IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

It currently offers undergraduate, postgraduate and research degrees across 16 disciplines in Engineering, Sciences, Humanities and Management. About 596 faculty belonging to science and engineering departments and centres of the Institute are engaged in teaching, research and industrial consultancy.

IIT Madras

Polyvinylidene fluoride (PVDF), a piezoelectric material has many useful applications like sensors, transducers and surface acoustic wave devices. Since PVDF is a polymer, it is possible that its mechanical response is likely to be frequency and time dependent. It is important therefore, to characterize the frequency or time dependent behavior of PVDF using appropriate models that are based on experimental observations. Dynamical Mechanical Tests were conducted on bi-axially stretched PVDF at different temperatures (35°C-70°C) and frequencies (0.33Hz-10Hz). It was observed from the experimental results that at higher temperatures, the rate of change of modulus with frequency reduces. Time-temperature superposition was done for the storage and the loss modulii to arrive at a single master curve at a temperature of 35°C. The results obtained by the superposition were then modeled using a simple viscoelastic three parameter model with ordinary dashpot. This model predicted the storage modulus well but was found short of the required predictive capability for the loss modulus. A four parameter model with fractional dashpot was used and it is found that it models the loss and storage modulus results better than the model with ordinary dashpot. © 2008 COMADEM International.

Department of Applied Mechanics

International Journal of COMADEM

Modeling of the viscoelastic properties of PVDF through the fractional differential model

The ultrasonic Time-of-Flight Diffraction (TOFD) technique is a well-known technique for defect sizing. This technique has been applied to thick sections (>15 mm). The application of the TOFD technique to thin sections like pressure vessels and piping requires simulation of this technique. Simulation gives ideas about the expected results from experiments where real experiments are not possible or are difficult to conduct. Further, simulation helps us to derive the optimum choice of experimental parameters. This paper discusses the application of the finite element technique to simulate the ultrasonic time-of-flight diffraction (TOFD) technique. The diffracted and reflected signals in TOFD techniques for vertical and inclined defects were simulated using plane strain elements. The simulated results are compared with the experimental observations. FEM simulation of wave propagation in complex joints such as a T-joint with embedded flaws is also discussed.

Department of Mechanical Engineering

Insight: Non-Destructive Testing and Condition Monitoring

Simulation of the TOFD technique using the finite element method

The main objective of this study is to introduce a 'dissipationless band' to model inner hysteresis loops of response of shape memory alloys (SMAs). Dissipation that occurs when the material undergoes phase transformation is critical to the modeling of hysteretic behavior. Emphasis is placed on modeling such dissipation in the proposed methodology. Using a dissipationless virtual response of the material, a logical framework for the onset transformation under reversal of cycles is presented. Characteristics of the material transformation with reference to a dissipationless band model the true inner hysteresis loops. It is identified that this dissipationless band occurs due to the difference between the starting states of forward and reverse transformations. The construction of the generalized driving force for the transformation along with the rate of dissipation function is formulated. A numerical example is presented to highlight the qualitative prediction capabilities of the model. The example involves simulating hysteresis loops for different kinds of partial and complete loading cycles in the pseudoelastic state of the material. The predictions show that the proposed one-dimensional model is capable of representing the actual hysteresis behavior of the stabilized shape memory alloys, by effectively incorporating the dissipation effects due to the loading history. © IOP Publishing Ltd.

GENERAL

Smart Materials and Structures

Thermomechanical modeling of hysteresis in SMAs using the dissipationless reference response

The fundamental solutions in axisymmetric elastostatic boundary element method (BEM) are more complicated compared to those in general three dimensional elastostatic BEM from which they are derived. Evaluating the integrals of these fundamental solutions over singular elements is a tedious process. Both the standard Gaussian quadrature as well as a specialized quadrature technique, capable of integrating logarithmically singular functions, need to be used in the computation. When plotted over singular elements however, the integrals corresponding to the off-diagonal components of the axisymmetric displacement fundamental solution are found to be non-singular. Exploiting this new finding, much of the extra computational effort in integrating them over singular elements is avoided by simply using the standard Gaussian quadrature technique. This modification does not cause any loss of accuracy; it yields results identical to those obtained conventionally. In addition, it also results in dramatic savings in the time spent in evaluating the boundary integrals over singular elements. The modified implementation is termed "semi-singular BEM for axisymmetric elasticity," since exactly two out of the four components of the fundamental solution are found to be non-singular. Results obtained by solving numerical problems using the semi-singular and conventional BEM are presented and compared, which validate these claims.

Journal of Structural Engineering (Madras)

Semi-singular BEM in axisymmetric elastostatics

Ultrasonic time of flight diffraction (TOFD) for sizing defects is based on the time of flight of the diffracted echo that is generated when a longitudinal wave is incident on a crack tip. This technique has the limitation during near-surface inspection due to signal superposition. Here, this limitation is overcome by using the shear wave-diffracted signal (instead of longitudinal wave) and hence called S-TOFD. Experiments were conducted on samples with defect tip closer to the surface of a flat plate sample to illustrate the utility of the S-TOFD technique. An increase in the flaw sizing accuracy, by using the shear wave-diffracted echoes from the tip and through the application of a signal processing technique (ESIT), was demonstrated. © 2006 Elsevier Ltd. All rights reserved.

NDT and E International

Shear-wave time of flight diffraction (S-TOFD) technique

This paper presents an enriched meshless method based on an improved moving least-square approximation (IMLS) method for fracture analysis of cracks in homogeneous, isotropic, linear-elastic, two-dimensional bimaterial solids, subject to mixed-mode loading conditions. The method involves an element-free Galerkin formulation in conjunction with IMLS and a new enriched basis functions to capture the singularity field in linear-elastic bi-material fracture mechanics. In the IMLS method, the orthogonal function system with a weight function is used as the basis function. The IMLS has higher computational efficiency and precision than the MLS, and will not lead to an ill-conditioned system of equations. The proposed enriched basis function can be viewed as a generalized enriched basis function, which degenerates to a linear-elastic basis function when the bimaterial constant is zero. Numerical examples are presented to illustrate the computational efficiency and accuracy of the proposed method. Copyright © 2006 by ASME.

Department Of Civil Engineering

American Society of Mechanical Engineers, Pressure Vessels and Piping Division (Publication) PVP

An efficient meshfree method for fracture analysis of cracks in BI-materials

It is necessary to size the cracklike defects accurately in order to extend the life of thin-walled (< 10 mm) components (such as pressure vessels) particularly for aerospace applications. This paper discusses the successful application of ray techniques to simulate the ultrasonic time-of-flight diffraction experiments for platelike structures. For the simulation, the diffraction coefficients are computed using the geometric diffraction theory. The A and B scans are simulated in near real time and the different experimental parameters can be interactively controlled due to the computational efficiency of the ray technique. The simulated results are applied to (1) defect signal identification for vertical defects, (2) inspection of inclined defects, and (3) study the effect of pulse width or probe frequency on experimental results. The simulated results are compared with laboratory scale experimental results. Copyright © 2005 by ASME.

Department Of Physics

Journal of Pressure Vessel Technology, Transactions of the ASME

Ray based model for the ultrasonic time-of-flight diffraction simulation of thin walled structure inspection

It is difficult to accurately size the defects that are oriented at an angle (that is not normal to the wave) using conventional amplitude based ultrasonic techniques. Since Time of Flight Diffraction (TOFD) is based on the diffraction of ultrasound at defect edges, defect sizing using this technique is amplitude independent. However, most of the TOFD based assessment relies on manual sizing, whose accuracy depends on quality of image and the operator's experience. Also, the utilization of TOFD for sections less than 15 mm has reportedly several difficulties. In this paper, we report our attempts to size the vertical and inclined defects using an in-house TOFD system built to inspect thin sections (6-10 mm). To improve sizing, automated defect sizing techniques termed Embedded Signal Identification Technique (ESIT) and Point Source Correlation Technique (PSCT) were developed. A ray tracing based model was also developed for a) optimizing the experimental parameters for thin sections, b) interpreting the received signals. Experiments were conducted on 10 mm thick samples with EDM defects and 6-7 mm welded maraging steel samples. The results obtained using manual and automated techniques were compared. Our comparisons lead us to believe that the automated defect sizing techniques can provide accurate and reliable results for thin sections. © 2005 American Institute of Physics.

AIP Conference Proceedings

Application of TOFD technique to thin sections using ESIT and PSCT

Experiments were carried out on Form I uniaxially and biaxially stretched Polyvinylidene Fluoride (PVDF) samples to characterize their electromechanical response. The piezoelectric constants were measured at different values of superimposed strains or loads and at different values of frequencies of dynamic strains, which were applied in the longitudinal and transverse direction. It was also found through experiments that, the piezoelectric constants of PVDF are affected by the frequency of loading and there seems to be some dependence of the phase difference between the electrical and mechanical response on the superimposed strains. The tests also confirm to some extent, the results obtained by other researchers, that the piezoelectric constants increase in the longitudinal direction with increasing superimposed strains and decrease in the transverse direction for uniaxially stretched PVDF.

Ferroelectrics

Electromechanical behavior of Form I uniaxially and biaxially stretched PVDF

Numerical simulation of wave propagation and its interaction with defects in a long range (1 m) thin sections (10 mm) using the regular finite element method is very costly, since it requires very fine meshes to accurately capture response signals. Spectral element method (SEM) is a numerical technique that solves the dynamic problem in frequency domain. SEM has been applied for detection of vertical and horizontal defects. This paper discusses application of SEM to any arbitrarily orientated crack in long range pressure vessels. The simulation of applications of Lamb wave for detection and sizing using TOFD technique is demonstrated. The advantage of the current formulation is quick detection of defects in thin bodies of any orientation using Lamb waves and sizing the same using TOFD techniques are illustrated. Copyright © 2005 by ASME.

Simulation of wave interaction with arbitarily oriented defects in thin sections using spectral element method

The flow of granular materials between rotating cylinders is studied using a continuum model proposed by Rajagopal and Massoudi (A method for measuring material moduli for granular materials: flow in an orthogonal rheometer, DOE/PETC/TR90/3, 1990). For a steady, fully developed condition, the governing equations are reduced to a system of coupled non-linear ordinary differential equations. The resulting boundary value problem is non-dimensionalized and is then solved numerically. The effect of material parameters, i.e., dimensionless numbers on the volume fraction and the velocity fields are studied. Published by Elsevier Science Ltd.

International Journal of Non-Linear Mechanics

Couette flow of granular materials

Active continua with integrated sensor and actuator have proved effective in damping the vibrations of any dynamic system. This work deals with experimentally implementing control algorithms, namely negative velocity and Lyapunov control to actively damp the vibrations of a cantilever beam. A Finite element scheme is used to predict the dynamic response of the beam to transient electrical signals and to compare them with the experimentally obtained result. A distributed PVDF (Polyvinylidene fluoride) actuator is used for actuation and an accelerometer is used as a tip velocity sensor.

Proceedings of SPIE - The International Society for Optical Engineering

Vibration control of a cantilever beam using distributed PVDF actuator

Distress due to water induced damage and bleeding accelerates the failure of an asphalt concrete pavement due to damage mechanisms such as fatigue cracking, rutting, stripping etc. One of the important variables used while modeling water induced damage for asphalt concrete pavement is permeability. This study uses the framework developed to model the movement of air voids in asphalt concrete in a previous study by the authors. The assumption of constant permeability in the theory of consolidation by Terzaghi is relaxed and a linear relation is proposed between permeability and air voids. Voids filled with asphalt (VFA) are proposed as a parameter to model bleeding of asphalt concrete, and its variation for different loading and mixture condition is studied.

International Journal of Engineering Science

Permeability and bleeding of asphalt concrete using mixture theory

Few investigations have been carried out with bamboo fibers despite its high strength, biodegradability, and low cost. The overall objective of this work was to investigate fiber extraction from bamboo and the use of these bamboo fibers as reinforcement in polymeric composites. A combination of chemical and mechanical methods was used for the extraction of bamboo fibers. Conventional methods of compression molding technique (CMT) and roller mill technique (RMT) were explored for the mechanical separation. Fiber population from both the techniques were characterized. Mechanical properties of the fibers also were evaluated. Bamboo fibers obtained from CMT and RMT were used to make unidirectional composites of polyester. High values of tensile strength were observed in all the composites. The predominant mode of failure for the composite was shown to be the cracking of the fiber-matrix interface. Quantitative results from this study will be useful for further and more accurate design of bamboo reinforced composite materials.

Department Of Chemical Engineering

Journal of Applied Polymer Science

Extraction of bamboo fibers and their use as reinforcement in polymeric composites

This paper deals with a novel test facility, developed to study surface crack propagation in a non-standard plate specimen. This specimen effectively simulates various aspects of the large diameter pipes and main vessel of a liquid metal fast breeder reactor (LMFBR). Experiments are carried out on SS 316L(N) plates, containing a semi-elliptical surface crack, subjected to pre-dominantly bending loads. This is a critical mode of loading for the associated full-scale components. The machine is ideal and cost-effective for pre-cracking the specimen, which has an initial notch. The experiments carried out on this machine have yielded interesting results, like evolution of the crack front across the thickness as a function of applied cycles and the difference in the crack propagation behaviour in the surface and depth directions. This system in conjunction with crack depth measurement techniques has got high potential for further applications in the domain of leak before break (LBB) investigations.

Journal of the Institution of Engineers (India): Aerospace Engineering Journal

A novel testing facility for fracture mechanics analysis of plates

Asphalt concrete used in flexible highway pavements has 5-8% air voids immediately after laying of the roadway. Constitutive laws for asphalt concrete developed till now have modelled the mix as a linear elastic or viscoelastic material and have not taken into account the effect of voids concentration on the mechanical behaviour of the material. In the present study the theory of mixtures is used to model asphalt concrete. Asphalt concrete is considered to be a mixture of aggregate matrix, asphalt and air in a purely mechanical system in which the thermal effects and chemical reactions are ignored. Constitutive relation for each component of the mixture is assumed to be dependent only on the kinematical quantities associated with each component. The resulting hyperbolic conservation equations are solved by an upwind finite volume scheme coupled with an operator splitting technique for a quasi-static type of loading. The numerical scheme is used to simulate the variation of air voids content across the thickness of a typical road pavement.