This Letter proposes the application of RF technology for the non-destructive detection of spoiled coconut at the pre-processing stage. Since the dielectric property of coconut water changes at different stages of coconut, the sensor measures the content of water inside a coconut sample non-destructively at 2.45 GHz. The sensor has been trained to determine the threshold water level between healthy and spoiled coconut which in turn can be used to sort the coconuts. The measurement results have been presented for different processing stages of coconut for different samples. Detecting coconut quality through dielectric sensing reveals some very impressive results and opens the doors for better consumption.

Compressive 2D near-field millimetre-wave (MMW) imaging is a compressed sensing-based technique, which reconstructs an image from a few under-sampled measurements. Although this technique can extremely improve imaging process efficiency, resolution of the reconstructed image is still limited by the scanning grid interval, which is always hard to be improved in practice due to hardware constraint. To reconstruct a higher resolution image from under-sampled measurements, a compressive 2D near-field MMW super-resolution (SR) imaging model is established. Meanwhile, the corresponding algorithm, which is based on the primal-dual framework, is also proposed. Experimental results show that the proposed algorithm can effectively reconstruct the SR MMW image in superior performance.

The development and performance evaluation of a thin, broadband, and tunable radar wave absorber with reduced complexity is a strenuous task. This Letter reports the design, fabrication, and non-destructive measurement of an active frequency selective surface (FSS) embedded tunable radar absorber. The proposed absorber design consists of periodically patterned, PIN diode mounted-active transmissive/reflective FSS with a straightforward bias network. The tunable absorption characteristics are obtained over a wide frequency range by controlling the bias voltage of PIN diodes. The features of the proposed tunable absorber are demonstrated through non-destructive free space, microwave measurement of a fabricated prototype. The measurement results show the achievement of a remarkable bandwidth of 3.0 GHz, which enables dynamic control of the absorption characteristics ranging from 8.2 to 12.4 GHz. Such an approach paves the way toward the realisation of smart materials and structures for electromagnetic wave manipulation.

The authors propose a new non-destructive inspection method for underground fiberglass-reinforced plastic mortar (FRPM) pipelines. Their method utilises time-domain responses of microwave guided-modes propagating along an FRPM pipe-wall. Through the time-domain analyses, they found that a microwave guided-mode signal propagating along a pipe-wall is spread out to various waves corresponding to straight and circumferential waves along the pipe-wall and that the position of an unwanted foreign object on the wall can be detected by examining the path of the wave interrupted by the foreign object. Additionally, they also derived a propagation equation that enables them to calculate a time-domain response of the spread microwave guided-mode signals propagating along the pipe-wall. This method can be applied to pipelines consisting of various types of materials.

In this Letter, a non-destructive microwave resonant sensor based on the modified complementary split-ring resonator has been proposed and analysed with respect to its sensitivity for detecting the aqueous biochemicals. This technique is based on the perturbation theory and only requires to measure resonant frequency and hence, achieving a substantial reduction in the computation time and cost. The proposed sensor is designed to achieve high sensitivity performance with a measured quality factor of the sensor in the bare state, Q _B = 631.5, at the centre frequency of f _o = 6.31 GHz. The proposed sensor has a maximum sensitivity of S = 6.23 MHz with respect to a unit change in the dielectric constant value. A numerical model is developed for the dielectric constant of the biochemicals as a function of the resonant frequency. The proposed sensor, therefore, provides high sensitivity, simple structure layout and real-time response with cost effectiveness. The proposed structure can be a potential candidate for the non-intrusive sensing of aqueous biochemicals.

The authors present a terahertz-band imaging system using rail-based synthetic aperture radar sensing and processing. In the proposed setup, the object is inclined with respect to the transmitter and is scanned in azimuth using a motorised linear stage. The single-transceiver system is based on one-port full-duplex frequency extender module coupled with a vector network analyser. Processing the reflection data recorded in the frequency domain, it is possible to reconstruct high-resolution two-dimensional images of objects and voids concealed behind material layers in the 220–355 and 500–750 GHz bands with sub-millimetre resolution.

Non-destructive testing and evaluation (NDT&E) plays a vital role in industrial quality control and structural health monitoring. Among various NDT&E approaches, infrared thermography plays a crucial role due to its merits such as fast, remote, safe, and easy to implement in testing and evaluation of various materials. This Letter presents the statistical post-processing approach for pulse compression favourable frequency modulated thermal wave imaging for inspection of mild steel samples having flat bottom hole defects. Furthermore, a detailed analysis has been carried out using independent component analysis and provides insight for choosing the number of independent components and number of eigenvalues to be retained in the whitening procedure.

Owing to the significantly installed capacity of solar energy during the past decades, the operation and maintenance of the large photovoltaic power station is a big challenge, as the manual inspection is labour-intensive and expensive. This Letter presents a solution for the intelligent inspection of the hotspot with the unmanned aerial vehicle in the large-scale photovoltaic plant. First, a traditional image processing method is presented to eliminate the noise and crop the infrared image, which can make the defected feature more obvious. Then, the module is extracted by the line segments detection. Finally, considering the great advance in the realm of the computer vision, a deep-learning-based method for automatic hotspot detection is proposed for locating the hotspot with the data augmentation. The deep-learning-based model can extract the hotspot feature by the training process in the data set. In the end, the performance of the method proposed is extensively evaluated and the numerical results prove the accuracy and precision of the model.

Active infrared imaging is one of the promising remote and whole field characterisation techniques for non-destructive testing and evaluation of various solids irrespective of their electrical and magnetic prosperities. This technique relies on a mapping of thermal response for a predefined incident heat flux over the test object to detect the presence of surface and subsurface anomalies. Due to its fast, non-contact, safe and quantitative testing capabilities, infrared thermography has gained significant importance in the testing of fibre reinforced polymers. This Letter highlights testing and evaluation of glass fibre-reinforced polymer (GFRP) specimen for detection of subsurface hidden defects using pulse compression favourable thermal wave imaging techniques (for an imposed digitised chirp as well as a 7-bit Barker coded modulated heat fluxes over the test specimen). Further depth scanning capabilities of the proposed schemes have been compared using a time-domain pulse compression based approach. Proposed analytical, as well as simulation studies, have been validated with the experimental results on GFRP material having flat bottom holes as defects.

InfraRed non-destructive testing and evaluation (IRNDT&E) has emerged as a promising approach for non-destructive testing and evaluation (NDT&E) of various materials because of its inherent merits such as remote, non-invasive, qualitative as well as quantitative inspection capabilities. Among the various IRNDT&E techniques, recently proposed modulated pulse compression favourable thermal wave imaging (PCTWI) techniques, especially frequency modulated thermal wave imaging (FMTWI) and its digitised version digitised FMTWI (DFMTWI) have gained popularity over the conventional sinusoidal modulated [lock-in thermography (LT)] and pulse-based thermographic techniques [pulse thermography (PT) and pulse phase thermography (PPT)] by providing better depth resolution, in less experimentation time, using low peak power excitation heat sources. The present work highlights a comparative study on highly depth resolved continuous depth scanning PCTWI techniques, with single frequency thermal excited LT, on the basis of defect detection probability. The proposed study has been carried out on a glass fibre reinforced polymer test sample with Teflon inserts as defects by considering peak side lobe ratio as a figure of merit. The experimental results clearly show that the probability of detection of defects for DFMTWI is far superior as compared to FMTWI and LT.

Recent developments in electronics have demonstrated some advantages of using carbon fibres in manufacturing flexible PCBs thus requiring to elaborate some new approaches for checking PCB quality. Barely visible impact damage (BVID) represents a typical type of defects in carbon-based composites. In this study, the two inspection techniques, namely active infrared thermography and laser vibrometry, were comparatively used for detecting BVID in composite materials manufactured by three-dimensional printing from kevlar and carbon fibres. The comparative analysis of the composite resistance towards multifold impacting with the total energy of up to 15 J has demonstrated advantages of the hybrid configuration obtained by using the technology of additive manufacturing. The measurement of damping characteristics of the composites in the 50–100 kHz frequency range has also confirmed the ability of the hybrid composite to absorb more energy than classical single component materials.

Active thermal non-destructive testing (TNDT) has emerged as a swift, robust and cost-effective non-contact inspection method used to detect the surface and sub-surface defects present in a wide verity of solid materials. To further increase the efficacy of the process, various pulse-compression-based post-processing techniques are in use. However, the applicability of pulse compression-based thermographic methods has limited due to the presence of side lobes that degrades the energy concentration capabilities within the main lobe. In order to address this limitation, this work proposes a poly-phase code (P4-code). P4 codes are very efficient and robust in the reduction of distribution of energy in side lobes by concentrating on the main lobe. This Letter proposes a numerical study on the applicability of P4 codes-based pulse compression favourable thermal wave imaging approach for TNDT for testing and evaluation of steel specimen for identification of flat-bottom hole defects located at different depths. Further performance of the proposed method is compared with the widely used linear frequency modulated thermal wave imaging by considering the signal-to-noise ratio as a figure-of-merit.

Barker coded independent component thermography (BCICT) approach known for exploiting the pulse compression properties with independent component analysis method was used to examine a mild steel sample with drilled flat-bottomed holes at various depths. This Letter emphasises the application of the probability of detection (POD) as a predictive assessment tool for the efficacy of the different methods to detect the defects at varying depths. The results exhibited that the aspect ratio of the deep defect with 90% POD and for 90% POD with 95% confidence that can be detected using the BCICT approach are 3.161 and 3.835, respectively. Besides, a relation between the POD of the defect at different depths as a feature of the defect aspect ratio (diameter/depth) has also been presented.

Active infrared thermography or thermal wave imaging is one of the promising non-contact, non-invasive, remote, patient friendly and safe imaging modality for the testing and evaluation of various bio-materials. Among various thermal wave imaging modalities recently proposed, frequency modulated thermal wave imaging gained its acceptance due to its high depth resolvability for visualising the surface and sub-surface features of the test sample. This is due to its continuous depth scanning capabilities with the imposed continuous band of thermal waves with significant and equal magnitudes into the test sample with moderate peak power imposed heat flux in a limited span of experimentation time. This Letter highlights a novel analytical approach for examination of the severity of the osteoporosis. Further, the proposed analytical treatment is validated with the results obtained from the commercially available finite-element based modelling and simulation.

This Letter demonstrates a non-destructive prediction methodology for determining the necessary magnetic field to be imposed in a porous fin satisfying a particular heat generation occurring within an electronic device. For the first time, all possible kinds of heat transfer have been incorporated here, which were otherwise ignored in other published studies of a similar kind. Just observing the surface thermal response, golden section search (GSS) solver, in conjunction with a forward numerical scheme, has been used in this work to determine the strength of the magnetic field. Estimations are done for various levels of additive white Gaussian noise and satisfactory reconstructions are noted for noise level even up to 12%. The numerical method has been convincingly validated with other schemes of the published literature. Sensitivity analysis reveals that the temperature distribution is per se a strong function of the fin porosity and governed by a mutual trade-off between the heat generation rate and the imposed magnetic field. The results obtained from the present analysis using GSS are proposed to offer assistance to design efficient porous fin based heat transfer surfaces for providing safety and better cooling in addition to a considerable weight reduction of the system.

Spectral imaging technique plays a very vital role in the field of chemical detection and identification. Conventional spectroscopic imaging techniques suffer from massive acquisition time. This limitation sometimes restricts it from many practical applications. The acquisition of a full spectral image requires huge acquisition time. In this Letter, a compressive sensing-based single-pixel camera architecture has been realised to acquire spectral images that can be used for non-destructive testing and classification of explosive materials. The compressive measurements for all the spectral images are done simultaneously thus reducing the acquisition time significantly. The spectro-spatial images were reconstructed using the basis pursuit algorithm and compared with least square solutions, which resulted in fast acquisition and improved image quality. The maximum compression rate achieved was 95.84%.

In this Letter, a wavelet packet energy-based algorithm is proposed to classify surface and non-surface slits through the pulsed alternating current field measurement technique. Wavelet packet decomposition is used to obtain the coefficient of each frequency band. The slit is classified as non-surface slit or surface slit according to the normalised energy of the first band. The quantification of the remaining ligament or depth of slits is performed using the entropy of the wavelet packet energy. The experiment results show that the non-surface and surface slits can be clearly classified. The entropy of a non-surface slit is linearly proportional to the remaining ligament, and the entropy of a surface slit is inversely proportional to slit depth.

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.

A non-destructive testing system requires multiple system configuration parameters during the operation process. For a given transducer, scanning frequency and number of measurements for averaging are just among those parameters. This work tests the central limit theorem to optimally set these parameters. The authors have designed a compact ultrasound computer tomography scanner from scratch just to test this criterion. It is shown that optimal frequency value changes with respect to the scanning angle for an object with a heterogeneous inner profile. The proposed criterion helps to estimate the multi-spectral combination before the recovery stage minimum error. Possible industrial applications of such a system are 2D/3D multiphase flow profile recovery or crack detection in the composite matter, non-invasively.

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.

The full-field pulse-echo ultrasonic propagation imaging (PE UPI) system using a two-axis linear stage has been successfully used to perform a non-destructive evaluation of flat specimens. This study used a six-axis robot arm to develop a robotic scanning algorithm that can be applied to the laser pulse-echo inspection of non-flat objects. The robotic PE UPI system was constructed and verified for both flat and curved specimens. The internal structures and defects of a honeycomb sandwich plate and a glass-fibre-reinforced polymer specimen were visualised by the robotic PE UPI system. The quality of the evaluation of the flat specimen was equivalent to that of a two-axis linear system, and the result of the curved specimen showed improvement compared to that of the linear system. The ability of the robotic scanning technique to evaluate the structural health performance of arbitrarily shaped surfaces was validated.