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	SENSORS FOR NDT
		GMS Transducer for Concrete Inspection
		Large Aperture PVDF Plane Wave Sensor

GMS Transducer for Concrete Inspection

Magnetostriction occurs in the most ferromagnetic materials and leads to many effects. The most useful one is referred to as the Linear magnetostrictive effect. This is responsible for the expansion (positive magnetostriction) or the contraction (negative) of a rod subjected to a longitudinal static magnetic field. In a given material, this magneto strain is quadratic and occurs always in the same direction irrespective of the direction of the field. Rare earth – iron “giant” magnetostrictive materials discovered by Clark (1980), produce magneto strains which are two orders larger than those produced by material systems such as nickel. Among them Tb0.3Dy0.7Fe2, at room temperature, presents an acceptable compromise between a large magneto strain and a low magnetic field.

Positive magnetostrains of 1000 (x10–6) – 2000 (x10–6) obtained with fields of 50 – 200 kA m–1 are reported for bulk materials. More recently, high magneto strains (in the range of 500 (x10–6) – 1200 (x10–6)) have also been obtained in rare earth – iron based thin films at IITM. However, such large strains are rarely used directly because most applications require a linear behavior. The linearity is obtained by applying a magnetic bias and a mechanical pre-stress in the active material. Moreover, in the case of applications based on resonance, pre-stress is a pre-requisite to produce “giant” dynamic strains, the peak–to–peak amplitude of which is higher than static magnetostrain. The static magnetostrain of this material permits building of linear actuators offering large displacements (20 – 200 μm) and large forces (500 – 5000 N) at relatively low input voltage.

For the first time 100 kHz giant magnetostrictive transducer was developed at the CNDE, IIT Madras, and was tested using both the ultrasonic through transmission technique as well as the pitch-catch method. It is observed that signal was able to detect through 3m concrete block without any signal amplification at the receiver end. . The ultrasonic wave velocity in the concrete was measured to 4341 m/s as expected and demonstrated the ability of the developed transducer to penetrate 3 m of concrete in a through-transmission mode. In Pitch-catch mode with transmitter-receiver spacing of D=250 mm (distance between GMS transducer and PZT receiver), the defective interface region shows a higher amplitude signal reflected from the delaminated interface, when compared to the non-delaminated region. All these experiments were carried in the presence of eddy currents and if reduced the giant magnetostrictive actuation efficiency may be further increased.

The GMS Transducer

Signals from a good and a delaminated
location on a concrete culvert sample.

References : 1. S Narayana Jammalamadakal, G. Markandeyulu, Ravi and K. Balasubramaniam, IEEE Trans. on Magnetics 42(10) 2793-95 (2006)
2. S Narayana Jammalamadakal, G. Markandeyulu, and K. Balasubramaniam J. App. Phy . (2007).
3. S Narayana Jammalamadakal, E. Kannan, K. Balasubramaniam, and G. Markandeyulu, App.
Physics Letters (communicated)

Large Aperture PVDF Plane Wave Sensor

PVDF (Polyvinylidene Fluoride) transducers offer several advantages over conventional PZT transducers for nondestructive ultrasonic inspection and characterization of materials. Features such as low density, small thickness, and flexibility allow PVDF transducers to be shaped for different applications. The flexibility in shaping the PVDF has been used in the development of lens-less line-focus transducers

Many of the “forward” analytical models that are used for the calculation of the transmission spectra from multi-layered plates, made up of isotropic and/or anisotropic layers, are based on plane wave approximation. While a large aperture transmitter is required to generate a beam with little divergence so as to provide a plane wave, a large aperture receiver is also needed to capture the spatially displaced signals as the waves undergo multiple refraction/reflection within and through the medium. The wide frequency bandwidth ensures good spectral coverage.

A low-cost, wide-band, wide-aperture PVDF sensor is proposed as a receiver in immersion based goniometry experiments to obtain high resolution ultrasonic transmission spectra of plates. The performance of a 52 mm thick PVDF receiver of dimensions 6.13 inch ´ 0.74 inch was demonstrated by obtaining ultrasonic transmission spectra of plate samples of Aluminum and Graphite-Epoxy fiber reinforced composites. The spatial extent, and hence the receiver aperture over which detectable transmitted signals can exist was determined from a simple ray-based model as a function of the oblique incidence and the sample thickness. Transmission spectra obtained with three types of receivers - a un-backed PVDF sensor, a 3 inch ´ 0.5 inch commercial “paintbrush” transducer (2.25 MHz) and a 0.375 inch diameter commercial regular transducer (3.5 MHz), deployed in a synthetic aperture mode, are compared. It is shown that the spectra obtained with the un-backed PVDF sensor provided better transmission spectra that compares well with the plane wave based analytical models.

The utility of the improved transmission spectra from plates was demonstrated while calculating the elastic constants of the samples from the ultrasonic data. The inherent wide band response of these PVDF sensors has been exploited for capturing the transmission spectra needed for the determination of elastic constants. Further, the sharp spectral features obtained from using the PVDF sensor may also help in improved measurements of material parameters such as thickness, density, and elastic moduli. While it is clear that the PVDF sensor has attractive features such as wide aperture and wide band at low cost, the low dielectric constant precludes high signal strength generation for transmission. Improvements in the transmit characteristics would help exploit the full bandwidth of the pulser-receiver system and enable high-resolution spectral characteristics to be determined for a broad range of materials.