Srinivasan M. Sivakumar

Bio: Srinivasan M. Sivakumar is an academic researcher from Indian Institute of Technology Madras. The author has contributed to research in topics: Shape-memory alloy & Finite element method. The author has an hindex of 10, co-authored 43 publications receiving 278 citations. Previous affiliations of Srinivasan M. Sivakumar include Louisiana State University.

Domain switching criteria for ferroelectrics

Abstract: Domains, which exist in ferroelectric ceramics, on the application of external loads, such as electric fields and stress, undergo reorientation known as domain switching. This domain switching results in nonlinear behavior of ferroelectric ceramics. In this work, a domain switching criterion applicable to a generalized electromechanical loading is proposed based on an assessment of some of the existing microscopic energetically motivated domain switching criteria for ferroelectrics. The criterion is based on an instability condition of the current domain state determined by a critical value of the input energy calculated with respect to the state of switch-worthiness. The switch-worthiness is a state at which the Gibbs free energy of the current variant of the domain is no longer the lowest amongst that of all the variants. The criterion highlights the importance of the loading sequence in response evaluation, which has rarely been given any importance in literature so far. In obtaining the macroscopic response, it is assumed that the material is composed of several domains, each of which is defined by its local coordinate relative to a fixed global coordinate. For simplicity, Reuss average has been used to obtain macroscopic material response. The model is able to capture the essential features and trends of experimental results found in literature. The results obtained from the proposed model, other energetically motivated models and experiments have been compared, highlighting the important features of the proposed model.

Two models to simulate rate-dependent domain switching effects - application to ferroelastic polycrystalline ceramics

Abstract: The aim of this paper is to study rate-dependent switching in ferroelastic materials. More specifically, a micro-mechanically motivated model is embedded into an iterative three-dimensional and electromechanically coupled finite element framework. An established energy-based criterion serves for the initiation of domain switching processes as based on reduction in (local) Gibbs free energy. Subsequent nucleation and propagation of domain walls is captured via a linear kinetics theory with rate-dependent effects being incorporated in terms of a deformation-dependent limit-time-parameter. With this basic model in hand, two different switching formulations are elaborated in this work: on the one hand, a straightforward volume-fraction-ansatz is applied with the volume-fraction-value depending on the limit-time-parameter; on the other hand, a reorientation-transformation-formulation is proposed, whereby the orientation tensor itself is assumed to depend on the limit-time-parameter. Macroscopic behaviour such as stress versus strains curves or stress versus electrical displacements graphs are obtained by applying straightforward volume-averaging-techniques to the three-dimensional finite-element-based simulation results which provides important insights into the rate-dependent response of the investigated ferroelastic materials. (Less)

Microstructure evolution in the equal channel angular extrusion process

Abstract: We apply a theory of single-crystal plasticity with microstructure to the simulation of the ECAE process. The specific microstructures considered in the simulations are of the sequential lamination type. The size of the microstructure is estimated a posteriori by means of a nonlocal extension of the theory which accounts for dislocation energies. Texture evolution is calculated simply by recourse to Taylor’s hypothesis. Calculations concerned with an FCC material (Al–Cu alloy) and 90° ECAE reveal a wealth of information regarding the geometry, size, and texture evolution of subgrain microstructures. The predicted sizes and textures are in good quantitative agreement with the available experimental data.

A combined r-h adaptive strategy based on material forces and error assessment for plane problems and bimaterial interfaces

Abstract: An r-h adaptive scheme has been proposed and formulated for analysis of bimaterial interface problems using adaptive finite element method. It involves a combination of the configurational force based r-adaption with weighted laplacian smoothing and mesh enrichment by h-refinement. The Configurational driving force is evaluated by considering the weak form of the material force balance for bimaterial inerface problems. These forces assembled at nodes act as an indicator for r-adaption. A weighted laplacian smoothing is performed for smoothing the mesh. The h-adaptive strategy is based on a modifed weighted energy norm of error evaluated using supercovergent estimators. The proposed method applies specific non sliding interface strain compatibility requirements across inter material boundaries consistent with physical principles to obtain modified error estimators. The best sequence of combining r- and h-adaption has been evolved from numerical study. The study confirms that the proposed combined r-h adaption is more efficient than a purely h-adaptive approach and more flexible than a purely r-adaptive approach with better convergence characteristics and helps in obtaining optimal finite element meshes for a specified accuracy.

A new failure criterion for materials exhibiting ratcheting during very low cycle fatigue

Abstract: Ratcheting or cyclic creep is the phenomenon of progressive accumulation of permanent deformation when a component is subjected to cyclic loads in the plastic strain range under stress controlled fatigue with non-zero mean stresses This accumulation of plastic strain will finally lead to a shakedown, or a constant rate of ratcheting or very large ratcheting strains leading to failure of the material This latter situation is encountered during the very low cycle fatigue (VLCF) of components when the stress levels are close to the ultimate strength and the component fails in less than 100 cycles Large strains are accumulated leading to plastic instability/necking and final fracture A failure criterion based on the initiation of plastic instability has been proposed in this work, for the life prediction of materials exhibiting ratcheting in the VLCF regime The proposed model is supported by experimental observations made from the stress controlled VLCF tests with tensile mean stresses on smooth cylindrical mild steel specimens

Texture evolution in equal-channel angular extrusion

Abstract: The focus of this article is texture development in metals of fcc, bcc, and hcp crystal structure processed by a severe plastic deformation (SPD) technique called equal-channel angular extrusion (ECAE) or equal-channel angular pressing (ECAP). The ECAE process involves very large plastic strains and is well known for its ability to refine the grain size of a polycrystalline metal to submicron or even nano-size lengthscales depending on the material. During this process, the texture also changes substantially. While the strength, microstructure and formability of ECAE-deformed metals have received much attention, texture evolution and its connection with these properties have not. In this article, we cover a multitude of factors that can influence texture evolution, such as applied strain path, die geometry, processing conditions, deformation inhomogeneities, accumulated strain, crystal structure, material plastic behavior, initial texture, dynamic recrystallization, substructure, and deformation twinning. We evaluate current constitutive models for texture evolution based on the physics they include and their agreement with measurements. Last, we discuss the influence of texture on post-processed mechanical response, plastic anisotropy, and grain refinement, properties which have made ECAE, as well as other SPD processes, attractive. It is our intent to make SPD researchers aware of the importance of texture development in SPD and provide the background, guidance, and methodologies necessary for incorporating texture analyses in their studies.

Mechanical properties of unidirectional nanocomposites with non-uniformly or randomly staggered platelet distribution

Abstract: Unidirectional nanocomposite structures with parallel staggered platelet reinforcements are widely observed in natural biological materials. The present paper is aimed at an investigation of the stiffness, strength, failure strain and energy storage capacity of a unidirectional nanocomposite with non-uniformly or randomly staggered platelet distribution. Our study indicates that, besides the volume fraction, shape, and orientation of the platelets, their distribution also plays a significant role in the mechanical properties of a unidirectional nanocomposite, which can be quantitatively characterized in terms of four dimensionless parameters associated with platelet distribution. It is found that, compared with other distributions, stairwise and regular staggering of platelets produce overall the most balanced mechanical properties, which might be a key reason why these structures are most widely observed in nature.

Integrating strong and weak discontinuities without integration subcells and example applications in an XFEM/GFEM framework

Abstract: Partition of unity methods, such as the extended finite element method, allows discontinuities to be simulated independently of the mesh (Int. J. Numer. Meth. Engng. 1999; 45:601-620). This eliminates the need for the mesh to be aligned with the discontinuity or cumbersome re-meshing, as the discontinuity evolves. However, to compute the stiffness matrix of the elements intersected by the discontinuity, a subdivision of the elements into quadrature subcells aligned with the discontinuity is commonly adopted. In this paper, we use a simple integration technique, proposed for polygonal domains (Int. J. Nuttier Meth. Engng 2009; 80(1):103-134. DOI: 10.1002/nme.2589) to suppress the need for element subdivision. Numerical results presented for a few benchmark problems in the context of linear elastic fracture mechanics and a multi-material problem show that the proposed method yields accurate results. Owing to its simplicity, the proposed integration technique can be easily integrated in any existing code. Copyright (C) 2010 John Wiley & Sons, Ltd.

Design and development of a new piezoelectric linear Inchworm actuator

Abstract: This paper presents a proof-of-concept design of an inchworm-type piezoelectric actuator of large displacement and force (or power) for shape control and vibration control of adaptive truss structures. Applications for such actuators include smart or adaptive structural systems, auto and aerospace industries. The proposed inchworm-type actuator consists of three main components with frictional clamping mechanisms: two clamping or braking devices and one expanding device. The two frictional clamping devices provide alternating braking forces when the moving shaft, which is pushed by expanding device, walks inside the PZT tubular stack and emulates an inchworm, summing small steps to achieve large displacements. Since the development of a robust clamping mechanism is essential to realize the high force capability, a considerable design effort has been focused on optimizing the clamping device to increase the output force. CATIA is used as a platform to model the whole actuator and ANSYS is used to analyze and optimize the performance of the actuator. The proposed design avoids the tight tolerance of the tube diameters and reduces the clearance between clamps and the moving shaft with the adjustment device. The moving shaft of the actuator could also be replaced by one member of a truss structure for vibration suppression and position control purposes. In the proposed actuator the flexure clamps can also be easily replaced to outfit different dynamic characteristics. The complete design of the proposed actuator has been performed using the finite element analysis. The simulation result confirms that the output force of 160 N and incremental displacement in each step of 8.3 μm can be achieved using the proposed actuator. A prototype of actuator has been fabricated and static tests have been performed to validate the simulation results.

Rietveld refinement, microstructure, conductivity and impedance properties of Ba[Zr0.25Ti0.75]O3 ceramic

Abstract: In this work, we report the Rietveld refinement, microstructure, conductivity and impedance properties of Ba[Zr0.25Ti0.75]O3 ceramic synthesized by solid state reaction. This ceramic was characterized by X-ray diffraction, Rietveld refinement, scanning electron microscopy and energy dispersive X-ray spectrometry. Impedance spectroscopy analyses reveals a non-Debye relaxation phenomenon being its relaxation frequency moving toward to positive side with increase of temperature. A significant shift in impedance loss peaks toward higher frequency side indicates conduction in material and favoring the long range motion of mobile charge carriers. The frequency dependent ac conductivity at different temperatures indicates that the conduction process is thermally activated. The variation of dc conductivity exhibited a negative temperature coefficient of resistance behavior. The ac conductivity data are used to evaluate the density of states at Fermi level and activation energy of this ceramic. The dc electrical and thermal conductivities of grain and grain boundary have been discussed.