Modeling the Effect of Surface Energy on Stressed Grain Growth in Cubic Polycrystalline Bodies

A recently-developed constitutive theory of stressed grain growth is augmented to include the effect of excess surface energy via a surface effect state variable. The new constitutive theory is implemented into a coupled finite-element and phase-field computational framework. Through three-dimensional simulations, the new constitutive model is shown to be capable of predicting the experimental data of the annealing-induced texture transition in polycrystalline copper thin films of different thicknesses attached to a polyimide substrate. Our simulations show that the grain growth driving force arising from the through-film thickness grain boundary curvature plays a prominent role in such a transitional behavior

Variant Reorientation in Single-crystal Shape-memory Alloys

In this work we model the variant reorientation in a single crystal NiMnGa magnetic shapememory alloy using the crystal-mechanics-based constitutive model of Thamburaja[1]. The model has been implemented in the ABAQUS/Explicit finite-element program by writing a user-material subroutine. Its numerical simulations quantitatively predict the mechanical response in simple compression and plain strain compression experiments to good accord

Constitutive equations for superelasticity in crystalline shape-memory materials

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2002.Includes bibliographical references (leaves 118-122).A crystal-mechanics-based constitutive model for polycrystalline shape-memory materials has been developed. The model has been implemented in a finite-element program. Finite-element calculations of polycrystal response were performed using two methods: (1) The full-finite element method where each element represents a single crystal chosen from a set of crystal orientations which approximate the initial crystallographic texture; (2) A simplified model using the Taylor assumption (1938) where each element represents a collection of single crystals at a material point. The macroscopic stress-strain responses are calculated as volume averages over the entire aggregate. A variety of superelastic experiments were performed on initially-textured Ti-Ni rods and sheets. The predicted stress-strain curves from finite-element calculations are shown to be in good accord with the corresponding experiments. For the Ti-Ni sheet, strain-temperature response at a fixed stress was also experimentally studied. The model was also shown to accurately predict the results from these important experiments. Further, by performing superelastic experiments at moderately high strain rates, the effects of self-heating and cooling due to the phase transformations are shown to be captured well by the constitutive model. The thermo-mechanically-coupled theory is also able to capture the resulting inhomogeneous deformations associated with the nucleation and propagation of transformation fronts. Finally, an isotropic constitutive model has also been developed and implemented in a finite-element program. This simple model provides a reasonably accurate and computationally-inexpensive tool for purposes of engineering design.Prakash Thamburaja.Ph.D

Anisotropic Superelasticity of Textured Ti-Ni Sheet

A recently developed crystal-mechanics-based constitutive model for polycrystalline shape-memory alloys (Thamburaja and Anand [1]) is shown to quantitatively predict the in-plane anisotropy of superelastic sheet Ti-Ni to reasonable accord.Singapore-MIT Alliance (SMA

Entropy-Based Approach for Fatigue Crack Growth Rate of Dual-Phase Steel

This paper presents an entropy-based approach for the fatigue crack growth of dual-phase steel under a constant amplitude loading. According to the degradation-entropy generation theorem, the degradation coefficient can be derived from the correlations of entropy and crack propagation.  The temperature evolutions induced for the duration of the fatigue crack growth tests on the as-received and dual-phase steel till it failed were measured to ensure their validity. The results of the present model and the calculated Paris-regime crack growth data were analysed to reach the conclusion that the temperature at the surface of a specimen during a fatigue crack growth test can be used for the assessment of fatigue crack growth by the intensity of the degradation coefficient. The predicted results showed that the present model could accurately predict the fatigue crack growth rate of dual-phase steel with a regression value (R2) of 0.9952

A generalized thermophysical model for materials from molecular clusters to bulk crystals

Evolution of properties of bulk materials from its molecular clusters has been a relatively unexplored topic. In this article, we present a generalized model for total thermal energy and heat capacity of solids by considering that a bulk solid is evolved from zero-dimensional molecular clusters through one-dimensional wires and two-dimensionalquantum sheets. The basic difference among these structures is shown to be the phonon density of states, which is continuous (unconfined) in a bulk solid, whereas it is confined in one, two, and three dimensions in QSs, QWs, and QDs, respectively. Considering a semi-empirically derived phonon density of states and the total unconfined and quantum confined dimensions in these structures, we arrived at a generalized equation. The generalized equation fits well to many experimental heat capacities including palladium and nickel nanoparticles, single-walled carbon nanotubes, and cadmium selenide quantum dots with high degree of accuracy (R2 > 0.99). This close agreement between experiments and the generalized equation derived herewith provide promising directions to understand the evolution of bulk properties of materials

An Investigation Of Modal Analysis For Al6061 Between Piezoelectric Film Sensor And Accelerometer

An experiment was conducted to determine modal parameters such as natural frequencies and mode shapes of aluminum 6061 (Al6061). A free dynamic vibration analysis was conducted to obtain the parameters. Al6061 was chosen as the experiment component mainly because of its wide application in automotive industries. Theoretically, if the component vibrates and produce frequency coherence with the natural frequency, resonance frequency will occur which can lead to structural failure. Modal analysis study was conducted by using both simulation and experimental method to compare their outcome. Simulation was conducted via ANSYS software while impact hammer testing was done for experimental work to determine the vibration parameter. Piezoelectric film and accelerometer were used as the sensor. The result obtained from simulation showed that frequencies for mode shape 1, 2 and 3 for circle shape were 134.60Hz, 324.73Hz and 727.52Hz. The result obtained from accelerometer showed that frequencies for mode shape 1, 2 and 3 for circle shape were 158.67Hz, 421.33Hz and 625.00Hz. Finally, the result captured from piezoelectric film sensor appeared that frequencies for mode shape 1, 2 and 3 for circle shape were 141.00Hz, 321.00Hz and 504.33Hz. There was a good results agreement between simulation and experimental work outcome

An Approach to Elastoplasticity at Large Deformations

Finite plasticity theories are still a subject of controversy and lively discussions. Among the approaches to finite elastoplasticity two became especially popular. The first, implemented in the commercial finite element codes, is based on the introduction of a hypoelastic constitutive law and the additive elastic-plastic decomposition of the deformation rate tensor. Unfortunately, the use of hypoelasticity may lead to a nonphysical creation or dissipation of energy in a closed deformation cycle. In order to replace hypoelasticity with hyperelasticity the second popular approach based on the multiplicative elastic-plastic decomposition of the deformation gradient tensor was developed. Unluckily, the latter theory is not perfect as well because it introduces intermediate plastic configurations, which are geometrically incompatible, non-unique, and, consequently, fictitious physically. In the present work, an attempt is made to combine strengths of the described approaches avoiding their drawbacks. Particularly, a tensor of the plastic deformation rate is introduced in the additive elastic-plastic decomposition of the velocity gradient. This tensor is used in the flow rule defined by the generalized isotropic Reiner-Rivlin fluid. The tensor of the plastic deformation rate is also used in an evolution equation that allows calculating an elastic strain tensor which, in its turn, is used in the hyperelastic constitutive law. Thus, the present approach employs hyperelasticity and the additive decomposition of the velocity gradient avoiding nonphysical hypoelasticity and the multiplicative decomposition of the deformation gradient associated with incompatible plastic configurations. The developed finite elastoplasticity framework for isotropic materials is specified to extend the classical -theory of metal plasticity to large deformations and the simple shear deformation is analyzed

Thermomechanical couplings in shape memory alloy materials

In this work we address several theoretical and computational issues which are related to the thermomechanical modeling of shape memory alloy materials. More specifically, in this paper we revisit a non-isothermal version of the theory of large deformation generalized plasticity which is suitable for describing the multiple and complex mechanisms occurring in these materials during phase transformations. We also discuss the computational implementation of a generalized plasticity based constitutive model and we demonstrate the ability of the theory in simulating the basic patterns of the experimentally observed behavior by a set of representative numerical examples

Nanometallic Glasses: Size Reduction Brings Ductility, Surface State Drives Its Extent

We report tensile experiments on Ni80P20 metallic glass samples fabricated via a templated electroplating process and via focused ion beam milling, which differed only in their surface energy states: Ga-ion-irradiated and as-electroplated. Molecular dynamics simulations on similar Ni80Al20 systems corroborate the experimental results, which suggest that the transition from brittle to ductile behavior is driven by sample size, while the extent of ductility is driven by surface state