Nonlinear Ultrasonic Imaging for Porosity Evaluation.

The influence of porosity on the mechanical behaviour of composite laminates represents a complex problem that involves many variables. Therefore, the evaluation of the type and volume content of porosity in a composite specimen is important for quality control and for predicting material behaviour during service. A suitable way to evaluate the porosity content in composites is by using nonlinear ultrasonics because of their sensitivity to small cracks. The main objective of this research work is to present an imaging method for the porosity field in composites. Two nonlinear ultrasound techniques are proposed using backscattered signals acquired by a phased array system. The first method was based on the amplitude of the half-harmonic frequency components generated by microbubble reflections, while the second one involved the frequency derivative of the attenuation coefficient, which is proportional to the porosity content in the specimen. Two composite samples with induced porosity were considered in the experimental tests, and the results showed the high accuracy of both methods with respect to a classic C-scan baseline. The attenuation coefficient results showed high accuracy in defining bubble shapes in comparison with the half-harmonic technique when surface effects were neglected.

A multifunctional ultra-thin acoustic membrane with self-healing properties for adaptive low-frequency noise control.

This paper proposes a novel multifunctional ultra-thin membrane based on a Polyborosiloxane-based gel with stimuli-responsive sound absorption and sound transmission loss (STL) and characterised by excellent self-healing properties. This adaptive behaviour is the result of a dynamically activated phase transition in the membrane's polymeric network which is given by the interaction with the travelling sound pressure wave. The presence and the extent of such phase transition in the material was investigated via oscillatory rheological measurements showing the possibility to control the dynamic response by modifying the Boron content within the polymer. Acoustic analyses conducted at different stimuli responses showed high and dynamic absorption (95%) at the absorption coefficient peaks and an adaptive shift to lower frequencies while sound amplitudes were increased. An average STL up to 27 dB in the frequency range between 500 to 1000 Hz was observed and an increased STL above 2 dB was measured as the excitation amplitude was increased. Results demonstrated that the new membrane can be used to develop deep subwavelength absorbers with unique properties (1/54 wavelength in absorption and 1/618 in STL) able to tune their performance in response to an external stimulus while autonomously regaining their properties in case of damage thanks to their self-healing ability.

Multifunctional Thermal, Acoustic, and Piezoresistive Properties of In Situ-Modified Composite Aerogels with Graphene Oxide as the Main Phase.

Automotive and aerospace industries require advanced materials capable of multifunctional abilities while guaranteeing limited weight and volume and simple processing. Cellular materials such as graphene-based aerogels represent a promising solution. In this study, chemical modification approaches of graphene oxide and polyvinyl alcohol (GOP) aerogels are presented. The combination of a plasticizing agent, glycerol, and a cross-linking agent, glutaraldehyde, is exploited to obtain a mechanically balanced and robust cellular structure. Modified GOP aerogels show high elastic resilience (energy loss coefficient of 29% and compressive strength of 5 kPa at 30% strain, after the 10th compression cycle), low thermal conductivity (0.0424 W mK-1), and high sound absorption (average coefficient of 0.72 between 500 and 1500 Hz) while maintaining a low density of 6.51 kg m-3 with a maximum thickness of 25 mm. Moreover, chemically reduced GOP (rGOP) aerogels are also synthesized. They are characterized by the additional feature of piezoresistive behavior, with only a marginal impact on the other properties. These results show that modified GOP and rGOP aerogels are promising candidates for the fabrication of multifunctional structures to be applied in advanced engineering applications.

Bioinspired Helicoidal Composite Structure Featuring Functionally Graded Variable Ply Pitch.

Composite laminated materials have been largely implemented in advanced applications due to the high tailorability of their mechanical performance and low weight. However, due to their low resistance against out-of-plane loading, they are prone to generate damage as a consequence of an impact event, leading to the loss of mechanical properties and eventually to the catastrophic failure of the entire structure. In order to overcome this issue, the high tailorability can be exploited to replicate complex biological structures that are naturally optimised to withstand extreme impact loading. Bioinspired helicoidal laminates have been already studied in-depth with good results; however, they have been manufactured by applying a constant pitch rotation between each consecutive ply. This is in contrast to that observed in biological structures where the pitch rotation is not constant along the thickness, but gradually increases from the outer shell to the inner core in order to optimise energy absorption and stress distribution. Based on this concept, Functionally Graded Pitch (FGP) laminated composites were designed and manufactured in order to improve the impact resistance relative to a benchmark laminate, exploiting the tough nature of helicoidal structures with variable rotation angles. To the authors' knowledge, this is one of the first attempts to fully reproduce the helicoidal arrangement found in nature using a mathematically scaled form of the triangular sequence to define the lamination layup. Samples were subject to three-point bending and tested under Low Velocity Impact (LVI) conditions at 15 J and 25 J impact energies and ultrasonic testing was used to evaluate the damaged area. Flexural After Impact (FAI) tests were used to evaluate the post-impact residual energy to confirm the superior impact resistance offered by these bioinspired structures. Vast improvements in impact behaviour were observed in the FGP laminates over the benchmark, with an average reduction of 41% of the damaged area and an increase in post-impact residual energy of 111%. The absorbed energy was similarly reduced (-44%), and greater mechanical strength (+21%) and elastic energy capacity (+78%) were demonstrated in the three-point bending test.

Nonlinear Ultrasound Crack Detection with Multi-Frequency Excitation-A Comparison.

Nonlinear ultrasound crack detection methods are used as modern, non-destructive testing tools for inspecting early damages in various materials. Nonlinear ultrasonic wave modulation, where typically two or more frequencies are excited, was demonstrated to be a robust method for failure indicators when using measured harmonics and modulated response frequencies. The aim of this study is to address the capability of multi-frequency wave excitation, where more than two excitation frequencies are used, for better damage identification when compared to single and double excitation frequencies without the calculation of dispersion curves. The excitation frequencies were chosen in such a way that harmonic and modulated response frequencies meet at a specific frequency to amplify signal energy. A new concept of nonlinearity parameter grouping with multi-frequency excitation was developed as an early failure parameter. An analytical solution of the one-dimensional wave equation was derived with four fundamental frequencies, and a total of 64 individual and 30 group nonlinearity parameters. Experimental validation of the approach was conducted on metal plates with different types of cracks and on turbine blades where cracks originated under service conditions. The results showed that the use of multi-frequency excitation offers advantages in detecting cracks.

Ultralight graphene oxide/polyvinyl alcohol aerogel for broadband and tuneable acoustic properties.

An ultralight graphene oxide (GO)/polyvinyl alcohol (PVA) aerogel (GPA) is proposed as a new class of acoustic materials with tuneable and broadband sound absorption and sound transmission losses. The interaction between GO sheets and PVA molecules is exploited in our environmentally friendly manufacturing process to fabricate aerogels with hierarchical and tuneable porosity embedded in a honeycomb scaffold. The aerogels possess an enhanced ability to dissipate sound energy, with an extremely low density of 2.10 kg m-3, one of the lowest values ever reported for acoustic materials. We have first experimentally evaluated and optimised the effects of composition and thickness on the acoustic properties, namely sound absorption and sound transmission losses. Subsequently, we have employed a semi-analytical approach to evaluate the effect of different processing times on acoustic properties and assessed the relationships between the acoustic and non-acoustic properties of the materials. Over the 400-2500 Hz range, the reported average sound absorption coefficients are as high as 0.79, while the average sound transmission losses can reach 15.8 dB. We envisage that our subwavelength thin and light aerogel-based materials will possess other functional properties such as fire resistance and EMI shielding, and will prove to be novel acoustic materials for advanced engineering applications.

Deep-Subwavelength-Optimized Holey-Structured Metamaterial Lens for Nonlinear Air-Coupled Ultrasonic Imaging.

Ultrasound non-destructive testing (NDT) is a common technique used for defect detection in different materials, from aluminium to carbon-fiber-reinforced polymers (CFRPs). In most cases, a liquid coupling medium/immersion of the inspected component is required to maximize impedance matching, limiting the size of the structure and materials. Air-coupled inspection methods have recently been developed for noncontact inspections to reduce contact issues in standard ultrasonic inspections. However, transmission of ultrasound in air is very inefficient because of the enormous impedance mismatch between solids and air, thus requiring a signal amplification system of high-sensitivity transducers. Hence, the captured signal amplitude may not be high enough to reveal any wave distortion due to defects or damage. This work presents a design of a holey-structured metamaterial lens with a feature size of λ/14 aiming at improvement of acousto-ultrasonic imaging using air-coupled transducers. The required effect is obtained by matching geometrical parameters of the proposed holey-structured metamaterials and the Fabry-Perot resonance modes of the structure. Transmission tests have been conducted on different fabricated metamaterial-based structures, to assess the frequency component filtering of the proposed method in both acoustic (f = 5 kHz, 20 kHz) and ultrasonic range (f = 30 kHz, 40 kHz). Results showed an improved sensitivity of damage imaging, with an increase in amplitude of the design frequencies of the lens by 11 dB. Air-coupled inspections were conducted on a stress-corrosion cracked aluminum plate and impacted CFRP plate using the holey-structured lens. Results showed an improvement in the damage-imaging resolution due to a wave-amplitude increase across the defective features, thus demonstrating its potential as an efficient and sensitive inspection tool for damage-detection improvement in geometrically complex components of different materials.

Graphene oxide and starch gel as a hybrid binder for environmentally friendly high-performance supercapacitors.

Alternative green binders processable in water are being investigated for the development of more efficient and sustainable supercapacitors. However, their electrochemical performances have fallen within or below the average of commercially available devices. Herein, an optimised gelled mixture of graphene oxide (GO) and starch, a biopolymer belonging to the family of polysaccharides, is proposed. The molecular interactions between the two components enhance electrodes structure and morphology, as well as their thermal stability. GO, thanks to its reduction that is initially triggered by reactions with starch and further progressed by thermal treatment, actively contributes to the charge storage process of the supercapacitors. The optimised electrodes can deliver a specific capacitance up to 173.8 F g-1 while providing good rate capabilities and long-term stability over 17,000 cycles. These are among the best electrochemical performances achieved by environmentally friendly supercapacitors using a biomaterial as a binder.

Carbon Black and Reduced Graphene Oxide Nanocomposite for Binder-Free Supercapacitors with Reduced Graphene Oxide Paper as the Current Collector.

Reduced graphene oxide (rGO) is an ideal candidate for the improvement of supercapacitor (SC) performances due to its industrial-ready manufacturing process and ease of processing. In this work, rGO was used as an active binder for the manufacture of carbon black (CB) and rGO-based SCs. Being able to form a stable suspension in water, graphene oxide (GO) was initially exploited as a dispersing agent to fabricate a homogeneous slurry with CB having exclusively water as a low-cost and environment-friendly solvent. After casting on a suitable substrate, the material was subjected to thermal treatment allowing the reduction of GO to rGO, which was successively confirmed by chemical-physical analysis. An innovative current collector, consisting of high-quality rGO paper, was also proposed ensuring an improved adhesion between the active material and the substrate and a reduction of the whole weight with respect to devices fabricated using common metallic current collectors. Due to the interesting electrochemical performances, with a high specific power of 32.1 kW kg-1 and a corresponding specific energy of 8.8 Wh kg-1 at a current of 1 A g-1, and the improved manufacturing process, the described "all-graphene-based" device represents a valuable candidate for the future of SCs.

A nonlinear ultrasonic SHM method for impact damage localisation in composite panels using a sparse array of piezoelectric PZT transducers.

Structural health monitoring techniques (SHM) for material damage identification have demonstrated higher sensitivity and accuracy when relying on the assessment of nonlinear features exhibited in the material response under ultrasonic wave propagation. In this paper, a novel nonlinear ultrasonic SHM method is introduced for localisation of impact damage in composite laminates using an array of surface-bonded sensors. Unlike existing algorithms, this method enables quick selection of a suitable signal transmission frequency based on the combined sensor-material response, it does not rely on baseline data or complex measurements of signal arrival time, and it allows identification of malfunctioning sensors to minimise damage localisation errors. The proposed technique is based on the transmission and reception of ultrasonic waves through the inspected panel. Initially, the functionality of the transducers is inspected by comparing the signal amplitude in both directions of sensor-to-sensor paths. Then a planar map of material nonlinearity parameter ß is created, and the damage position is defined as the point of highest ß amplitude. Experimental tests on three CFRP panels confirmed successful positioning of barely visible impact damage (BVID) within a range of 4-22 mm. Sensor functionality check was demonstrated on one of the composite laminates, and a malfunctioning transducer was detected. The results suggested that the presented method could be considered an improved alternative to existing SHM techniques for localisation of BVID in composite panels.

A Review of NDT/Structural Health Monitoring Techniques for Hot Gas Components in Gas Turbines.

The need for non-destructive testing/structural health monitoring (SHM) is becoming increasingly important for gas turbine manufacturers. Incipient cracks have to be detected before catastrophic events occur. With respect to condition-based maintenance, the complex and expensive parts should be used as long as their performance or integrity is not compromised. In this study, the main failure modes of turbines are reported. In particular, we focus on the turbine blades, turbine vanes and the transition ducts of the combustion chambers. The existing monitoring techniques for these components, with their own particular advantages and disadvantages, are summarised in this review. In addition to the vibrational approach, tip timing technology is the most used technique for blade monitoring. Several sensor types are appropriate for the extreme conditions in a gas turbine, but besides tip timing, other technologies are also very promising for future NDT/SHM applications. For static parts, like turbine vanes and the transition ducts of the combustion chambers, different monitoring possibilities are identified and discussed.

A combined linear and nonlinear ultrasound time-domain approach for impact damage detection in composite structures using a constructive nonlinear array technique.

Discovery and evaluation concerns of barely visible impact damage in composite materials is a well-known issue in industries using these materials. This work proposes a frequency sweep method where damage assessment is conducted with respect to the time domain. Firstly, a combined linear and nonlinear ultrasound imaging technique is proposed, which focuses on the excitation of damage/defect regions using a frequency sweep methodology from multiple transducer locations. Secondly, the method deconstructs time domain signals, which allows for the visualisation of linear and nonlinear ultrasound components independently. While, a filtering and frequency band separation method was used to exploit defect responses over different frequency ranges and provide time domain visualisation at the damage region. Finally, image segmentation was employed to automate the damage sizing procedure, while a binary imaging method was used to remove false positive damage regions produced by material vibration mode excitation (fundamental frequency responses) by using the nonlinear responses as a baseline-free tool. The results showed that the combined linear and nonlinear results provided more accurate results than a purely linear or nonlinear approach, furthermore the results were shown to be equivalent to those of a standard phased array system. The ability of the method to visualise nonlinear outputs in time can improve the understanding of nonlinear ultrasound mechanisms while provides a clear argument that a complete approach, incorporating both linear and nonlinear methods should be regarded as the future of NDT/E systems.

Nonlinear elastic imaging of barely visible impact damage in composite structures using a constructive nonlinear array sweep technique.

Linear and nonlinear ultrasound imaging methods highlight different damage features: the linear method detects large stiffness changes, while the nonlinear technique identifies small impedance mismatches, such as microcracks or closed delaminations. Typically, nonlinear ultrasound techniques detect damage/defects in materials by measuring higher order harmonics. These harmonics can be difficult to measure due to low magnitude and signal to noise ratios (SNR): hence large excitation amplitudes are needed, which can further complicate the reliability of these methods as equipment nonlinearities can be generated. To overcome these issues, exciting at specific frequencies, known as local defect resonances (LDR), produce a much larger displacements at the damaged regions. However, estimation of LDR is time-consuming, complex and not an easily automated process. A coupled baseline-free linear and nonlinear ultrasonic imaging approach is proposed, using a Constructive Nonlinear Array Sweep excitation and an image subtraction method for identifying damage in layered materials. The signal sweep method uses a narrow band frequency excitation to increase the probability of detection of a LDR frequency. The novel imaging approach was employed using laser vibrometry measurements in various complex composite structures to assess barely visible impact damage, critical for the aircraft industry. The results showed better estimation of impact damage when compared to classical linear or nonlinear ultrasonic methods leading to improved reliability of aircraft inspections.

Recent Advances in Active Infrared Thermography for Non-Destructive Testing of Aerospace Components.

Active infrared thermography is a fast and accurate non-destructive evaluation technique that is of particular relevance to the aerospace industry for the inspection of aircraft and helicopters' primary and secondary structures, aero-engine parts, spacecraft components and its subsystems. This review provides an exhaustive summary of most recent active thermographic methods used for aerospace applications according to their physical principle and thermal excitation sources. Besides traditional optically stimulated thermography, which uses external optical radiation such as flashes, heaters and laser systems, novel hybrid thermographic techniques are also investigated. These include ultrasonic stimulated thermography, which uses ultrasonic waves and the local damage resonance effect to enhance the reliability and sensitivity to micro-cracks, eddy current stimulated thermography, which uses cost-effective eddy current excitation to generate induction heating, and microwave thermography, which uses electromagnetic radiation at the microwave frequency bands to provide rapid detection of cracks and delamination. All these techniques are here analysed and numerous examples are provided for different damage scenarios and aerospace components in order to identify the strength and limitations of each thermographic technique. Moreover, alternative strategies to current external thermal excitation sources, here named as material-based thermography methods, are examined in this paper. These novel thermographic techniques rely on thermoresistive internal heating and offer a fast, low power, accurate and reliable assessment of damage in aerospace composites.

On the generation of nonlinear damage resonance intermodulation for elastic wave spectroscopy.

Recent nonlinear elastic wave spectroscopy experiments have shown that the nonlinear ultrasonic response of damaged composite materials can be enhanced by higher vibrations at the local damage resonance. In this paper, the mathematical formulation for the generation of nonlinear wave effects associated with continuous periodic excitation and the concept of local defect resonance is provided. Under the assumption of both quadratic and cubic approximation, the existence of higher harmonics of the excitation frequency, superharmonics of the damage resonance frequency and nonlinear wave effects, here named as nonlinear damage resonance intermodulation, which correspond to the nonlinear intermodulation between the driving and the damage resonance frequencies, is proved. All these nonlinear elastic effects are caused by the interaction of propagating ultrasonic waves with the local damage resonance and can be measured at locations different from the material defect one. The proposed analytical model is confirmed and validated through experimental transducer-based measurements of the steady-state nonlinear resonance response on a damaged composite sample. These results will provide opportunities for early detection and imaging of material flaws.

Nonlinear imaging (NIM) of flaws in a complex composite stiffened panel using a constructive nonlinear array (CNA) technique.

Recently, there has been high interest in the capabilities of nonlinear ultrasound techniques for damage/defect detection as these techniques have been shown to be quite accurate in imaging some particular type of damage. This paper presents a Constructive Nonlinear Array (CNA) method, for the detection and imaging of material defects/damage in a complex composite stiffened panel. CNA requires the construction of an ultrasound array in a similar manner to standard phased arrays systems, which require multiple transmitting and receiving elements. The method constructively phase-match multiple captured signals at a particular position given multiple transmit positions, similar to the total focusing method (TFM) method. Unlike most of the ultrasonic linear techniques, a longer excitation signal was used to achieve a steady-state excitation at each capturing position, so that compressive and tensile stress at defect/crack locations increases the likelihood of the generation of nonlinear elastic waves. Moreover, the technique allows the reduction of instrumentation nonlinear wave generation by relying on signal attenuation to naturally filter these errors. Experimental tests were carried out on a stiffened panel with manufacturing defects. Standard industrial linear ultrasonic test were carried out for comparison. The proposed new method allows to image damages/defects in a reliable and reproducible manner and overcomes some of the main limitations of nonlinear ultrasound techniques. In particular, the effectiveness and robustness of CNA and the advantages over linear ultrasonic were clearly demonstrated allowing a better resolution and imaging of complex and realistic flaws.

Modelling of multiscale nonlinear interaction of elastic waves with three-dimensional cracks.

This paper presents a nonlinear elastic material model able to simulate the nonlinear effects generated by the interaction of acoustic/ultrasonic waves with damage precursors and micro-cracks in a variety of materials. Such a constitutive model is implemented in an in-house finite element code and exhibits a multiscale nature where the macroscopic behavior of damaged structures can be represented through a contribution of a number of mesoscopic elements, which are composed by a statistical collection of microscopic units. By means of the semi-analytical Landau formulation and Preisach-Mayergoyz space representation, this multiscale model allows the description of the structural response under continuous harmonic excitation of micro-damaged materials showing both anharmonic and dissipative hysteretic effects. In this manner, nonlinear effects observed experimentally, such as the generation of both even and odd harmonics, can be reproduced. In addition, by using Kelvin eigentensors and eigenelastic constants, the wave propagation problem in both isotropic and orthotropic solids was extended to the three-dimensional Cartesian space. The developed model has been verified for a number of different geometrical and material configurations. Particularly, the influence of a small region with classical and non-classical elasticity and the variations of the input amplitudes on the harmonics generation were analyzed.

Nonlinear elastic imaging using reciprocal time reversal and third order symmetry analysis.

This paper presents a nonlinear imaging method for the detection of the nonlinear signature due to impact damage in complex anisotropic solids with diffuse field conditions. The proposed technique, based on a combination of an inverse filtering approach with phase symmetry analysis and frequency modulated excitation signals, is applied to a number of waveforms containing the nonlinear impulse responses of the medium. Phase symmetry analysis was used to characterize the third order nonlinearity of the structure by exploiting its invariant properties with the phase angle of the input waveforms. Then, a "virtual" reciprocal time reversal imaging process, using only one broadcasting transducer and one receiving transducer, was used to insonify the defect taking advantage of multiple linear scattering as mode conversion and boundary reflections. The robustness of this technique was experimentally demonstrated on a damaged sandwich panel, and the nonlinear source, induced by low-velocity impact loading, was retrieved with a high level of accuracy. Its minimal processing requirements make this method a valid alternative to the traditional nonlinear elastic wave spectroscopy techniques for materials showing either classical or non-classical nonlinear behavior.

Baseline-free estimation of residual fatigue life using a third order acoustic nonlinear parameter.

Prediction of crack growth and fatigue life estimation of metals using linear/nonlinear acousto-ultrasound methods is an ongoing issue. It is known that by measuring nonlinear parameters, the relative accumulated fatigue damage can be evaluated. However, there is still a need to measure two crack propagation states to assess the absolute residual fatigue life. A procedure based on the measurement of a third-order acoustic nonlinear parameter is presented to assess the residual fatigue life of a metallic component without the need of a baseline. The analytical evaluation of how the cubic nonlinear-parameter evolves during crack propagation is presented by combining the Paris law to the Nazarov-Sutin crack equation. Unlike other developed models, the proposed model assumes a crack surface topology with variable geometrical parameters. Measurements of the cubic nonlinearity parameter on AA2024-T351 specimens demonstrated high sensitivity to crack propagation and excellent agreement with the predicted theoretical behavior. The advantages of using the cubic nonlinearity parameter for fatigue cracks on metals are discussed by comparing the relevant results of a quadratic nonlinear parameter. Then the methodology to estimate crack size and residual fatigue life without the need of a baseline is presented, and advantages and limitations are discussed.