Showing papers by "Prabhu Rajagopal published in 2020"

Detection of interfacial weakness in a lap-shear joint using shear horizontal guided waves

Abstract: This study aims to develop a shear horizontal guided wave based technique to evaluate the interfacial adhesion of aluminium-epoxy-aluminium tri-layer in a lap shear joint. A 3-D Multi-physics finite element model was developed to investigate the physics of the interaction of SH modes with a tri-layer structure. By employing the boundary stiffness approach, different cases of interfacial adhesion-ranging from perfect bond, intermediate and weak bond, were simulated. Frequency-wavenumber analysis reveals that at the bond overlap region, the incident SH0 wave mode-converts to fundamental (SH0-like) and first-order(SH1-like) modes. The dispersion characteristics of first-order mode (SH1-like) was found to be dependent on the adhesion level, and this influences the time responses collected on a receiver plate in guided wave through-transmission configuration. Experiments were carried out on aluminium-epoxy-aluminium lap shear joints using PPM-EMAT transducers. The analysis shows that this technique can detect and quantify different levels of adhesion, rather than merely classifying as good or bad bonds.

Additively manufactured integrated slit mask for laser ultrasonic guided wave inspection

Abstract: Guided ultrasonic waves are attractive for inspection of additively manufactured plate-like components. Illumination of a slit mask by a pulsed laser is one method by which guided ultrasonic waves can be generated. This work proposes a method for generating narrowband ultrasonic guided waves using an additively manufactured slit mask that is integrated onto the component during selective laser melting (SLM) process. Multiple guided wave modes with a dominant wavelength but with different frequencies were generated using the slit mask fabricated using AlSi12 material. The generated modes were identified using the time frequency response of the received signals and dispersion plots. Identifying the modes and its characteristics (frequency, wavelength, phase and group velocity) beforehand facilitates material and defect characterization. A multiphysics numerical model was developed to simulate laser generation of ultrasound and the model was validated using experimental results. The numerical model developed aided in understanding the physics of line arrayed laser ultrasonic generation and was used as a tool to optimize laser parameters. The developed model was used to study the effect of pulse width of the laser on Lamb wave mode generation. It was observed that a pulse width of 100 ns reduced the overall ultrasonic bandwidth to 4.5 MHz thereby limiting the modes to the fundamental modes A0 and S0 for the given wavelength of 0.8 mm. Rayleigh wave studies using a slit mask showed that the rate of decay of the fundamental frequency component was steeper than the rate of decay of the second harmonic component.

Elastic metamaterial rod for mode filtering in ultrasonic applications

Abstract: The proposed 1D elastic metamaterial rod finds application in non-destructive evaluation, as a filter in the transduction side, where only the torsional and flexural modes need to be generated by isolating longitudinal modes. In this Letter, this is achieved by obtaining a wide bandgap in low-frequency ultrasound regime. The novelty of the proposed design arises from the use of a high impedance mismatch between the materials selected for the rod. The proposed concept is demonstrated and verified with numerical simulations. The effectiveness of the proposed technique is confirmed by comparing the results obtained from the rod made of a single material.

Structured channel metamaterials for deep sub-wavelength resolution in guided ultrasonics

Abstract: Experimental results on deep subwavelength resolution of defects are presented for the first time in the context of guided ultrasonic wave inspection of defects, using novel “structured channel” metamaterials. An Aluminum bar with side-drilled holes is used as a test sample, interrogated by the fundamental bar-guided symmetric mode. Simulations were conducted to optimize dimensional parameters of the metamaterial structure. Experiments using metamaterials fabricated accordingly demonstrate a resolution down to 1/72 of the operating wavelength, potentially bringing the resolution of guided wave inspection to the same range as that of bulk ultrasonics. This work has much promise for remote inspection in industry and biomedicine.

Semiconductor-based thermal wave crystals

Abstract: One-dimensional phononic crystals made of silicon (Si) and germanium (Ge), both of which are materials commonly used in semiconductor devices, are shown to be effective in inducing bandgaps in the dispersion of heat flow at the nanoscale. Numerical approaches are used to understand the dispersion and propagation of thermal waves in Si–Ge phononic crystals. The results show for the first time how nanostructuring could yield band gaps in the dispersion of thermal phonons in the GHz range. We arrive at conditions that can yield bandgaps as high as 40 GHz; this is a bandgap that exceeds the value reported thus far. Variations in the unit cell dimensions are studied to understand the corresponding evolution in the bandgap frequencies. The control of heat using such proposed media holds promise for better heat management solutions for modern electronic devices, nanoscale sensing as well as for novel applications including the development of thermal diodes and thermal cloaks.

Novel hermetically sealed device to realize unconventional phonon blockade at near-micron dimensions and milli Kelvin temperatures

Abstract: We propose a novel design for a hermetically sealable device consisting of charged linear and nonlinear membranes driven in the Gigahertz range in vacuum setting, as a source of antibunched single phonons. Constraints for effecting phonon antibunching are found using the stationary Liouville-von Neumann master equation. Using analytical calculations, material and geometry optimization we show that sizes of the proposed system can be upscaled to near-micrometer range, in a trade-off with the system operating temperature. The results are significant to realize quantum Phononics which has much promise as a modality for sensing and computing applications.

Ultrasonic imaging beyond diffraction limit using conventional transducers with conical baffles

Abstract: Conventional ultrasonic imaging systems suffer from poor resolution imposed by the diffraction limit. The authors have recently demonstrated the use of holey-structured metamaterials (HSMs) to enable resolution beyond the diffraction limit in the ultrasonic regime. However, imaging with HSM requires acquisition of data at fine spatial intervals. Although the use of laser Doppler vibrometers (LDV) as a receiver can be a solution for this as reported in earlier experimental studies, they are highly sensitive to ambient disturbances, suffer from low signal-to-noise ratio and are also expensive, hampering a wider practical implementation. This Letter presents experimental results using hollow cone attachments coupled with conventional ultrasonic transducers for achieving subwavelength resolution imaging at a relatively higher speed and minimal cost. The proposed methodology can be implemented easily for practical inspections and has great potential for commercialisation.

Effect of flapping orientation on caudal fin propelled bio-inspired underwater robots

Abstract: Aquatic animals and mammals in nature, in particular, the Body and/or Caudal Fin (BCF) swimmers swim either by flapping their fins in the sideways direction or the dorso-ventral direction. Not much literature is available on the effects of the performance of these robots based on the choice of its flapping orientation. In this research, it is found that dorso-ventral flapping could lead to better self-stabilizing effects and lesser energy consumption compared to sideways flapping. It is also found that the choice of dorso-ventral flapping offers the possibility of controlling the body’s oscillation amplitude while flapping. This is an appealing advantage for underwater surveying robots carrying cameras and sensors as controlled body oscillations could yield better results from its payloads. The main body of results is obtained with simulations for underwater vehicle dynamics with the coefficients of the REMUS underwater vehicle, while stability analysis for a generalised case is also presented.

Feasibility of ultrasonic guided waves for detecting surface-breaking cracks in laminated composite plate structures

Abstract: Ultrasonic guided waves are attractive for rapid inspection of laminated composite structures, where the cracks developed transverse to the loading direction are the serious type of damage. In this context, the paper presents studies of guided wave interaction with transverse cracks in symmetric and non-symmetric cross-ply composite laminates. In view of this, a two-dimensional finite element approach is used to gain physical insight into the reflection regime. Reflection ratios for GFRP/CFRP cross-ply laminates with various surface-breaking/transverse crack sizes are calculated, and results are presented in frequency spectrum, which shows that the minimum crack depth of 0.0093 of the operating wavelength has been successfully detected. Effect of different ply layup orientations is also considered. This work will be useful for practical guided wave-based inspection of composite plate structures. These points have been added at the abstract section of the revised manuscript.

Rapid nondestructive evaluation of defects in GFRP composites using terahertz line scanner

Abstract: Terahertz (THz) imaging is an attractive alternate to ultrasonic based Non-destructive Evaluation (NDE) especially for Fiber Reinforced Polymers (FRPs) such as Glass FRP (GFRP) composites as the latter demands proximity and additional coupling medium for the best performance. Typically, THz imaging system uses a single emitter-detector configuration employing raster scan method for image acquisition. The image acquisition speed is greatly limited by the speed of the mechanical stages and hence its usage in real-time industrial NDT applications such as in-line quality control has been limited. Alternatively, having an array of detectors will significantly increase the system cost. As an optimal compromise for speed and cost, line scanners are highly desirable. In this work, rapid imaging performance of a THz line scanner has been studied by imaging closely spaced defects in GFRP composites using a 100 GHz source. The total acquisition time for imaging the GFRP sample of dimensions 55× 35 mm2 is 10 s, which is >100 times faster compared to a conventional raster scanning technique. In addition, image deconvolution techniques such as Lucy Richardson and Weiner deconvolution have been adopted to improve the quality of the acquired THz images. The results show that the THz line scanners can successfully be employed for rapid defect detection in GFRP composites.

Effect of Conical Profile on the Transmission of Elastic Waves From Cylindrical Waveguide to Bulk

Abstract: This paper studies the transmission of elastic waves into test specimens for cylindrical waveguide-based ultrasonic transducers. However, to achieve better mode focusing, topographical waveguides with conical profiles were studied. Finite element simulations were used to study wave propagation and transmission into specimen samples using such “transmission horns.” Fundamental longitudinal mode L(0,1) was generated in cylindrical rods. Results from both finite element simulations and experiments show that a 50-deg conical transition profile help achieve better transmission and beam directionality into the specimen and also better reception of the back wall reflections at the waveguide as compared to a simple cylindrical rod waveguide.