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Abstract In recent years, solar energy becomes one of the most popular renewable energy resources. Among the solar energy systems, solar cells are considered to be one of the most critical components, which are naturally fragile during either fabrication or service. Thus, nondestructive examination of solar cells is required practically. In this research, solar cells made from monocrystalline silicon are selected as the typical samples. The air-coupled ultrasonic nondestructive testing system, based on the generation and reception of the A0 mode Lamb waves, will be adopted for defects detection. Firstly, the dispersion curves of Lamb waves in anisotropic monocrystalline silicon are calculated theoretically, which provides a theoretical foundation for analyzing the behavior of Lamb waves propagating in silicon solar cells. Then, experiments on monocrystalline silicon solar cells are carried out by introducing the air-coupled ultrasonic Lamb waves. The amplitude distributions of Lamb waves propagating along various rotation angles in silicon solar cells are obtained. It shows a strong anisotropic properties of the tested samples. Finally, artificial cracks in solar cells are scanned circularly and laterally. The results of amplitude distributions give the scattering pattern of cracks, which can be used to determine whether the crack breaks through or not. And the extracted amplitude cross-correlation coefficients can help to estimate the length of them. [ABSTRACT FROM AUTHOR]

Abstract In the search for ever more sensitive non-destructive testing (NDT) techniques to detect progressively smaller defects in increasingly complex industrial materials, environments and geometries, conventional eddy-current testing (ECT) methods, have reached their sensitivity limit. Fortunately, advanced resonance-based techniques, demonstrated in recent years, promise to help extend that limit, but, before such techniques can be implemented with confidence, a detailed understanding of their sensitivity and stability must be achieved. In this study, statistical probability-of-detection (PoD) analysis was performed to assess the sensitivity of a novel single-frequency, near electrical resonance signal enhancement (SF-NERSE) technique relative to an equivalent conventional probe operation. This study was performed on 36 real fatigue defects in Titanium 6V-4Al (Ti6-4) with defects ranging from 0.10 − 6.48 mm in surface extent. In addition, a critical evaluation of background noise stability in the SF-NERSE technique (a common concern with NERSE-based methods) was also performed to establish the viability of such a technique in relation to industrial inspection and assessment criteria. The results of this study demonstrate a sensitivity enhancement of up to 20% for the SF-NERSE method over conventional operation and, through controls, confirmed that the effect is a result of the resonance-shifting phenomenon and not just an increase in operational frequency. In addition, an examination of background noise implies that a SF-NERSE method exhibits more stable background noise than conventional excitation. This study validates the SF-NERSE technique and methodology as a viable industrial inspection technique, able to significantly improve detection capabilities. [ABSTRACT FROM AUTHOR]

Accurate non-destructive testing and evaluation of cracks is important for the quality assurance and life-cycle management of key components in aerospace and energy industries. This paper presents a novel imaging method for cracks detection in complex curved structure , combining virtual source technique, multi-mode total focusing method and phase coherence imaging method. The virtual source on the sample surface is employed as the emitter and receiver for the ultrasonic array imaging. In this way, the calculation complexity of imaging algorithm due to beam path search can be reduced, and the detection sensitivity in the region of interest can be improved. In addition, the phase coherence factor is used to suppress the background noise and imaging artifacts, which significantly improves the quality of ultrasonic array imaging. A complex curved sample with variable thickness and non-developable surface was used to examine the effectiveness of the virtual source total focusing method. The signal-to-noise ratio of the crack defect is increased by more than 25 dB using the novel method compared with that of the conventional total focusing method, and the quantitative evaluation error of defect is less than 2.67%. • A novel virtual source total focusing method is proposed for crack detection in the complex curved structure. • The phase coherence factor is used to suppress background noise and artifacts. • The detection sensitivity of the region of interest is significantly improved. • The effect of virtual source spacing on post-processing imaging is analyzed. [ABSTRACT FROM AUTHOR]

In this study, laser-generated surface acoustic wave (SAW) interaction with surface-breaking cracks is numerically investigated to identify the relationship between crack depth and features of SAWs. The Scanning Laser Source (SLS) technique is utilized to detect and quantify cracks by monitoring the changes of the SAWs as a laser source scans over the uniform and detective area. The simulation results show that crack depth can be described by several important features of the transmitted waves and reflected waves. These features are used as inputs to a quantitative machine learning approach for crack-depth evaluation based on a Neural Network (NN) optimized with Particle Swarm Optimization (PSO) algorithm. The results show the feasibility of the proposed machine learning method to estimate the crack depth rapidly and accurately using SLS data. [ABSTRACT FROM AUTHOR]

In this paper, an in situ technique for the localisation and the sizing of cracks in plate-like structures is presented. The excitation and the reception of the ultrasonic diagnostic signal – Lamb waves – are achieved by permanently installed piezoelectric ceramic disks. The technique simplifies the underdetermined problem of the full characterisation of cracks to an over-determined problem of the localisation of crack tips. Also, it overcomes the difficulties associated with using the pitch-catch excitation scheme for crack characterisation. The development of the technique was done systematically using analytic models and the finite element method. The technique has been validated by an experiment and assessed by extensive parametric studies. While it was designed to be a convenient approach, highly accurate crack characterisation has been attained as evident through the results of both numerical studies and physical experiments. [ABSTRACT FROM AUTHOR]

A significant requirement of low-cost sensor systems for defect detection and characterization is to bridge the gap of nondestructive testing & evaluation (NDT&E) and structural health monitoring (SHM). This paper introduces an ultra-high frequency (UHF) passive RFID sensor system for crack detection and potential structural monitoring. The working principles and major research challenges are presented for the new concept of NDT&E sensor systems. One novel approach using 3D antenna and kernel principal component analysis (PCA) is investigated and validated in conjunction with two case studies for evaluation of open and closed cracks. This proposed method can partly alleviate the nonlinearity from wireless channel and demonstrate the feasibility of the in-situ motoring through amplitude and phase of tag’s backscattered signals. [ABSTRACT FROM AUTHOR]

Steel T-welded joint structures have been largely applied in aerospace and marine infrastructures. The weld length in T-weld joint structures is usually long, and it is difficult to access the weld conditions for damage detection purposes. This is due to the T-welded joint structure possessing a more complex cross-section than the conventional welded plate-like structures. Rather than formed by two plates and joined with a butt weld, the joint regions in the T-welded joint structure are connected to the sides of three plates to form a T-welded joint, and hence cannot be considered as plate-like structures. This makes guided wave modes difficult to be identified and exploited. This paper assesses the feasibility to detect weld defects on a steel T-welded joint structure using the feature guided wave (FGW) technique. FGW technique can provide a fast and long-range inspection for local topographical features, such as welded joints, offering great potential for effective screening of damages. Semi-Analytical Finite Element (SAFE) method is applied to acquire modal solutions in the waveguide. A new algorithm is developed to obtain FGW modes with both high energy concentration and low attenuation from the solutions of SAFE eigenvalue problem. One particular FGW mode with similar mode shape as the Rayleigh surface wave is identified, of which the propagation occurs only in the joint region with small attenuation. This FGW, labelled Rayleigh T-welded wave (RTW), is explored in comprehensive experimental and numerical studies to investigate the propagation and backward scattering characteristics regarding optimal excitation frequency and different sizes of weld defect. The numerical and experimental results are in good agreement. The outcome of the study confirms the capability and robustness of RTW in detecting small weld defects, which allows a long-distance and sensitive screening for the small weld defects at the joint region in the T-welded joint structure. [ABSTRACT FROM AUTHOR]

An investigation was performed into the effects of operating an absolute eddy-current testing (ECT) probe at frequencies close to its electrical resonance. A previously undocumented defect signal enhancement phenomenon, resulting from associated shifts in electrical resonant frequency, was observed and characterized. Experimental validation was performed on three notch defects on a typical aerospace superalloy, Titanium 6Al-4V. A conventional absolute ECT probe was operated by sweeping through a frequency range about the electrical resonance of the system (). The phenomenon results in signal-to-noise ratio (SNR) peak enhancements by a factor of up to 3.7, at frequencies approaching resonance, compared to those measured at 1 MHz. The defect signal enhancement peaks are shown to be a result of resonant frequency shifts of the system due to the presence of defects within the material. A simple, operational approach for raising the sensitivity of conventional industrial eddy-current testing is proposed, based on the principles of the observed near electrical resonance signal enhancement (NERSE) phenomenon. The simple procedural change of operating within the NERSE frequency band does not require complex probe design, data analysis or, necessarily, identical coils. Therefore, it is a valuable technique for improving sensitivity, which complements other ECT methods. [ABSTRACT FROM AUTHOR]

This paper presents the fundamental mathematics to determine the minimum crack width detectable with a terrestrial laser scanner in unit-based masonry. Orthogonal offset, interval scan angle, crack orientation, and crack depth are the main parameters. The theoretical work is benchmarked against laboratory tests using 4 samples with predesigned crack widths of 1–7mm scanned at orthogonal distances of 5.0–12.5m and at angles of 0°–30°. Results showed that absolute errors of crack width were mostly less than 1.37mm when the orthogonal distance varied 5.0–7.5m but significantly increased for greater distances. The orthogonal distance had a disproportionately negative effect compared to the scan angle. [ABSTRACT FROM AUTHOR]

The detection of crack and their measurement in the packed ceramic tiles is an important procedure for quality monitoring and testing in the tile manufacturing industry. In the investigation of undercover targets high-definition (HD) cameras or surface crack detection methods like eddy current testing, etc. fail but millimeter-wave imaging can be useful. In our experimental work, cracked tiles are covered with the packaging cardboard sheet, and the unknown crack is detected using Millimeter-Wave Imaging. In this paper, an adaptive and statistical approach is proposed for crack detection and a computer-aided non-invasive technique is developed to measure a crack length. After critically analyzing various available signal and image processing techniques, a line enhancement known as Frangi filtering is selected for crack detection. In the current scenario; the decision parameter for the Frangi filter known as the Frangi scale ratio is required to be fixed manually, to minimize the selection error automation of this process is done using a Genetic Algorithm (GA). In literature, the focus is limited to only the detection of cracks in the tile, but the estimation of the crack length is also essential. The possible use of image morphing techniques for the measurements of the crack length is explored. It is observed that the conventional Hough transform fails to measure the length of the individual crack segment, hence the pixel connectivity enforcement for hole filling and local intensity distribution equalization is proposed to extract the approximate coefficients belonging to the crack segment. The raw data collected previously from our laboratory is used and processed using the proposed methodology, the obtained results are satisfactory and very encouraging as compared to earlier results reported in the literature. [ABSTRACT FROM AUTHOR]

Abstract: Detecting the orientation of deep oriented cracks is a major challenge in the development of Eddy-Current (EC) Non-Destructive Testing (NDT). In fact, the detection sensitivity of EC-NDT depends on the interaction between the crack length direction and the EC flowing in the materials. In conventional EC-NDT systems, the induced currents are primarily generated along a single direction in the tested sample. This paper presents a new excitation method for generating a pseudo-rotating ac magnetic field and, consequently, pseudo-rotating eddy currents. This method significantly improves the detection of deep cracks of any orientation. The detected signal due to the crack is measured by an Improved Giant Magneto-Resistance Magnetometer (IGMRM). The magnetic flux density signature of the crack is studied using a 3D finite element model. Numerical and experimental results demonstrate the potential and the efficiency of this method. [Copyright &y& Elsevier]

Abstract: This paper proposes an approach to detecting and characterizing structural cracks emanating from rows of rivet holes in thin metallic plates using lamb wave, wavelet transform and neural networks. When lamb waves propagate through rows of holes, there exist strong wave interferences among different transmitted waves which make health diagnosis complicated. An active sensing network is mounted on the plate and wavelet transform is used to extract a robust and effective feature called energy ratio change from time domain signals. The relationship between energy ratio change and the crack characteristics is analyzed and the effective detection paths are found out. Neural networks are then developed using the feature to diagnose health condition in stages. One neural network is first used to diagnose plate integrity. If cracks are detected, then the second neural network is called to determine their locations. The method is first examined in simulation data, then the NNs trained by simulation data are used to diagnose real plates. The results shows that the method can effectively detect cracks and identify their locations in both simulation and experimental data, hence demonstrating the feasibility of using the method to develop built-in real time intelligent diagnosis systems. [Copyright &y& Elsevier]

Abstract: Fatigue cracks are common and potentially critical defects in metallic plate-like structures, and ultrasonic guided wave methods provide an efficient and relatively low-cost means of crack detection and monitoring. However, widely used baseline subtraction methods may fail under mismatched environmental and operational conditions. In particular, varying applied loads change not only the contact state of a crack but also specimen dimensions and wave speeds, which affect the ultrasonic signal response. The load dependence of crack opening provides a possibility for enhanced crack detection, which is well-known for higher frequency bulk waves. A load-differential method is proposed in this paper whereby guided wave signals obtained at different loads under the same damage state are compared without utilizing previously recorded damage-free data. To demonstrate this method, a fatigue test was performed on an aluminum plate specimen instrumented with a sparse array of piezoelectric transducers. Signal changes due to crack opening effects caused by increasing tensile loads are visualized using delay-and-sum imaging. The results show that the load-differential method is capable of detecting cracks and visualizing their locations. [Copyright &y& Elsevier]

Abstract: The performance of electromagnetic-thermal non-destructive inspection is investigated experimentally by eddy current thermography. The experiments concerned the detection of the crack in ten square aluminum plates at various positions and orientations. For the excitation are employed two circular coils with an outer diameter either smaller or comparable to the plate side. Crack detection is based not only on the thermograms, but on data processing techniques as well. Despite the fact that the performance of the infrared camera used is relatively low and the exciting frequency was only 50Hz, the coil with small section detected effectively the nine cracks out of the ten while the coil with large section detected effectively seven out of ten. [Copyright &y& Elsevier]

Abstract: Computed tomography (CT) technology is very versatile and is not restricted by the size, shape or composition of an object as long as adequate X- or γ-ray transmission can be achieved. Fatigue cracking usually starts as microcracks that later develop to form macrocracks that propagate due to tensile or shear stress, or combinations of both, causing disintegration and final failure of material because of unstable crack growth. Pavement serviceability is reduced as these cracks propagate and disintegrate occurs. Mixtures resistant to crack development and propagation affect the cracking performance of asphalt pavements. Therefore, it is necessary to acquire more insight into the crack behavior of asphalt concrete mixtures in order to obtain a better understanding of the cracking mechanism of asphalt pavements and to have a practical and reliable system to determine the resistance of mixture to crack development and propagation. In this study, which has applied CT to the study of asphaltic mixture, it was required to detect and follow the evolution of a crack for mixture submitted to fatigue tests. The fatigue tests were done through use of diametral compression test, which provided repeated loading. The trajectory of a crack is influenced by original voids in the specimen. It may be noted that voids exist at the same positions in all tomograms. With sufficient sequence of applied loadings these voids can be transformed into small crack that may increase in size, causing the collapse of the specimen. [ABSTRACT FROM AUTHOR]

Abstract: Comparison of recent theoretical estimates with experiments has indicated that the ultimate sensitivity of the conventional modulation technique of crack detection is mainly determined by the background modulation produced by the quadratic component of the atomic nonlinearity of the matrix material. Much smaller level of masking nonlinear effects is typical of higher-order interactions due to cubic and higher-order components in the power-series expansion of the background nonlinearity of the solid. In contrast, the level of formally higher-order components originated due to nonlinearity of crack-like defects can be comparable with that of the first-order components. Such strongly increased efficiency of higher-order interactions is due to the fact that crack-like defects often demonstrate non-analytic (non power-law) nonlinearity even for moderate acoustic amplitudes. Besides the increased level, the higher-order components arisen due to non-analytic nonlinearity of cracks can demonstrate significantly different functional behavior compared to manifestations of the atomic nonlinearity. This difference can also help to discriminate the contributions of the defects and the background atomic nonlinearity. Here, we focus on the main differences between the modulation components arisen due to cubic terms in the power-series expansion of the atomic nonlinearity and similar components generated by clapping Hertzian nonlinearity of inner contacts in cracks. We also examine experimental examples of higher-order modulation interactions in damaged samples. These examples clearly indicate non-analytical character of the defects’ nonlinearity and demonstrate that the use of higher-order modulation effects can significantly improve the ultimate sensitivity and reliability of the modulation approach to detection of crack-like defects. [ABSTRACT FROM AUTHOR]

Abstract: Imaging-based inspection methods are increasingly being employed for crack detection in concrete structures, because they provide quantitative information compared to inspections based solely on conventional visual approaches. However, efficient image analysis methods are needed. This study proposes the application of the grey level co-occurrence matrix (GLCM) texture analysis approach and an artificial neural network (ANN) classifier to obtain surface damage information, such as the total amount of superficial cracking, as well as the total length, and range of crack widths. These methods were applied to thermographic, visual colour and greyscale images of concrete blocks from CANMET that were exposed outdoors for ten years, as well as slabs from GRAI that were kept indoors, all specimens exhibiting various levels of alkali-aggregate reaction (AAR) damage. Results of the classifications show that the greyscale imagery performed fairly well, with an overall classification accuracy range of 72.3–76.5% for the CANMET blocks, and 68.7–75.3% for the GRAI slabs. Classifications using the colour imagery were slightly better than the greyscale imagery, with accuracies ranging from 71.4% to 75.2% for CANMET blocks and 70.9–72.0% for the GRAI slabs. The thermographic imagery, however, produced the highest overall classification accuracies, which range from 73.1% to 76.3% for the CANMET blocks and 74.2–76.9% for the GRAI slabs. The results show that all three types of imagery are relatively effective in characterizing and quantifying crack damage; however, the infrared thermography produced more accurate results compared to the visual colour, and greyscale images. [Copyright &y& Elsevier]

Abstract: A statistical non-contacting and non-intrusive method for revealing the presence of cracks in unsintered (green) parts manufactured by powder metallurgy (PM) technology was developed based on photothermal radiometry (PTR). The technique relies on the interaction of a modulated laser-generated thermal wave with the crack resulting in change of amplitude and phase of the detected signal. The crack existence at points in high stress regions of a group of green sprockets was evaluated through the proposed method. The results were validated by an independent destructive technique—microscope observation of the tested green sprockets following sintering, sectioning, and polishing at the locations where signal changes were observed in the green state. Statistical analysis confirmed the excellent sensitivity (91%) of the method in detecting the presence of hairline (∼5–10μm) cracks. This PTR diagnostic technique may lead to a simple and reliable on-line inspection methodology in high-stress locations of PM manufactured industrial steel products. Ultimately, the method can be developed for non-destructive quality and feedback control of the metal forming process of green automotive parts. [Copyright &y& Elsevier]

Abstract: Despite the increasing number of impressive demonstrations of the high sensitivity of nonlinear-acoustic methods of damage detection, the physical factors which limit the ultimate sensitivity of this approach are not yet clearly understood. In this paper we perform the corresponding analysis and formulate criteria determining the minimal size of detectable cracks. The relation of the nonlinear-modulation technique to the linear frequency-shift technique is discussed. The analysis is particularly focused on the nonlinear-modulation resonance technique. It is based on rather general properties of the defects consistently predicted by various rigorous cracks models. The obtained conclusions are compared with experimental data obtained in nonlinear-modulation experiments on the detection of cracks in railway axles and wheels. [Copyright &y& Elsevier]

Abstract: This contribution deals with the use of coupled Rayleigh-like waves for the remote detection of defects in stiffened plates similar to aerospace structures, where access is restricted by regularly spaced features such as stiffeners or stringers. Selecting appropriate excitation frequency and position, it is shown experimentally that a significant part of the energy can be transmitted past multiple obstacles, which act as a band-pass filter. Using a standard Rayleigh angle beam wedge transducer in a pulse-echo configuration, a surface defect (notch) located behind three stiffeners more than half a meter from the measurement position is detected experimentally. The detection sensitivity for different defect depths and locations relative to the structural features is discussed based on finite-difference simulations. [Copyright &y& Elsevier]

Abstract: Surface-breaking cracks affect the material and structural properties of concrete members, e.g. concrete plates. Therefore, the non-destructive measurement of the crack depths is important to assess the structural strength and serviceability of concrete plates. In this paper, a theoretical transfer function for a pulse propagating as Lamb modes is used to study the effects from the dispersion of Lamb modes and the participation of high-order Lamb modes. It is found that the fundamental symmetric Lamb mode dominates when a vertical Morlet pulse is applied at the surface; thus, wavelet transform is used for the analysis of the surface responses to extract information of the fundamental Lamb mode. The recently proposed wavelet transmission coefficient (WTC) is sensitive to normalized crack depths up to one-third of the wavelength. The crack depth is successfully evaluated not only in numerical simulations but also in experimental tests. [Copyright &y& Elsevier]

Abstract: Numerical experiment results are processed by various techniques and the improvements in electromagnetic-thermal non-destructive inspection are investigated. The performances of image subtraction, depiction of the norm of the spatial derivatives of temperature and the Fourier transform are evaluated. Compared to the other methods, the cases for which the results are improved by image subtraction are relatively small. The depiction of the norm of the spatial derivatives is more effective, with the 2nd derivative giving better results than the 1st one. The most effective technique is the Fourier transform, which gives satisfactory results even for very distant cracks. The phase images are sharper than the amplitude ones only in cases where the crack is perpendicular to the current flow. Otherwise, the amplitude images are sharper. [Copyright &y& Elsevier]

Abstract: High-speed ACFM tests were carried out using a rotary test piece that contained spark-eroded notches. The ACFM sensor detected the induced notches during inspection at 121.5km/h. The recorded signal remained unaffected by the increases in inspection speed under constant lift-off. To simulate actual rail inspection conditions at high speed, further tests were carried out using a spinning rail rig and a set of rails that contained spark-eroded notches of various shapes and sizes up to a speed of 32km/h. Although, the ACFM sensor successfully detected the majority of the notches, the signal obtained was affected by lift-off variations. [Copyright &y& Elsevier]

Abstract: Structures are weakened by cracks. When the crack size increases in course of time, the structure becomes weaker than its previous condition. Finally, the structure may breakdown due to a minute crack. Therefore, crack detection and classification is a very important issue. In this study, free and forced vibration analysis of a cracked beam were performed in order to identify the crack in a cantilever beam. Single- and two-edge cracks were evaluated. The study results suggest that free vibration analysis provides suitable information for the detection of single and two cracks, whereas forced vibration can detect only the single crack condition. However, dynamic response of the forced vibration better describes changes in crack depth and location than the free vibration in which the difference between natural frequencies corresponding to a change in crack depth and location only is a minor effect. [Copyright &y& Elsevier]

Abstract: The performance of circular and square coils in electromagnetic–thermal non-destructive inspection of conductive plates is investigated numerically for various configurations. A coil is most effective when placed at an optimum distance from the plate, equal to 1/4 of the diameter for a circular coil or 1/4 of the side for a square coil. At this distance, the coils considered are equally efficient in detection of cracks. Substantial differences exist only when the coils are placed very close to the plate but then become less effective. For that case, the planar coils give better results. A coil with a section comparable in area with the inspected surface cannot inspect the whole surface effectively in a single inspection. The support of a smaller section coil is necessary in order to clarify the shape of the cracks that are not clear. [Copyright &y& Elsevier]

Abstract: The effectiveness of an electromagnetic-thermal method for non-destructive testing of conductive materials is investigated numerically in thin conductive plates. The detection region is related to the diameter of the exciting coil while the detection period is affected from the crack length and its distance from the coil. Both, the detection range and detection period depend on the crack orientation with respect to the heat flow, the magnetic flux density and the heating time. By proper selection of the coil positions, during a second inspection it is possible to elongate the detection period for the ambiguous cracks and make clear their shape. [Copyright &y& Elsevier]

Abstract: Novel nonlinear-modulation methods for crack detection are discussed. The approach is based on the so-called cross-modulation effect consisting of the modulation transfer from an intensive, initially slowly amplitude-modulated stronger (pump) excitation to the probe signal. Advantage of this technique is a very flexible choice of the operation frequencies, since their ratio for both carriers and the modulation may be rather arbitrary. This in its turn allows one to effectively use the sample resonances in order to achieve the necessary level of the pump excitation and to ameliorate conditions for detection of the modulation sidelobes for the probe wave. Unlike higher harmonic-generation methods the initial nonlinear distortions of the pump and probe excitations (e.g. due to nonlinearities in the electronics) are not critical for this technique. In the paper we summarise results of recent test experiments that indicated high sensitivity of the new technique. [Copyright &y& Elsevier]

Abstract: In this paper, a new method for crack detection in beams based on instantaneous frequency and empirical mode decomposition is proposed. The dynamic behaviour of a cantilever beam with a breathing crack under harmonic excitation is investigated both theoretically and experimentally. A simple single-degree-of-freedom system with varying stiffness is employed to simulate the dynamic behaviour of the beam. The time-varying stiffness is modelled using a simple periodic function. Both simulated and experimental response data are analysed by applying empirical mode decomposition and Hilbert transform and the instantaneous frequency of each oscillatory mode is obtained. It is shown that the instantaneous frequency oscillates between frequencies corresponding to the open and closed states revealing the breathing of the crack. The variation of the instantaneous frequency increases with increasing crack depth following a polynomial law and consequently can be used for estimation of crack size. Using the intrinsic modes of the system, the harmonic distortion of the distorted sinusoidal response is calculated. It follows that the harmonic distortion increases with crack depth following definite trends and can be also used as an effective indicator for crack size. The proposed time–frequency approach is superior compared to Fourier analysis and can be used to improve the effectiveness of vibration-based crack detection techniques. [Copyright &y& Elsevier]

Abstract: Defects influence in a negative way the service life of structures. Thus, detection of them even at a very small size is a very important point of view to guarantee structural safety and to safe costs. The objective of this study is to obtain information about the location and depth of cracks in cracked beams. For this purpose, the vibrations as a result of impact shocks were analyzed. The signals obtained in defect-free and cracked beams were compared in the frequency domain. The results of the study suggest to determine the location and depth of cracks by analyzing the from vibration signals. Experimental results and simulations obtained by the software ANSYS are in good agreement. [Copyright &y& Elsevier]

In this paper the dynamic behaviour of a cantilever beam with a breathing crack is investigated both theoretically and experimentally. The primary aim is to reveal the nonlinear behaviour of the system by using time frequency methods as an alternative to Fourier analysis methodology. A simple single-degree-of freedom lumped system is employed to simulate the dynamic behaviour of the beam. The time varying stiffness is modelled using a simple periodic function. Both simulated and experimental response data are analysed by applying empirical mode decomposition and Hilbert transform and the instantaneous frequency (IF) is obtained. It is shown that the IF oscillates between the frequencies corresponding to open and closed states revealing the physical process of crack breathing. The variation of the IF follows definite trends and therefore can be used as an indicator of the crack size. It allows an efficient and accurate description of the nonlinearities caused by the presence of a breathing crack. Consequently, it can be used to improve vibration based crack detection techniques. [Copyright &y& Elsevier]

A new method for monitoring the evolution of gear faults based on the calculation of local energy density is presented. A theoretical model for a gear pair with a tooth root crack was developed. Experimental results from a test rig were analyzed using the continuous wavelet transform and the energy density of the vibration signal associated with the meshing of defected gears was calculated. It was shown that the local energy is a sensitive feature for assessing the fault magnitude. An empirical law, which relates the energy content to the depth of the crack, was established. [Copyright &y& Elsevier]

The detection of crack-like defects using ultrasound is a problem widely studied by the non-destructive evaluation community. In this work, a technique of how to detect the presence of a crack-like defect and estimate its orientation in an anisotropic material by means of two orthogonal shear waves is presented. A detailed study of shear wave propagating and splitting at any incident angle, and its interaction with crack-like defects in anisotropic materials, is also presented. In this paper it is shown that by the use of two orthogonal shear waves with linear polarisation the authors could: 1) estimate the material level of anisotropy within approximately 0.5 % of the measured value; 2) estimate the angular position of the principal shear directions of the material with respect to the orthogonal shear wave sources within ± 5 ∘ accuracy; 3) excite a shear wave purely polarised along one of the principal shear directions with two orthogonal shear waves placed at an arbitrary angular position; 4) detect the presence of a crack-like defect in an anisotropic material based on the back-wall echoes of two orthogonal shear waves, 5) estimate the orientation of a crack-like defect in an anisotropic material within ~ ± 5 ∘ of the theoretical value. The simulation and experimental results obtained show that the technique presented in this paper has potential to become an effective tool for crack-like defect detection and material characterisation. [ABSTRACT FROM AUTHOR]

The use of polarised shear waves to detect the presence of crack-like defects seems to have received little or no attention in the past. The authors believe that the main reason for this appears to be the lack of a device with the capability to excite shear waves of different polarisations. In this paper, the authors, first, present the design of an EMAT that permits the excitation of two orthogonally polarised shear waves in metallic materials by means of two coils that are orthogonal with respect to each other. This is then followed by a 3D finite element analysis of the wavefield generated by the EMAT and its interactions with crack-like defects of different sizes, positions and orientations. Then a methodology of how this EMAT can be used to simultaneously measure material thickness and detect crack-like defects in pulse-echo mode is introduced. Good agreement between the finite element simulation and experimental results was observed which makes the presented technique a potential new method for simultaneous thickness measurements and crack detection. [ABSTRACT FROM AUTHOR]

Automatic segmentation of cracks using crack detection methods fails when applied on concrete with embedded reinforcements; it results in artifacts called false-detected cracks. We present a two-step approach to trace the cracks which reduces significantly detecting the false ones. In the first step, the reinforcements were found and removed from the 3D data set, and in the second step, the cracks were detected in the 3D data set free from reinforcements. To visualize and determine the structure and the degree of damages nondestructively, planar tomography as a special case of laminography was used. [ABSTRACT FROM AUTHOR]

This study proposed a long-range non-destructive pipe inspection approach using linearly-polarized circular TE 11 mode microwaves, enabling rapid detection of both axial and circumferential cracks on the inner surface of a metallic pipe. The applicability of the TE 11 mode microwaves for crack detection was discussed in the light of the currents induced on the inner surface of a pipe due to the propagation of the TE 11 mode microwaves. A dual-bend structured mode converter was designed to propagate linearly-polarized TE 11 mode microwaves inside a pipe with a certain inner diameter, based on the theoretical and numerical analysis of mode conversion due to a bend. A 3.5 GHz working bandwidth of the mode converter was achieved with a conversion efficiency of the TE 11 mode of over 90%. Experimental verification was conducted using stainless-steel pipes with an inner diameter of 23 mm and a total length of 7 m. An axial and a circumferential slit were introduced to simulate cracks. The experimental results confirmed reflections from both the axial and circumferential slits, indicating that the TE 11 mode microwaves are applicable to crack detection. The results also confirmed the correlation between TE 11 mode polarization and its detection sensitivity toward slits with different orientations along the circumferential direction. Furthermore, the axial and circumferential slits situated at different longitudinal and circumferential locations in the pipe were successfully detected by orthogonally deploying the mode converter to change the polarization of the TE 11 mode by 90°. [ABSTRACT FROM AUTHOR]