Showing papers by "Prabhu Rajagopal published in 2018"

Porous metamaterials for deep sub-wavelength ultrasonic imaging

Abstract: This paper reports the application of a porous medium as an aperiodic metamaterial lens for ultrasonic imaging in the context of nondestructive evaluation and non-invasive diagnostics. Experimental results are presented, demonstrating a deep sub-wavelength imaging down to 1/36th of the operating wavelength, which is the highest resolution demonstrated worldwide using bulk ultrasound. The improvement in the resolution is shown to be linked to aperiodicity overcoming the Wood anomaly, which sets limits on wave transmission by holey structured lenses.

Probability of Detection (PoD) Curves Based on Weibull Statistics

Abstract: Probability of detection (PoD) curves are a popular metric for the reliability assessment of Nondestructive Testing (NDT) procedures. However, the classical Berens method for signal response PoD analysis strongly relies on the hypothesis of Gaussian residuals which can be violated in practical conditions. In particular, data from sparse field trials can be scattered and or skewed. Hence, this paper studies the feasibility of assuming a Weibull distribution, which is known for versatility in representing several fundamental statistical states, for regression residuals without modifying the overall Berens framework for PoD curve determination. The proposed ‘Weibull-Berens’ PoD statistics is first shown to compare well with the classical Berens method for an ideal case of Gaussian residuals. The advantages of the method are further demonstrated using a synthesised dataset, as well as a practical case of non-Gaussian residuals arising from reduced number of experimental trials.

Selective modal excitation for optimization of waveguide based bulk ultrasonic transducers

Abstract: The use of waveguides for ultrasonic transduction is advantageous in numerous applications such as measurement of flow, temperature and material properties under harsh environments. However, typically waveguide transducers do not use any mode control. This paper describes aspects of mode selection for waveguide based transduction of bulk longitudinal ultrasonic waves in a target metallic specimen. It is proposed that specific waveguide modes with suitable modal structure and matched acoustical impedance can significantly enhance the transduction of bulk ultrasonic waves in the specimen sample. Using circular cylindrical waveguides excited by magnetostriction, experimental results guided by finite element simulations and analysis are presented, demonstrating the feasibility of the approach. An example application in the form of a bulk longitudinal wave ultrasonic transducer is also provided illustrating the improvement in the signal quality using the proposed concept.

Comparison of bio-inspired flapping foil propulsion systems with rotary propulsion

Abstract: Conventional rotary thrusters used in underwater Remotely Operated Vehicles (ROV) suffer from disadvantages such as high noise, entangling with floating objects and limited efficiency. Our group has studied bio-inspired propulsion with a single caudal fin which overcomes some of these disadvantages. However, it still faces some problems such as low thrust density (thrust generated per unit volume of the thruster), pitch/yaw and centre of mass (COM) oscillations in the body increasing the energy cost of transport. The solution being proposed is a novel dual flapping foil arrangement actuated using a single motor which tackles the limitations of single flapping foil. Comparison of efficiency, maximum thrust and thrust density has been performed between single and dual flapping foil propulsion. Also, this solution is benchmarked against commercially available rotary thrusters. The results showed that thrust density of bioinspired propulsion was not at par with rotary thrusters. However, the efficiency obtained was comparable.

Non-destructive evaluation of GFRP-wood sandwich structure composite using terahertz imaging

Abstract: Engineering of novel structures with high strength to weight ratio for applications in aerospace, renewable energy and naval industries has resulted in an increased popularity of sandwich structured composites. A sandwich-structured composite is fabricated by bonding a thick lightweight core between two stiff, thin skins such as Glass Fiber Reinforced Plastic (GFRP). Balsawood is a type of homogeneous core which is widely used for renewable energy structures, such as wind turbine blades. In this paper, a GFRP-balsawood sandwich structure is evaluated non-destructively for internal defects such as holes, using a CW Terahertz system in transmission mode. Internal defects will give rise to differential THz transmission and hence can be identified using THz imaging. The imaging studies are carried out with a central frequency of 0.35 THz and the sample is raster scanned using 2-D translational stages controlled by high precision stepper motors in x-y directions to obtain the THz image. The image acquired using CW THz system clearly identifies the defects in the GFRP-balsawood composite structure with good contrast demonstrating the potential of THz imaging for non-destructive testing of sandwich composite structures.

Distributed temperature sensors development using an stepped-helical ultrasonic waveguide

Abstract: This paper presents the design and development of the distributed ultrasonic waveguide temperature sensors using some stepped-helical structures. Distributed sensing has several applications in various industries (oil, glass, steel) for measurement of physical parameters such as level, temperature, viscosity, etc. This waveguide incorporates a special notch or bend for obtaining ultrasonic wave reflections from the desired locations (Gage-lengths) where local measurements are desired. In this paper, a multi-location measurement wave-guide, with a measurement capability of 18 locations in a single wire, has been fabricated. The distribution of these sensors is both in the axial as well as radial directions using a stepped-helical spring configuration. Also, different high temperature materials have been chosen for the wave-guide. Both lower order axi-symmetric guided ultrasonic modes (L(0,1) and T(0,1)) were employed. These wave modes were generated/received (pulse-echo approach) using conventional longitu...

Detection of Guided Waves in a Composite Plate Using Surface Bonded Fiber Bragg Gratings Sensor

Abstract: Ultrasonic guided waves based non-destructive testing (NDT) is attractive due to its potential for long range inspection. In NDT, fiber Bragg gratings (FBG) sensor is widely adopted due to its rugged wavelength encoded information and immunity to electromagnetic interference (EMI). In this paper we discuss the reception of ultrasonic guided waves in a glass fiber reinforced plastic (GFRP) composite plate using a surface bonded FBG sensor. Experimentally deduced values for wave velocity and attenuation are compared with the theoretical values.

Ultrasonic transducers based on spray coated PZT film for in situ operation for temperatures up to 200 °C

Abstract: Ultrasonic testing is widely used for assessing the integrity of structures, and increasingly, there is much interest in the inspection of components operating at elevated temperatures. Degeneration of acoustic couplant and active element are among the challenges widely reported to develop ultrasonic piezoelectric transducers operating at higher temperatures. In our study, the development of an in situ ultrasonic transducer using spray deposited PZT slurry as the active element to perform bulk ultrasonic inspection/monitoring of components and structures operating at elevated temperatures (up to 200° C) is described. This method allows deposition of PZT coatings of thickness > 500 µm. The operation of the developed ultrasonic transducer performing pulse-echo ultrasonic measurements at 1 MHz up to 200 °C is demonstrated experimentally and compared with conventional methods. Merits and limitations to practical realisation are also discussed.

Deep sub-wavelength ultrasonic imaging

Abstract: There is much interest in improving the resolution of ultrasonic inspection, which suffers from large wavelengths typically in the range of millimeters, due to low value of speed of sound in solid media. The authors are interested in achieving this through holey structured metamaterial lenses, and have recently demonstrated an experimental subwavelength resolution of λ/25. However the previous work was in through-transmission mode with reception using Laser Doppler Vibrometer (LDV), which may not be suitable for practical applications. This paper discusses the use of optimized holey structured metalens to achieve a deep sub-wavelength imaging up to λ/18 in through-transmission mode, but using commercially available piezoelectric ultrasonic transducers for both generation and reception of ultrasound.There is much interest in improving the resolution of ultrasonic inspection, which suffers from large wavelengths typically in the range of millimeters, due to low value of speed of sound in solid media. The authors are interested in achieving this through holey structured metamaterial lenses, and have recently demonstrated an experimental subwavelength resolution of λ/25. However the previous work was in through-transmission mode with reception using Laser Doppler Vibrometer (LDV), which may not be suitable for practical applications. This paper discusses the use of optimized holey structured metalens to achieve a deep sub-wavelength imaging up to λ/18 in through-transmission mode, but using commercially available piezoelectric ultrasonic transducers for both generation and reception of ultrasound.

Demonstration of control and inspection by circumferentially guided ultrasonic waves using novel Remotely Operated Underwater Vehicle

Abstract: This article presents a cost efficient semi-autonomous remotely operated underwater vehicle for inspecting water-submerged pipeline networks using circumferential guided ultrasonic waves. Robot navigation is implemented using inertial measurement unit and pressure sensor, with motion control achieved through proportional–integral–differential controller specially tuned to take angled turns. The robot is equipped with pneumatic grippers for clamping on-to a pipe and uses thrusters to move along its length. The paper describes the electronics and control design of this mechanism and presents results from practical experimental trials and ultrasonic measurements.