Showing papers in "Journal of Nondestructive Evaluation in 2014"

Resonant Acoustic Nonlinearity of Defects for Highly-Efficient Nonlinear NDE

Abstract: In this paper, the effect of local defect resonance (LDR) on the nonlinear ultrasonic responses of defects is studied and applied for enhancement of sensitivity of nonlinear NDE. Unlike the resonance of the whole specimen, the LDR provides an efficient energy pumping from the wave directly to the defect and causes an efficient generation of the higher harmonics and wave mixing even at moderate input signals. At higher levels of excitation, a combined effect of LDR and nonlinearity results in qualitatively new “nonclassical” features characteristic of the nonlinear and parametric resonances. The resonant nonlinear defects demonstrate threshold dynamics of instable vibrations, hysteresis, super- and subharmonic resonances. Under nonlinear LDR conditions nearly total input energy can be converted into higher harmonic or subharmonic vibrations of the defect. This proposes nonlinear LDR application as an extremely efficient and sensitive mode for nonlinear imaging and NDE.

Guided Wave Damage Characterization via Minimum Variance Imaging with a Distributed Array of Ultrasonic Sensors

Abstract: Guided wave imaging with a distributed array of inexpensive transducers offers a fast and cost-efficient means for damage detection and localization in plate-like structures such as aircraft and spacecraft skins. As such, this technology is a natural choice for inclusion in condition-based maintenance and integrated structural health management programs. One of the implementation challenges results from the complex interaction of propagating ultrasonic waves with both the interrogation structure and potential defects or damage. For example, a guided ultrasonic wave interacts with a surface or sub-surface defect differently depending on the angle of incidence, defect size and orientation, excitation frequency, and guided wave mode. However, this complex interaction also provides a mechanism for guided wave imaging algorithms to perform damage characterization in addition to damage detection and localization. Damage characterization provides a mechanism to help discriminate actual damage (e.g. fatigue cracks) from benign changes, and can be used with crack propagation models to estimate remaining life. This work proposes the use of minimum variance imaging to perform damage detection, localization, and characterization. Scattering assumptions used to perform damage characterization are obtained through both analytical and finite element models. Experimental data from an in situ distributed array are used to demonstrate feasibility of this approach using a through-hole and two through-thickness notches of different orientations to simulate damage in an aluminum plate.

Damage Classification of Sandwich Composites Using Acoustic Emission Technique and k-means Genetic Algorithm

Abstract: In this study acoustic emission (AE) technique was used for monitoring mode I delamination test of sandwich composites. Since, during mode I delamination test various damage mechanisms appear, their classification is of major importance. Hence, integration of $$k$$ -means algorithm and genetic algorithm was applied as an efficient clustering method to discriminate different failure modes. Performing primary experiments to find the relationship between AE parameters and damage mechanisms, the AE signals of obtained clusters were assigned to distinct damage mechanisms. Also, the dominance of damage mechanisms was determined based on the distribution of AE signals in different clusters. Finally SEM observation was employed to verify obtained results. The results indicate the efficiency of the proposed method in damage classification of sandwich composites.

Effect of Localized Microstructure Evolution on Higher Harmonic Generation of Guided Waves

Abstract: The use of nonlinear ultrasonics to characterize microstructural evolution is investigated with the aim of enabling earlier remaining useful life prediction and thereby greatly improving condition based maintenance. Higher harmonic generation is sensitive to microstructural features, whose evolution is indicative of ongoing damage processes. Localized plastic deformation is controlled in an aluminum sample by varying the notch length, which dictates the extent of the plastic zone. The essentials of higher harmonic generation analysis for ultrasonic guided waves are highlighted to provide a means to select a primary mode that generates a strong higher harmonic. Experimental methods to use magnetostrictive transducers for third harmonic generation measurements are described. Experimental results on aluminum plates indicate that plastic deformation increases the third harmonic by up to a factor of five and that the harmonic amplitude ratio $$A_{3}$$ / $$A_{1}^{3}$$ is sensitive to the plastic strain magnitude. These initial results show that when the plastic strain is localized, the $$A_{3}$$ / $$A_{1}^{3 }$$ ratio appears to be proportional to the plastic zone-to-propagation distance ratio.

Detecting Localized Plastic Strain by a Scanning Collinear Wave Mixing Method

Abstract: When the frequencies of a pair of collinear shear and longitudinal waves satisfy the resonant condition, mixing of these two primary waves generates a third, resonant shear wave that propagates in the direction opposite to the propagating direction of the primary shear wave. In this study, experiments were conducted to demonstrate that the acoustic nonlinearity parameter at the location of the mixing zone can be obtained by measuring the resonant shear wave. Since the location of the mixing zone can be controlled by adjusting the trigger time of the transducers that generate the primary waves, this collinear wave mixing technique enables the scanning of a bar sample to measure the distribution of acoustic nonlinearity along the bar. To demonstrate this scanning capability, bar samples with non-uniform acoustic nonlinearity parameters were fabricated by inducing localized plastic deformation at known locations. Scanning collinear wave mixing tests conducted on such bar samples clearly identified the locations of the plastic zone. These results show that collinear wave mixing is a promising method for scanning the test sample to map out the distribution of localized plastic deformation.

Investigation of Magnetic Memory Signals Induced by Dynamic Bending Load in Fatigue Crack Propagation Process of Structural Steel

Abstract: Metal magnetic memory effect, induced by applied stress under the excitation of the geomagnetic field, has attracted a lot of attentions due to its unique advantages of stress concentration identification and early damage detection for ferromagnetic materials. To further investigate the regularity of magnetic memory signals in the fatigue crack propagation process under the dynamic bending load, the surface magnetic field intensity $$H_{p}(y)$$ of ferromagnetic structural steel was measured throughout the dynamic three-point bending fatigue tests; variation of $$H_{p}(y)$$ and its maximum gradient $$K_{max}$$ were studied; meanwhile the possibility of using $$K_{max}$$ to predict the fatigue crack propagation was discussed. The results showed that $$H_{p}(y)$$ was relatively stable at different loading cycles and its maximum value appeared at the fatigue crack area before the specimen fractured; instead the $$K_{max}$$ increased exponentially with the increase of loading cycles, and an approximate linear relationship was found between $$K_{max}$$ and crack length 2a. The cause for this phenomenon was also discussed.

Comparing Linear Viscoelastic Properties of Asphalt Concrete Measured by Laboratory Seismic and Tension–Compression Tests

Abstract: Seismic measurements and conventional cyclic loading have been applied to a cylindrical asphalt concrete specimen to compare the complex modulus and complex Poisson’s ratio between the two testing methods. The seismic moduli and Poisson’s ratio have been characterized by optimizing finite element calculated frequency response functions to measurements performed at different temperatures. An impact hammer and an accelerometer were used to measure the frequency response functions of the specimen which was placed on soft foam for free boundary conditions. The cyclic loading was performed by applying both tension and compression to the specimen while measuring the displacements in the axial and radial direction. The Havriliak–Negami and the 2S2P1D model have been used to estimate master curves of the complex modulus and complex Poisson’s ratio from the seismic and the tension–compression tests. The seismic measurements performed at a lower strain level than the tension–compression test give a higher absolute value of the complex moduli (e.g. \({\sim }12\,\%\) at 100 Hz) and a lower phase angle compared to the tension–compression results.

An Intelligent Methodology for Railways Monitoring Using Ultrasonic Guided Waves

Abstract: It is far from trivial to inspect railways for defections. In particular, for the foot area of the rail non destructive testing methods are known to be difficult to apply. In this paper, an ultrasonic guided wave method is considered along with classification methods for automated rail foot defect detection. In effect, given a set of gathered ultrasonic signals, multiple features are extracted from time-, frequency- and time–frequency domains. Next, a robust feature selection method is performed, to collect a small set of complementary features. The classification task is accomplished by means of a kernel-based support vector machine. To demonstrate the performance capabilities of our approach, an extensive experimental setup is designed under representative environmental and operational conditions. The sensitivity and the resolution of the proposed defect detection system are reported. A study on the influence of rail fastening on the proposed method is also reported where robust defect detection rates, greater than 93 %, are achieved assuming that a compact feature subset is considered. However, it is evident in experiments that even in the case of large defects, changes in the environmental conditions (temperature and humidity) increase the interpretation of the acquired signals, thus making the detection task more difficult.

Dynamic Acousto-Elasticity in a Fatigue-Cracked Sample

Abstract: Dynamic acousto-elasticity (DAE) provides a unique way to observe nonlinear elastic features over an entire dynamic stress cycle including hysteresis and memory effects, detailing the full nonlinear behavior under tension and compression. This supplemental information cannot be observed with conventional nonlinear ultrasonic methods such as wave frequency mixing or resonance measurements, since they measure average, bulk variations of modulus and attenuation versus strain level. Where prior studies have employed DAE in volumetrically nonlinear materials (e.g., rocks, bone with distributed micro-crack networks), here we report results of DAE on the application to a single localized nonlinear feature, a fatigue crack, to characterize the nonlinear elastic response in regions of the crack length, tip, and undamaged portions of an aluminum sample. Linear wave speed, linear attenuation and third order elastic moduli (i.e., nonlinear parameters) each indicate a sensitivity to the presence of the crack, though in unique manners. The localized nature of the DAE measurement and its potential for quantifying all of the third order elastic constants makes it a promising technique for both detecting cracks, as well as providing quantitative information on the effect of the cracks on the material integrity.

Non-linear Ultrasonic NDE of Titanium Diffusion Bonds

Abstract: Diffusion bonding is an attractive solid-state welding technique that represents a valuable tool for reducing weight and improving performance in the aerospace industry. However, its full exploitation in titanium components is currently limited by a lack of robust NDE techniques capable of overcoming the crystallographic anisotropy of these important materials. Advanced ultrasonic techniques have been explored previously, but their sensitivity to imperfections is limited by the linear acoustic phenomena on which they depend. Non-linear ultrasonic methods have been shown to be significantly more sensitive than their linear counterparts to these types of imperfections, but suppressing extraneous contributions to the non-linear response of the interface is not trivial. An approach that succeeds in suppressing such contributions is presented here. The technique, which is based on the non-collinear mixing of ultrasonic waves to generate a spectrally, modally and spatially dissociable third wave, was used to reliably characterise a set of samples whose bond quality was indeterminable using conventional ultrasonic methods.

Frequency Dependence of Second-Harmonic Generation in Lamb Waves

Abstract: The frequency dependence of the second-harmonic generation in Lamb waves is studied theoretically and numerically in order to examine the role of phase matching for sensitive evaluation of material nonlinearity. Nonlinear Lamb wave propagation in an isotropic plate is analyzed using the perturbation technique and the modal decomposition in the neighborhood of a typical frequency satisfying the phase matching. The results show that the ratio of the amplitude of second-harmonic Lamb mode to the squared amplitude of fundamental Lamb mode grows cumulatively in a certain range of fundamental frequency for a finite propagation distance. It is also shown that the frequency for which this ratio reaches maximum is close but not equal to the phase-matching frequency when the propagation distance is finite. This feature is confirmed numerically using the finite-difference time-domain method incorporating material and geometrical nonlinearities. The fact that the amplitude of second-harmonic mode becomes high in a finite range of fundamental frequency proves robustness of the material evaluation method using second harmonics in Lamb waves.

Automated Defect Recognition and Identification in Digital Radiography

Abstract: Weld quality assurance is important for the safe exploitation of many products and constructions. This paper summarizes work on an advanced system for automated radiogram analysis. The most important parts of the process of radiogram analysis such as segmentation, thresholding and defect recognition and classification are discussed. A complex classifier composed of artificial neural networks and a fuzzy logic system is proposed and discussed in detail. The proposed classifier shows better performance and flexibility than the normal neural networks classifiers.

Infrared Lock-in Thermography Crack Localization on Metallic Surfaces for Industrial Diagnosis

Abstract: Optical lock-in thermography with a modulated laser excitation is used for the qualitative assessment of surface cracks in metallic samples. In order to identify and localize an open defect, a novel dedicated image processing of the recorded IR amplitude sequence is proposed. The obtained results demonstrate the potentiality of active lock-in thermography as a contactless measurement tool for the localization of breaking cracks located into specific regions difficult to reach by other conventional non-destructive testing (NDT) techniques such as eddy currents or ultrasound techniques. Crack localization without a prior preparation of the inspected surface can be a possible alternative to penetrant inspection in industrial processes. Various applications illustrating the proposed procedure are presented.

Automatic Inspection of Spot Welds by Thermography

Abstract: The interest for thermography as a method for spot weld inspection has increased during the last years since it is a full-field method suitable for automatic inspection. Thermography systems can be developed in different ways, with different physical setups, excitation sources, and image analysis algorithms. In this paper we suggest a single-sided setup of a thermography system using a flash lamp as excitation source. The analysis algorithm aims to find the spatial region in the acquired images corresponding to the successfully welded area, i.e., the nugget size. Experiments show that the system is able to detect spot welds, measure the nugget diameter, and based on the information also separate a spot weld from a stick weld. The system is capable to inspect more than four spot welds per minute, and has potential for an automatic non-destructive system for spot weld inspection. The development opportunities are significant, since the algorithm used in the initial analysis is rather simplified. Moreover, further evaluation of alternative excitation sources can potentially improve the performance.

Discovering the Defects in Paintings Using Non-destructive Testing (NDT) Techniques and Passing Through Measurements of Deformation

Abstract: The present study is focused on two topics. The former is a mathematical model useful to understand the deformation of paintings, which uses straining devices, adjustable and micrometrically controlled through a pin implanted in a hollow cylinder. Strains were analyzed by holographic interferometry (HI) technique using an appropriate frame. The latter concerns the need to improve the conservator’s knowledge about the defect’s detection and defect’s propagation in acrylic painting characterized by underdrawings and pentimenti. To accomplish this task, a sample was manufactured to clarify the several uncertainties inherent the influence of external factors on their conservation. Subsurface anomalies were also retrieved by near-infrared reflectography and transmittography techniques, using LED lamps and several narrow-band filters mounted on a CMOS camera, working at different wavelengths and in combination with UV imaging. In addition, a sponge glued on the rear side of the canvas was impregnated with a precise amount of water by means of a syringe to verify the stretcher effect by digital speckle photography (DSP) technique (using MatPIV). The same effect also affects the sharp transition of the canvas at the stretcher’s edge. In this case, the direct mechanical contact between stretcher and canvas was investigated by HI technique. Finally, advanced algorithms were successfully applied to the square pulse thermography data to detect three Mylar $$^{\textregistered }$$ inserts simulating different types of defects. These fabricated defects were also identified by optical techniques: DSP and laser speckle imaging.

Thermal Degradation Evaluation of HP40Nb Alloy Steel After Long Term Service Using a Nonlinear Ultrasonic Technique

Abstract: Nonlinear ultrasonic analysis has been proposed to characterize the thermal degradation of HP40Nb steel associated with microstructure evolution. Measurements of ultrasonic nonlinear signals were performed in the thermally degraded damaged HP40Nb specimens and the results showed a clear change of increase-plateau-decrease tendency of the normalized nonlinear parameter with respect to the different thermal loading time. Based on metallographic studies, the variation in the measured acoustic nonlinearity reveals that the normalized acoustic nonlinearity increases due to the increases of the second phase precipitates and dislocation density in the early stage and it decreases as a combined result of the reduction of dislocation density, coarsening of precipitates and initiation of micro-voids after long-term high temperature exposure. Moreover, an analytical model calculation of precipitate–dislocation interaction has been performed to interpret the correlation between microstructural evolutions and the measured ultrasonic nonlinearity. Consequently, ultrasonic nonlinearity is found to be strongly sensitive to the microstructure evolutions during thermal degradation of HP40Nb steels.

Detection and Localization of Delaminations in Thin Carbon Fiber Reinforced Composites with the Ultrasonic Polar Scan

Abstract: In this paper, the hybrid compliance-stiffness matrix method for simulating wave propagation in (delaminated) multilayered media with viscoelastic anisotropy has been confronted with high-quality amplitude and phase experiments on delaminated composites, obtained using the ultrasonic polar scan setup (UPS) in transmission by considering harmonic as well as pulsed ultrasound. Results are presented for multiple thin carbon/epoxy laminates with an artificial edge delamination induced by a foil insert, showing a good agreement between experimental recording and numerical modeling. The obtained results further reveal the feasibility of the harmonic UPS to detect and even locate the depth-position of multiple delaminations in fiber reinforced composites. Considering that the harmonic UPS method does not rely on the detection of different echoes like the classical C-scan, but rather expounds the conditions for efficient stimulation of guided waves in the solid, the method is found to be highly suited for inspecting thin composite materials for the presence of delaminations.

Image Fusion for Improved Detection of Near-Surface Defects in NDT-CE Using Unsupervised Clustering Methods

Abstract: The capabilities of non-destructive testing (NDT) methods for defect detection in civil engineering are characterized by their different penetration depth, resolution and sensitivity to material properties. Therefore, in many cases multi-sensor NDT has to be performed, producing large data sets that require an efficient data evaluation framework. In this work an image fusion methodology is proposed based on unsupervised clustering methods. Their performance is evaluated on ground penetrating radar and infrared thermography data from laboratory concrete specimens with different simulated near-surface defects. It is shown that clustering could effectively partition the data for further feature level-based data fusion by improving the detectability of defects simulating delamination, voids and localized water. A comparison with supervised symbol level fusion shows that clustering-based fusion outperforms this, especially in situations with very limited knowledge about the material properties and depths of the defects. Additionally, clustering is successfully applied in a case study where a multi-sensor NDT data set was automatically collected by a self-navigating mobile robot system.

Optimized Dynamic Acousto-elasticity Applied to Fatigue Damage and Stress Corrosion Cracking

Abstract: The dynamic acousto-elasticity (DAE) technique uniquely provides the elastic (speed of sound and attenuation) behavior over a dynamic strain cycle. This technique has been applied successfully to highly nonlinear materials such as rock samples, where nonlinear elastic sources are present throughout the material. DAE has shown different nonlinear elastic behavior in tension and compression as well as early-time memory effects (i.e. fast and slow dynamics) that cannot be observed with conventional dynamic techniques (e.g. resonance or wave mixing measurements). The main objective of the present study is to evaluate if the DAE technique is also sensitive to (1) fatigue damage and (2) a localized stress corrosion crack. A secondary objective is to adapt the DAE experimental setup to perform measurements in smaller specimens (thickness of few cm). Several samples (intact aluminium, fatigued aluminium and steel with a stress corrosion crack) were investigated. Using signal processing not normally applied to DAE, we are able to measure the nonlinear elastic response of intact aluminium, distinguish the intact from the fatigued aluminium sample and localize different nonlinear features in the stress corrosion cracked steel sample.

Detection and Modelling of Nonlinear Elastic Response in Damaged Composite Structures

Abstract: In this paper the nonlinear material response of damaged composite structures under periodic excitation is experimentally and numerically investigated. In particular, the nonlinear wave propagation problem was numerically analysed through a finite element model able to predict the nonlinear interaction of acoustic/ultrasonic waves with damage precursors and micro-cracks. Such a constitutive model is based on the Landau’s semi-analytical approach to account for anharmonic effects of the medium, and is able to provide an understanding of nonlinear elastic phenomena such as the second harmonic generation. Moreover, Kelvin tensorial formulation was used to extend the wave propagation problem in orthotropic materials to the 3D Cartesian space. In this manner, the interaction of the stress waves with the 3D crack could be analysed. This numerical model was then experimentally validated on a composite plate undergone to impact loading. Good agreement between the experimental and numerical second harmonic response was found, showing that this material model can be used as a simple and useful tool for future structural diagnostic applications.

Relation of Electromagnetic Emission and Crack Dynamics in Epoxy Resin Materials

Abstract: Electromagnetic emission (EME) testing and acoustic emission (AE) testing are applied to investigate the failure of a brittle, dielectric material under mechanical load. A setup for three point flexure tests comprising simultaneous monitoring of EME and AE was used to induce fracture of epoxy resin specimens. The influences of the orientation and the distance of the crack surface on the detectable EME signals are the subjects of investigation. As EME sensor a capacitive sensor was used. Tests with an artificial test source are carried out to characterize the system response of the sensor, the attached amplifier and acquisition cards as well as the included bandpass filters. We propose an EME source based on the surface charge density modelled at the position of the fracture plane. Results of finite element method modelling of the EME source are compared to experimental results and show very good agreement. The experimental results show a clear directional character of the emitted electromagnetic field and a strong dependence of the detected signals amplitude on source-sensor distance. A significant influence of the measurement chain on the detected electromagnetic signals bandwidth was found. Furthermore it is shown that the electromagnetic signals consist of three contributions originating from different source mechanisms. These are attributed to the separation and relaxation of charges during crack growth and to the vibration of the charged crack surfaces.

Eddy Current Testing and Evaluation of Far-Side Corrosion Around Rivet in Jet-Engine Intake of Aging Supersonic Aircraft

Abstract: This study proposes an automated inspection system to inspect far-side corrosion around rivets in the F-5 supersonic aircraft. The system includes a linear integrated Hall sensor array, a C-type exciter, and an automated scanning system. The inspection system was tested on a modified elliptical intake having artificial defects with 10-mm diameter and 0.12–1.11-mm depth. An algorithm was produced to detect the defects. The probability of detection (POD) curve was obtained with hits/misses data using a log–logistics model. Defect with depths of 0.67 and 0.98 mm were detected with 90 % POD and lower 95 % confidence bound.

Pulse-Echo Harmonic Generation Measurements for Non-destructive Evaluation

Abstract: Ultrasonic harmonic generation measurements have shown great potential for detecting nonlinear changes in various materials. Despite this, the practical implementation of the technique in non-destructive evaluation (NDE) has typically been limited to the through transmission setup case, with which problems arise in certain situations. Recently, works in the fields of nonlinear fluids and biomedical imaging have reported different application of the harmonic generation theory by making use of reflective boundaries and beam focusing. It is thought that such techniques may be similarly applied in the field of NDE to enable single-sided nonlinear inspection of components. In this paper, we initially describe a numerical model which has been used to determine the effects of attenuation and acoustic beam diffraction on measurements of the nonlinear parameter β. We then extend the model to incorporate first the effects of multiple reflecting boundaries in the propagation medium, then of focused source excitation. Simulations, supported by experimental data, show that nonlinear pulse-echo measurements have the potential to provide a viable (and practical) alternative to the usual through-transmission type as a means of measuring β in solids. Furthermore, it is shown that such measurements may be optimised, both by adjusting the excitation frequency, and by focusing the acoustic source at a certain point relative to the specimen boundary.

FRP-to-Masonry Bond Durability Assessment with Infrared Thermography Method

Abstract: The bond behavior between FRP composites and masonry substrate plays an important role in the performance of externally bonded reinforced masonry structures. Therefore, monitoring the bond quality during the application and subsequent service life of a structure is of crucial importance for execution control and structural health monitoring. The bond quality can change during the service life of the structure due to environmental conditions. Local detachments may occur at the FRP/substrate interface, affecting the bond performance to a large extent. Therefore, the use of expedite and efficient non-destructive techniques for assessment of the bond quality and monitoring FRP delamination is of much interest. Active infrared thermography (IR) technique was used in this study for assessing the bond quality in environmentally degraded FRP-strengthened masonry elements. The applicability and accuracy of the adopted method was initially validated by localization and size quantification of artificially embedded defects in FRP-strengthened brick specimens. Then, the method was used for investigating the appearance and progression of FRP delaminations due to environmental conditions. GFRP-strengthened brick specimens were exposed to accelerated hygrothermal ageing tests and inspected periodically with the IR camera. The results showed environmental exposure may produce large progressive FRP delaminations.

Air-Coupled Impact-Echo Delamination Detection in Concrete Using Spheres of Ice for Excitation

Abstract: The aims of this paper are to demonstrate that ice can be used as a suitable impactor to excite the acoustic modes in concrete associated with delaminations and to compare ice sphere impacts with traditional steel ball impacts. Simultaneous acoustic recordings and high-speed photography of representative low-velocity impacts with parametric analysis compare impact characteristics of steel balls and ice spheres on intact and delaminated concrete. These results agree qualitatively with Hertzian contact theory for low-velocity impacts. Excitation of concrete using continuous impacts of ice spheres of multiple sizes and a frequency analysis allows the acoustic signature of delaminations to be classified. The use of ice as an impactor for excitation of acoustic modes in concrete is thus demonstrated.

Post-Processing of Phased-Array Ultrasonic Inspection Data with Parallel Computing for Nondestructive Evaluation

Abstract: Phased-array ultrasonic nondestructive evaluation is an effective tool of safety assurance for key structural components. The paper presents a general post-processing methodology for phased-array ultrasonic inspection data. The methodology is developed to integrate three components: mapping of sampling points to structure model, re-sampling from non-uniformly distributed sampling points in phased-array to a uniform volume, and data fusion strategies for multiple channels. An adaptive method called spatially adaptive Gaussian splatting is proposed for data re-sampling and fusion considering the reconstruction resolution and local characteristics of ultrasonic sound paths. This adaptivity provides a viable approach to minimize the effects of under-sampling, over-sampling, and holes which are introduced by the non-uniformly distributed sampling points. The processing of large scale data through segmentation and parallelization techniques is discussed in detail. The effectiveness and performance of the proposed methodology are investigated using actual phased-array ultrasonic testing data.

Effect of the Temperature on the Nonlinear Acoustic Behavior of Reinforced Concrete Using Dynamic Acoustoelastic Method of Time Shift

Abstract: During field tests, there is almost no effective way to control thermal changes in concrete structures. It is obvious that temperature fluctuations influence nonlinear acoustic behavior of concrete, which may lead to incoherent results during field investigations. The research presented herein was conducted to assess the effects of temperature changes on the nonlinear acoustic behavior of the reinforced concrete using Time Shift method. The Time Shift method, based on dynamic acoustoelastic principle, takes its roots from the coda wave interferometry method and combines it with the study of the nonlinear behavior in cementitious materials in a methodological manner that allows field investigations. Near-to-field environmental conditions were simulated in the laboratory using an automatic climatic room. The specimens were subjected to temperature changes ranging from $$-$$ 10 to 40 $$^{\circ }$$ C. Such a thermal regime is close to the thermal conditions prevailing for most concrete structures. The effect of the temperature variations was assessed in both sound and damaged concrete elements affected by alkali–silica reaction (ASR). The test-results demonstrate that the nonlinear acoustic responses of concrete depend on the temperature. However, the nonlinear parameter seems to get minimized in low temperatures ranging from $$-$$ 10 to 10 $$^{\circ }$$ C. Moreover, the state of medium (i.e. intact or damaged) can alter the sensitivity level of the nonlinear behavior to temperature variations.

The Application of Nonlinear Reverberation Spectroscopy for the Detection of Localized Fatigue Damage

Abstract: Nonlinear reverberation spectroscopy (NRS) is a non destructive evaluation method exploiting the amplitude dependent change of the resonance frequency in samples made of a nonlinear elastic material. After a sample has been excited near resonance, the immediate vibration frequency of the decaying reverberation signal decays with the decreasing amplitude. The frequency-amplitude dependence can be used to quantify the nonlinearity of the material, typically caused by the damage level. This paper handles the possibilities and difficulties of using NRS to detect a single fatigue crack in specimens made of an linear material like a composite or steel. In this case, the nonlinearity is concentrated in a small zone which is not necessarily affected by the low frequency vibrations used for NRS. First, a proof of concept is given by testing composite beams with increasing levels of fatigue damage. Tests prove that the nonlinear frequency change is more efficient for quantifying early damage than its linear counterparts such as the damping coefficient, at least when the crack is located in a vibration antinode. The method is subsequently used to test steel industrial samples with a complex geometry, showing that efficient damage detection indeed depends on the used vibration mode.

Discrimination Between Cracks and Recrystallization in Steel Using Nonlinear Techniques

Abstract: One major problem in ultrasonic NDT for steel products and welding inspection is that standard linear methods are often unable to distinguish the nature of signals. Partially recrystallized grains, voids, small cracks or inclusions in the piece under investigation could produce indications very similar in terms of acoustic energy reflected and ultrasonic peaks envelope. Here, we analyze the nonlinear response to ultrasonic excitations of steel bars with both kind of imperfections purposefully generated. Using the Scaling Subtraction Method as a tool for the analysis, we show differences in the nonlinear signature, which can be used to distinguish nondestructively a crack/delamination from a region with imperfect grains formation, with possible applications of this technique in the production cycle.

Ultrasonic Imaging in Hot Liquid Sodium Using a Plate-Type Ultrasonic Waveguide Sensor

Abstract: This paper reports the first set of results from ultrasonic measurements for determining the imaging capability of a plate-type ultrasonic waveguide sensor in $$200\,^{\circ }$$ C liquid sodium. This 10-m long plate-type waveguide sensor has been developed for viewing objects in opaque liquid sodium coolant for the applications in a sodium-cooled fast reactor (a next generation nuclear reactor). Various imaging capabilities of the waveguide sensor have already been demonstrated in water including ultrasonic beam steering, high resolution C-scan, and so on. However, water and liquid sodium have different acoustic properties and, more importantly, different wetting characteristics with stainless steel—the material for the waveguide sensor. For applications of the developed waveguide sensor in a real reactor environment, this research performs a set of necessary ultrasonic measurements in liquid sodium. The end section of the waveguide sensor which radiates an ultrasonic beam into the liquid sodium is coated with thin beryllium and nickel layers which can significantly improve the ultrasonic beam quality and wetting property of the stainless steel. A liquid sodium facility that consists of a glove box system, a sodium test tank, and an argon purification system has been built. The resolution and beam property are determined from ultrasonic C-scan experiments; a signal-to-noise ratio of over 10 dB and the resulting detection of a 1 mm wide slit can be achieved. The inherent issues associated with wetting of the waveguide sensor in liquid sodium are discussed based on the ultrasonic imaging results.