION
TEACHING
RESEARCH
PROJECTS
PUBLICATIONS
SERVICE
LANGUAGE
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EDUCATION:
Ph.D., Mechanics, 1994, University of Minnesota
B.Tech.(Honors), 1988, Mechanical Engineering, Indian Institute of Technology, Kharagpur, India
TEACHING:
1988-1994 (various periods), Teaching Assistant, U. of Minnesota
1997-present, Assistant Professor, University of Miami
RESEARCH:
My goal is to understand and optimize the macroscopic behavior of materials with microstructure such as shape memory alloys (SMA), zirconia toughened Ceramics (ZTC) and soft tissues. In order to model the behavior of these materials properly, it is often necessary to extract relevant information from various length scales. For instance, the atomic displacements during a phase transformation provide the transformation strains for a continuum model for SMA, which in turn predicts the twinned microstructure. I am interested both in the analysis at the scale of the microstructure and in developing polycrystaline and composite models. By understanding the processes that operate in the deformation and damage of these materials, I hope to provide guidelines for the fabrication of new materials. The phenomena involved are wideranging (e.g. surface-diffusion in thin films and phase transformations in SMA) and often violate some of the basic assumptions of classical mechanics (convex free energy); in addition, these materials are often used in configurations, such as thin films, where the mechanics is not completely understood. Many model problems, to gain insight into the mechanics of these materials, do not have analytical solutions, and hence I hope to use both computational and experimental techniques.
PROJECTS:
FATIGUE OF Ni-Ti SHAPE MEMORY ALLOYS.
Biomedical applications of Nitinol such as Aortic and coronary Stents have to meet FDA requirements of a working life of 10 years (about 400 million pulsatile cycles). Although existing stents last about 400 million cycles under accelerated device testing many fundamental questions about the fatigue of Nitinol remain. The effect of mean cycling stress on fatigue life and multiaxial fatigue behavior need to be examined. The stress-strain curve corresponding to the first cycle is drastically different from the curve corresponding to the 100th cycle. This phenomena makes it hard to design reliable actuators. Systematic experiments are underway not only to obtain design data and develop design tools, but also to understand the basic issues such as the effects of dislocation, twinning, texture and heat-treatment. Lastly, constitutive models that account for polycrystalline texture are being developed for stress analysis of Nitinol structures and components.
APPLICATIONS OF CONFIGURATIONAL FORCES TO SOME PROBLEMS IN MECHANICS
Recently Gurtin (1995) has proposed the concept of configurational forces, which is a powerful technique to study various phenomena such as phase boundary propagation, crystal and grain growth, growth of thin films as well as crack propogation. This approach was used succesfully in my work on phase boundary propagation (Simha and Bhattacharya, 1998). Currently I am using this approach to study the following:
(a) Crack propagation in Zirconia-toughened-ceramics (ZTC) and Shape memory alloys (SMA).
Path independence of the J-integral in fracture mechanics applies in a non-linear, homogeneous material in the absence of surfaces of strain-discontinuities. In a pioneering work, Budiansky et al. (1983) extended the J-integral approach to examine sub-critical ZTC where there are no phase-boundaries. However, in super-critical ZTC and SMA phase-boundaries are inevitable and the dissipation due to their propagation contributes to toughening.
(b) Kinetics of Singular lines in three dimensions.
Based on my work on propagating phase boundaries in two dimensions, I am studying the kinetics of a singular line formed by intersecting surfaces of discontunities. Since the geometry of surfaces is a non-trivial extension of the geometry of curves, the entire kinematics will be rederived. Applications of this to crack-propagation and dislocations in three-dimensions will be examined.
(c) Growth and Remodeling of collagen films.
The evolution of residual stress in growing soft tissues is reasonably understood. However, the mechanical and chemical driving forces for growth and remodeling of tissues has not been examined. In this exploratory work, Prof. M. Sacks of the Biomedical Department, UM, and I are examining the effects of stress on growth of collagen films.
CURRENT EXPERIMENTAL RESEARCH
The Mechanics and Materials laboratory at UM has various testing facilities and equipment such as polishing wheels, furnaces, an optical microscope (up to 500X) with a Panasonic CCT Camera and Sony TV, a MTS 810 material testing machine (up to 20,000 lbf) and two PCs for digital control and data acquisition.
The following design projects are in progress:
(a) Tension-Torsion Testing Machine. As part of an undergraduate design project a combined torsion-tension device is being fabricated at the Mechanics and Materials Laboratory. Static forces and torques are applied by using pulleys and weights, while the linear extension and twist are measured at the grips. This will allow for measurements of shear-stress/shear-strain curves with superimposed tensile loading. Since the rotation is not restricted, longer specimens can be tested on this set-up.
(b) Electrical resistivity measuring device A simple way to characterize the four martensitic transformation temperatures of shape memory alloys is by measuring resistivity as a function of temperature. A tall open Dewar flask with dry ice at the bottom will have temperatures ranging from room temperature at the top to dry ice temperature at the bottom. The temperature of small specimens will be controlled essentially by varying their height inside the container by using a stepper motor. A simple electrical circuit measures the resistivity, while thermocouples will be used to measure specimen temperature. This device can also be used to assess heat treatments and recovery following cold working.
The following research projects are currently in progress:
Mean Stress Effects on Fatigue of NiTi
Motivated by the loading situation relevant to stents, a graduate student Mr. Tabanli, Dr. Berg, Scimed Inc. and I have recently started examining mean stress effects on the S-N curves for the pseudoelastic behavior of NiTi. Mean stress will be chosen such that the specimens contain material in the austenite phase, martensite phase or in both phases. This project will provide design tools such as Goodman or Soderberg diagrams.
Kinetic contact angles.
Propagating phase boundaries play an important role in various situations such as coherent precipitation, martensitic transformation, recrystallization and grain growth. Although isolated phase boundaries have been extensively studied, the kinetics of junctions, where two or more phase boundaries meet, and edges, where a phase boundary meets a physical boundary, have only recently been examined. Using an analysis based on the second law of thermodynamics, Simha and Bhattacharya (1998) have shown that a constitutive relation for the edge velocity needs to be specified in addition to a constitutive relation for the normal velocity of the interface. For instance, the edge kinetic relation can relate the edge velocity to the contact angle, while the interface kinetic relation can relate the normal velocity to the curvature. Existing experiments do not appreciate the need for two different kinetic relations. The commonly used relation due to Hoffmann measured the edge angle as a function of normal interface velocity, which is not relevant to the edge. Using the simplified setting of a spreading liquid drop on a substrate, I propose to measure appropriate edge and interface kinetic relations.
Effect of Texture on the shape memory effect in NiTi.
When a shape memory alloy is subjected to a large deformation, it deforms in a plastic manner. Upon heating, the alloy can recover most of this apparent plastic deformation. CuAlNi and NiTi are two examples of shape memory alloys. In single crystal form CuAlNi recovers a strain of about 2-9% whereas polycrystals recover only about 2%; single crystal NiTi recovers about 3-10% strain while polycrystals recover about 5-8%. Shu and Bhattacharya (1998) have shown that polycrystalline texture plays an important role in determining the recoverable strain; their analysis predicts lower and upper bounds on the recoverable strain using energy minimization and composite materials theory. Experiments are needed to confirm the theoretical predictions. Moreover, there is a considerable difference between the lower and upper bounds, for instance in combined tension-torsion of thin tubes; hence systematic experiments to search for textures that will be closer to the upper bound are necessary to optimize the recoverable strain in polycrystals.
Fracture of NiTi.
The controlled temperature of the human body offers an excellent environment for applications of shape memory alloys such as Nitinol. To date coronary and aortic stents as well as various orthopedic devices have been fabricated using Nitinol. However, the full potential of Nitinol has not been realized mainly because of insufficient design data. Fundamental design parameters such as the critical fracture toughness have not been measured for Nitinol; on the other hand such data are available in handbooks for the common steels. Hence, I propose experiments to measure the critical tear energy for crack propagation in Nitinol based on the classical work of Rivlin for rubber. Since processing conditions such as heat-treatment and cold working drastically change the thermomechanical properties of Nitinol, it is expected that they will also affect crack propagation. Consequently the proposed experiments will also examine these issues. From a more basic scientific viewpoint, the phase transformation in Nitinol makes crack propagation very interesting. For instance, following the analyses of ZTC (Simha and Truskinovsky, 1994), one can expect toughening due to martensitic transformations in the vicinity of the crack tip in Nitinol. This will result in a R-curve behavior and the steady state critical tear energy will depend on the polycrystalline texture.
Mechanics of Thin film NiTi.
Among all actuator materials NiTi performs the most work per cycle per unit mass. Since NiTi is activated by heat-transfer, cycling is typically low in bulk materials. However, in the thin film configuration, cycling can be considerably increased. I plan to study the thermo-mechanical response, crack propagation as well as fatigue properties of thin films.
PUBLICATIONS:
Simha, N. K. & Truskinovsky, L., Shear induced transformation toughening in ceramics, Acta Metallurgica et Materialia, 42, pp. 3827, 1994
Simha, N. K., Twin and habit plane microstructures due to the tetragonal to monoclinic transformation of zirconia, Journal of the Mechanics and Physics of Solids, 45, pp. 261-292, 1997
Simha, N. K. & Bhattacharya, K., Equilibrium conditions at corners and edges of an interface in a multiphase solid. Materials Science and Engineering A, 238, pp. 32-41, 1997
Simha, N. K., Fedewa, M., Leo, P.H., Lewis, J., and Oegema, T., Correlations between the elastic properties and collagen content of chondrocyte culture tissue, (submitted to Journal of Biomechanics), 1997
Simha, N. K. & Bhattacharya, K., Kinetics of phase boundaries with edges and junctions. Journal of the Mechanics and Physics of Solids (in print) 1998
SERVICE:
Coordinator for the Mechanical Engineering Graduate Research Seminar Series
LANGUAGE:
Fluent in English, German, Hindi and Kannada
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