Vikram K. Kinra

On the role of thermoelectric heat transfer in the design of SMA actuators: theoretical modeling and experiment

A combined theoretical/experimental study of the heat transfer in thermoelectric shape memory alloy (SMA) actuators is undertaken in this paper. A one-dimensional model of a thermoelectric unit cell with a SMA junction is developed first and the transient temperatures in the SMA are evaluated for different applied electric current densities. As a first step towards the design of an actuator, a thermoelectric module is assembled in the laboratory for cooling/heating the SMA. Transient temperature profiles are recorded for the monotonic heating and cooling runs for two different materials-copper and SMA (with or without the phase transformation). These recorded profiles are then compared with the predictions from the model; the agreement is reasonable, particularly during the cooling process. Temperature profiles are also recorded for cyclic cooling and heating of copper at a frequency of 0.5 Hz and a good comparison is obtained. Theoretical predictions for thermal cycling of SMA show that it is possible to achieve a frequency of 2 Hz on full phase transformation and 17 Hz on partial transformation of 25%.

Analysis of elastothermodynamic damping in particle-reinforced metal-matrix composites

When a composite material is subjected to a homogeneous or inhomogeneous stress field, different phases undergo different temperature fluctuations due to the well-known thermoelastic effect. As a result, irreversible heat conduction occurs and entropy is produced. This entropy production is the genesis of elastothermodynamic damping. Recently, taking the second law of thermodynamics as a starting point, a general methodology for calculating the elasto-thermodynamic damping was presented by Kinra and Milligan. Using this method, we calculate the elastothermodynamic damping for two canonical problems concerning particle-reinforced metal-matrix composites: (1) a single spherical inclusion in an unbounded matrix and (2) anN layer finite concentric composite sphere. In both cases, a uniform radial time-harmonic loading is considered.

Simultaneous measurement of the acoustical properties of a thin‐layered medium: The inverse problem

This paper presents a frequency‐domain ultrasonic technique for a simultaneous determination of the thickness (h) and wave speed (c) of the individual layers comprising a multilayered medium. The layers may be ‘‘thin’’; by thin we mean that the successive reflections of an ultrasonic pulse from the two faces of a layer are nonseparable in the time domain. Plane longitudinal waves which are normally incident upon the medium are considered. A systematic analysis of the sensitivity of the complex‐valued transfer function to the acoustical parameters of each layer has been carried out. An inverse algorithm, which utilizes either the Newton–Raphson or the Simplex method in conjunction with the incremental search method, has been developed to reconstruct simultaneously the thickness and phase velocity of each layer by minimizing the difference between the theoretical and the experimental results in the mean‐sum‐square sense; the entire complex spectrum, i.e., the amplitude as well as the phase spectrum, was use...

A new technique for ultrasonic-nondestructive evaluation of thin specimens

Combining standard FFT methods with conventional ultrasonics, a method has been developed for measuring the phase velocity, the group velocity and the attenuation in ultrathin specimens (submillimeter or subwavelength in thickness). A detailed description of this technique is given. The technique was used on four disparate materials: aluminum, an epoxy, a particulate composite and a graphite-fiber/epoxy composite. The method works equally well for thin or thick specimens, and for dispersive as well as nondispersive media.

An experimental investigation of pass bands and stop bands in two periodic particulate composites

Abstract An important property of the periodic composites is that the dispersion curve is characterized by pass bands and stop bands. In the past these have been demonstrated, analytically and experimentally, for layered and fibrous composites. The purpose of the present experimental investigation is to show that the same phenomena exist in periodic particulate composites.

Leaky Lamb waves in an anisotropic plate. II: Nondestructive evaluation of matrix cracks in fiber-reinforced composites

This paper is concerned with the use of leaky Lamb waves for the nondestructive evaluation (NDE) of damage in anisotropic materials such as fiber-reinforced composites. Two fundamental acoustic properties of the material, namely, the wave speed and attenuation have been measured. Stiffness is deduced from the wave speed. The damage mode selected for this study is matrix cracking. As expected, the in-plane stiffness decreases and the attenuation increases with an increase in the linear crack density.

Elastothermodynamic damping in laminated composites

When a composite material is subjected to a time-harmonic stress field (homogeneous or inhomogeneous), different phases undergo different temperature fluctuations due to the well-known thermoelastic effect. As a result irreversible heat conduction occurs within each phase and between phases, and entropy is produced. This entropy production is the genesis of elastothermodynamic damping, and manifests itself as a conversion of work into heat. This is one of a large number of sources of damping in a real composite. Mechanics-based analytical methods for predicting damping due to various relaxation mechanisms have not been reported. Therefore, it becomes difficult to divide the total (experimentally measured) damping into its various constituents. This defines the objective of the present work: we present a methodology for predicting the elastothermodynamic damping of an N-layer laminated composite. The stress field may be quite general so long as the resulting heat conduction occurs only in the direction orthogonal to the laminae. By way of illustration, numerical results are presented for a symmetric three-layer plate in biaxial bending.

Diffraction of Rayleigh waves in a half‐space. I. Normal edge crack

This paper is concerned with the diffraction of Rayleigh surface waves by an edge‐crack normal to the free surface of a half‐space. An experimental technique was developed to yield accurate and reproducible measurements of the scattered field on the free surface both in the vicinity of the crack and far away from it. The results show that the nearfield scattered field is very sensitive to the crack length and possesses many characteristic features which are shown to be potentially useful in the area of nondestructive testing and evaluation. The farfield transmission and reflection coefficients AT and AR were measured under both the steady time‐harmonic motion and the transient motion of the half‐space. In the short‐wavelength limit (l/λR≥2), both AR and AT achieve the crack‐length‐independent values, namely AR=0.41 and AT=0.06; the former agrees with the reflection coefficient of a quarter‐space and the latter agrees with the combined transmission coefficient through the leading corner, the tip, and the t...

Ultrasonic Nondestructive Evaluation of Thin (Sub-Wavelength) Coatings

This paper describes a technique for ultrasonic nondestructive evaluation (NDE) of a thin coating on a thick substrate; here thin means the thickness of the coating is less than the wavelength of the ultrasonic wave used to interrogate it. A plane longitudinal wave that is normally incident upon the coating is considered. Transfer functions have been derived for both the coating‐side and the substrate‐side insonification. A systematic analysis of the sensitivity of the transfer functions to the thickness and wave speed has been carried out. An inverse algorithm, which utilizes the well‐known Newton–Raphson method, has been developed to reconstruct the thickness and the phase velocity through a comparison of the theoretical and the measured transfer functions; fortunately, this algorithm is independent of the substrate thickness. Using this technique both the thickness and the wave speed of the coating can be extracted from the same measurement, without knowing either. The technique is fully automated and ...

A new technique for the ultrasonic detection of internal transverse cracks in carbon-fibre/bismaleimide composite laminates

Through-transmission ultrasonic C-scan imaging with inclined focusing transducers in confocal configuration has been used for the detection of partial and internal cracks in a cross-ply, [0/90/0] carbon/epoxy laminate. A number of partial cracks (intersecting only one edge of a coupon) as well as completely internal cracks were found ultrasonically after the sample was subjected to static loading. The validation of the result by cutting the sample along two lines in the 0° direction and checking the presence of the internal cracks with an optical microscope shows a good correlation between the ultrasonic image and the optical data. Experimental results show that in the IM7/5250-4 composite, which contains a toughened BMI resin, initiation of transverse cracks under static loading occurs with the formation of partial cracks not necessarily originating at the edge of the sample.