Showing papers in "Journal of Nondestructive Evaluation in 2016"

Thermal (IR) and Other NDT Techniques for Improved Material Inspection

Abstract: Thermal nondestructive testing (TNDT) may be considered to be a more widely applicable method than many traditional techniques, such as X ray, ultrasonic, eddy current, liquid penetrant, etc. It can be applied to both metals and non-metals containing subsurface defects such as cracks, foreign inclusions, disbonds, delaminations, variations in thermal properties, etc. This is especially true for composite materials, and TNDT is very appropriate for screening purposes. TNDT test results may be analyzed by advanced image processing algorithms. This paper provides a concise review of composite NDT using TNDT in combination with other inspection techniques, providing an opportunity for data fusion.

A Novel Methodology for Spatial Damage Detection and Imaging Using a Distributed Carbon Nanotube-Based Composite Sensor Combined with Electrical Impedance Tomography

Abstract: This paper describes a novel non-destructive evaluation methodology for imaging of damage in composite materials using the electrical impedance tomography (EIT) technique applied to a distributed carbon nanotube-based sensor. The sensor consists of a nonwoven aramid fabric, which was first coated with nanotubes using a solution casting approach and then infused with epoxy resin through the vacuum assisted resin transfer molding technique. Finally, this composite sensor is cured to become a mechanically-robust, electromechanically-sensitive, and highly customizable distributed two-dimensional sensor which can be adhered to virtually any substrate. By assuming that damage on the sensor directly affects its conductivity, a difference imaging-based EIT algorithm was implemented and tailored to offer two-dimensional maps of conductivity changes, from which damage location and size can be estimated. The reconstruction is based on a newly defined adjacent current–voltage measurement scheme associated with 32 electrodes located along the boundary of the sensor. In this paper, we evaluate our methodology first by introducing well-defined damage where sections are either removed or narrow cuts are made on a series of sensor specimens. Finally, a more realistic damage scenario was investigated to show the capability of our methodology to detect impact damage on a composite laminate. The resulting EIT maps are compared to visual inspection and thermograms taken with an infrared camera.

Active IR-Thermal Imaging in Medicine

Abstract: In this paper we summarize results of several research projects devoted to development of new diagnostic methods and procedures based on quantitative infrared thermography in medical applications. First, basics of active dynamic thermography are presented. Described are both, instrumentation and software comprising measurement procedures including collection of series of IR-images after external excitation, data treatment with displacement corrections, reduction of noise and artifacts and finally presentation of parametric diagnostic images. The thermal tomography method is also shortly discussed. As the most important for medical diagnostics parametric images of thermal time constants are discussed. For illustration of the value of presented methods chosen research and clinical applications performed in several clinics of Gdansk Medical University are presented. In vivo experiments on animals, mainly domestic pig, as well as clinical diagnostic procedures are discussed in skin burn and cardiosurgery applications. Specific requirements of IR-thermal investigation in medical applications are discussed in comparison to IR-thermal imaging in technical non-destructive evaluation. It should be underlined that medical applications of IR-diagnostics seem to be the most difficult among other applications as heat transfers in living tissues are far more complicated comparing to technical evaluation where nondestructive testing methods based on thermal IR-imaging are already well developed.

Fast Characterization of the Width of Vertical Cracks Using Pulsed Laser Spot Infrared Thermography

Abstract: In-service non-destructive detection of cracks is a challenging task for industries to prevent failures. In the last decades several methods based on infrared thermography have been proposed to detect vertical cracks. In a recent paper, the authors used a lock-in thermography setup with focused laser excitation to characterize the width of infinite vertical cracks accurately. As this method is very time consuming, we propose in this work to measure the width of an infinite vertical crack using pulsed laser spot infrared thermography. A semi-analytical solution for the surface temperature of a sample containing such a crack when the surface is illuminated by a pulsed Gaussian laser spot close to the crack is obtained. Measurements of the surface temperature on samples containing calibrated cracks have been performed using an infrared camera. A least square fit of the surface temperature is used to retrieve the thickness of the crack. Very good agreement between the nominal and retrieved thickness of fissure is found, even for widths down to 1 $$\upmu $$ m, confirming the validity of the model.

Corrosion Detection in Pipelines Using Infrared Thermography: Experiments and Data Processing Methods

Abstract: This article summarizes the main results of an investigation about the corrosion detection in pipelines by infrared thermography, a non-destructive testing and evaluation technique that allows a reliable and fast analysis of large surfaces. The experimental work has been carried out in laboratory on a specimen that has been manufactured using a piece of a real pipeline system for oil transportation. Defects of different kinds have been artificially introduced in such a system to be tested by thermography. The objective is the detection and analysis of the presence of water in the pipeline jacketing system, that is the cause of the corrosion under insulation. Standards indicate thermography as a technique for the detection of this last phenomena, even though a precise procedure is not defined up today. This work aims at contributing in the specification of such a procedure.

Finite Element Method Applied in Electromagnetic NDTE: A Review

Abstract: The paper contains an original comprehensive review of finite element analysis (FEA) applied by researchers to calibrate and improve existing and developing electromagnetic non-destructive testing and evaluation techniques, including but not limited to magnetic flux leakage (MFL), eddy current testing, electromagnetic-acoustic transducers (EMATs). Premium is put on the detection and modelling of magnetic field, as the vast majority of ENDT involves magnetic induction, either as a primary variable MFL or a complementary phenomenon (EC, EMATs). FEA is shown as a fit-for-purpose tool to design, understand and optimise ENDT systems, or a Reference for other modelling algorithms. The review intentionally omits the fundamentals of FEA and detailed principles of NDT. Strain-stress FEA applications in NDT, especially in ultrasonography and hole-drilling methodology, deserve as well a separate study.

Estimation of Remaining Useful Life of Slow Speed Bearings Using Acoustic Emission Signals

Abstract: Various direct and indirect sensing methods for machine condition monitoring have been reported in the literature Among these methods, acoustic emission technique is one of the effective means of monitoring rolling element bearings during industrial processes Today, many machines use computerized classification in a wide range of applications Further, recent developments indicate the drive towards integration of diagnosis and prognosis algorithms in future integrated machine health management systems With this in mind, this paper concentrates on the estimation of the remaining useful life for bearings whilst in operation To implement this, a linear regression classifier and multilayer artificial neural network model have been proposed to correlate the selected AE features with corresponding bearing wear throughout laboratory experiments Results showed that the proposed models exhibit good prediction performance This paper also presents the use of a new representative fault indicator, signal intensity estimator, employed for AE signals originating from natural degradation of slow speed rolling element bearings It is concluded that the obtained results were promising and selecting this appropriate signal processing technique can significantly affect the defect identification

A Non-Invasive Technique for Online Defect Detection on Steel Strip Surfaces

Abstract: During the production of steel strips, a large amount of surface defects can be generated, due to harsh environmental conditions. A high number of surface defects can lead to rejection by the customer, which represents significant economic losses to the production plant. Thus, it is very important to detect the presence and type of the defects generated during the production of each steel strip. Using this information, it is possible to determine whether a strip is suitable for sale, and it may also be useful to determine the origin of defects and, if possible, prevent them from being generated in subsequent strips. To perform these tasks, non-invasive inspection techniques are usually used, carried out automatically by artificial vision systems. Although the inspection conducted by humans is more accurate, they become fatigued quickly, or may even be unable to carry out the inspection correctly when the forward speed of the strip is high. In this paper, a new detection technique is proposed, based on the division of an image into a set of overlapping areas. The optimum values for the configuration parameters of the detection technique are automatically determined using a genetic algorithm. After the detection phase, all the defects are classified using a neural network. A very satisfactory success rate has been achieved in both detection and classification phases.

Examination of Spot Welded Joints with Active Thermography

Abstract: The method described here allows to determine the size of the thermal contact between two metal sheets joined by spot welding. This size is a measure for the size of the weld nugget, i.e. the zone melted during the welding process, and thus the quality of the welded joint. The method applies active thermography in transmission or reflection setup. Especially the reflection setup offers an attractive possibility for non-destructive testing when components can be accessed from one side only. The spot weld region is optically heated by laser or flash light radiation. The weld nugget provides the mechanical joint, but also constitutes a thermal bridge between the two welded sheets. The latter will be exploited in this method. The better thermal contact at the weld nugget contrasts with the surrounding material, where the heat transfer between the two sheets is comparatively low. A major advantage of the described method is the applicability on sheets without any surface treatment. This is achieved by a proper normalization of the data, allowing for a correction of the varying surface emissivity.

Manifold Learning Using Linear Local Tangent Space Alignment (LLTSA) Algorithm for Noise Removal in Wavelet Filtered Vibration Signal

Abstract: A denoising procedure is proposed to remove both out-band and in-band noise for extraction of weak bursts in signal obtained from defective bearing. Energy of continuous wavelet scalogram is computed and the band having higher energy is selected to remove the out-band noise. Signals of selected band are brought together to form a high-dimensional waveform feature space. Further, low dimensional waveform manifold is formed using linear local tangent space alignment (LLTSA) algorithm to remove in-band noise. A criterion, entitled as frequency factor is also proposed to determine the optimum neighbour size of LLTSA. The two complicated conditions are chosen to demonstrate the effectiveness of the technique in the extraction of bursts in the noisy situations. A significant improvement in the signal to noise ratio is observed when in-band noise is removed using manifold learning by LLTSA algorithm. The experimental result reveals the success of the proposed denoising procedure in extraction of defect features, even in the case of noisy condition.

The Sensitivity of Acoustic-Laser Technique for Detecting the Defects in CFRP-Bonded Concrete Systems

Abstract: This paper presents an experimental study to evaluate the sensitivity of acoustic-laser technique in defect detection. The technique is particularly useful towards the detection of near-surface defects in fiber reinforced polymer-bonded concrete by vibrating the material with an acoustic excitation and measuring the vibration signals with a laser beam. However, relatively little is known about the sensitivity of acoustic-laser technique. More research work should be conducted to evaluate the effectiveness of the technique when adopted for defect detection. It is also important to investigate the limits of the technique performance with respect to varying operational conditions so as to determine ways of improving the detectability. For this purpose, operational conditions in terms of acoustic excitation and laser beam incidence are investigated for their effectiveness in detecting near-surface defects and a reliable defect detection scheme using our portable equipment is therefore recommended. This work provides a basis for further improving such technique which can be used in other engineering applications including quality control of materials and product development process.

Noncontact Sonic NDE and Defect Imaging Via Local Defect Resonance

Abstract: A selective acoustic activation of defects based on the concept of local defect resonance enables to enhance considerably the intensity of defect vibrations and makes it possible to reduce the input acoustic powers to the levels permissible for noncontact nondestructive inspection. Since for cm-size defects in composite materials, the LDR frequencies lie in the low kHz-range, the resonant noncontact activation shifts to an audible frequency range and can be provided by conventional sonic equipment. In this paper, the feasibility of the resonant noncontact inspection is validated for the most “problematic” methodologies of nonlinear, thermosonic and shearosonic NDE that usually require an elevated acoustic power and, therefore, a reliable contact between the specimen and the transducer. In contrast, the noncontact versions developed employ commercial loudspeakers which can simultaneously insonify large areas and be applied for a contactless sonic inspection of different materials and various scale components.

Geometry Determination of a Foundation Slab Using the Ultrasonic Echo Technique and Geophysical Migration Methods

Abstract: The ultrasonic echo technique is a frequently used method in non destructive testing for geometry determination of concrete building elements. Important tasks are thickness measurements as well as the localization and characterization of built-in components and inhomogeneities. Currently mainly the synthetic aperture focusing family of techniques (SAFT) is used for imaging. These algorithms have difficulties in imaging steeply dipping interfaces and complicated structures such as steps and lower boundaries of voids. As an alternative two geophysical migration methods, pre-stack Kirchhoff depth migration and pre-stack Reverse-time migration (RTM) were tested in this paper at a reinforced concrete foundation slab. The slab consists of various reinforcement contents, different thicknesses and two pile heads. In a first step, both methods were evaluated with synthetic 2D data. In the second step, ultrasonic measurement data recorded with shear wave transducers on a line profile on the foundation slab were processed. The use of an automatic scanner simplified the measurements. A comparison of the geophysical migration results with those of SAFT shows, in particular for RTM, a significant improvement in the imaging of the geometry of the foundation slab. Vertical borders were reconstructed and the location and structure of the lower boundary of the foundation slab were reproduced better. Limitations still exist in imaging the piles below the slab.

Compensation of the Skin Effect in Low-Frequency Potential Drop Measurements

Abstract: Potential drop measurements are routinely used in the non-destructive evaluation of component integrity. Potential drop measurements use either direct current (DC) or alternating current (AC), the latter will have superior noise performance due to the ability to perform phase sensitive detection and the reduction of flicker noise. AC measurements are however subject to the skin effect where the current is electromagnetically constricted to the surface of the component. Unfortunately, the skin effect is a function of magnetic permeability, which in ferromagnetic materials is sensitive to a number of parameters including stress and temperature, and consequently in-situ impedance measurements are likely to be unstable. It has been proposed that quasi-DC measurements, which benefit from superior noise performance, but also tend to the skin-effect independent DC measurement, be adopted for in-situ creep measurements for power station components. Unfortunately, the quasi-DC measurement will only tend to the DC distribution and therefore some remnant sensitivity to the skin effect will remain. This paper will present a correction for situations where the remnant sensitivity to the skin effect is not adequately suppressed by using sufficiently low frequency; the application of particular interest being the in-situ monitoring of the creep strain of power station components. The correction uses the measured phase angle to approximate the influence of the skin effect and allow recovery of the DC-asymptotic value of the resistance. The basis of the correction, that potential drop measurements are minimum phase is presented and illustrated on two cases; the creep strain sensor of practical interest and a conducting rod as another common case to illustrate generality. The correction is demonstrated experimentally on a component where the skin effect is manipulated by application of a range of elastic stresses.

Experimental Structural Analysis of Hybrid Composite-Concrete Beams by Digital Image Correlation (DIC) and Acoustic Emission (AE)

Abstract: The use of composites such as textile reinforced cements (TRCs) and fibre reinforced polymers (FRPs) enables the development of lightweight structures. Such a lightweight solution for floor renovation consists of a hybrid composite-concrete cross section: prefabricated beams (TRC–CFRP reinforced hollow boxes with concrete on top) support sandwich panels together with a finishing concrete compression layer creating a monolithic hybrid floor. As the hybrid beams are the main structural element of this floor system, their load-bearing and failure behaviour should be fully understood. In order to examine the optimal design of these structures in terms of load bearing capacity, the beams are separately tested in four point bending while the amount of CFRP reinforcement and the concrete thickness are varied. The digital image correlation (DIC) and acoustic emission (AE) measuring techniques are applied in a complimentary way to monitor the bending and failure behaviour of the full scale hybrid beams. DIC visualises the development of surface strain fields together with the exact cracking patterns in relation to the applied load. AE contributes in defining the load at the onset of serious cracking activity. Furthermore, AE characterizes the contribution of the different fracture modes that may vary from concrete cracking, delamination between the successive layers of the TRC or debonding at the interphase between the TRC hollow box and the concrete on the one hand and the CFRP on the other hand.

Global and Local Thresholding Methods Applied to X-ray Microtomographic Analysis of Metallic Foams

Abstract: X-ray based computed microtomography is a non-destructive, well established tool for a three-dimensional characterization of open-cell metallic foams. Macroscopic physical and chemical properties of these materials stay in close relation to their micro-structure parameters. The purpose of the paper is to present two types of thresholding methods so-called global and local thresholding for evaluating the structural parameters of open-cell metal foams based on X-ray microtomography data. Two different methods were chosen: automatic Otsu thresholding (global) and adaptive (mean of minimal and maximal grey values of grayscales within a selected radius). The key parameters of aluminum and nickel-chromium foams fine structure calculated using Otsu and locally thresholded images were significantly different. The proper image segmentation is the key point in metallic foam morphometry. The influence of a radius of the image processing region on the results obtained is discussed for the local thresholding method. Examples of the images artifacts generated by local thresholding method to demonstrate possible results misinterpretation are also given. The optimization of local thresholding parameter (radius of the image processing region) was presented.

Object Recognition in X-ray Testing Using Adaptive Sparse Representations

Abstract: In recent years, X-ray screening systems have been used to safeguard environments in which access control is of paramount importance. Security checkpoints have been placed at the entrances to many public places to detect prohibited items such as handguns and explosives. Human operators complete these tasks because automated recognition in baggage inspection is far from perfect. Research and development on X-ray testing is, however, ongoing into new approaches that can be used to aid human operators. This paper attempts to make a contribution to the field of object recognition by proposing a new approach called Adaptive Sparse Representation (XASR+). It consists of two stages: learning and testing. In the learning stage, for each object of training dataset, several patches are extracted from its X-ray images in order to construct representative dictionaries. A stop-list is used to remove very common words of the dictionaries. In the testing stage, test patches of the test image are extracted, and for each test patch a dictionary is built concatenating the ‘best’ representative dictionary of each object. Using this adapted dictionary, each test patch is classified following the Sparse Representation Classification methodology. Finally, the test image is classified by patch voting. Thus, our approach is able to deal with less constrained conditions including some contrast variability, pose, intra-class variability, size of the image and focal distance. We tested the effectiveness of our method for the detection of four different objects. In our experiments, the recognition rate was more than 97 % in each class, and more than 94 % if the object is occluded less than 15 %. Results show that XASR+ deals well with unconstrained conditions, outperforming various representative methods in the literature.

Remaining Useful Life Estimation with Lamb-Wave Sensors Based on Wiener Process and Principal Components Regression

Abstract: This paper proposes a new approach for predicting the remaining useful life of a structure from Lamb wave sensor data using principal component regression and Wiener process degradation modeling. Principal component regression is used for extracting damage sensitive features of a Lamb wave sensor signal and establish a relation between the features and measured areas. A Wiener process is developed to model the random delamination growth and predict future damage and failure time probability. We use minimum squared prediction error to find the optimum number of features and the window inside a Lamb wave signal that is most sensitive to the fatigue damage. Method is illustrated on real sensor data collected under run-to-failure fatigue loading conditions.

Nondestructive In-situ Permittivity Measurement of Liquid Within a Bottle Using an Open-Ended Microwave Waveguide

Abstract: This research demonstrates an effective in-situ testing approach to measure permittivity of liquid samples within a container using microwaves. An open-ended waveguide is employed as a sensor in the experiment. This apparatus is easy-to-use because it does not require special calibration and is not limited by sample shape or dimensions. After positioning the bottle inside the open-ended waveguide and adding a layer of surrounding medium that fills the space between waveguide and the bottle, the waveguide functions as a resonant cavity. By analyzing the $$S_{11}$$ amplitude waveform over the frequency range from 250 to 550 MHz, the resonant frequencies can be obtained for a certain propagating mode. The measured values are found to match the theoretical ones extremely well; this result implies that the new establishment is highly precise and sensitive.

Multi-Objective Optimization of a Magnetic Circuit for Magnetic Flux Leakage-Type Non-destructive Testing

Abstract: The magnetic flux leakage technique is a widely used method for non-destructive testing of pipe-lines. The inspection of pipelines is typically performed with the assistance of a robotic tool called PIG, which is equipped with an array of magnetic circuits responsible for inducing a magnetic field in the pipeline wall. This magnetic field leaks out of the pipeline wall at the locations where potential anomalies are present. The optimization of the geometrical configuration of these magnetic circuits, as a method to improve the probability of detection of the technique, has been a question of great interest in recent studies. Drawing on the concept of Kirchhoff’s laws and the application of the finite elements method, this paper makes use of the forward analysis of the magnetic circuit to suggest a methodology for its design optimization. A lumped parameter model was proposed and calibrated to yield similar results as compared to the finite elements model. Following a multi-objective approach, a Genetic Algorithm was implemented in order to minimize the dimensions of the magnetic circuit while looking at the same time for the maximum magnetic flux leakage at locations with pipeline damage. The optimum design obtained by means of the Genetic Algorithm was experimentally validated. The results demonstrate the superior performance of the optimal magnetic circuit in comparison with two other non-optimal circuits.

The Second Harmonic Generation in Reflection Mode: An Analytical, Numerical and Experimental Study

Abstract: The procedure to measure the second harmonic generation has typically been restricted to relatively simple setups such as through transmission of longitudinal waves or Rayleigh surface waves on one side of a component. Since these types of setups are not always applicable for in-service components, this research investigates the second harmonic wave generation in longitudinal and shear waves reflected from a stress-free surface. This particular measurement setup potentially provides information about the local damage state in an in-service component with only single-sided access. Therefore, this measurement setup is evaluated analytically, numerically and experimentally with an aluminum specimen as an example. The setup being considered proposes two possible measurement positions, where the second harmonic and the fundamental wave amplitude can be measured to determine the nonlinearity parameter of the specimen. This proposed “reflection mode” setup is first analyzed analytically, and then is implemented in a commercial finite element code, using increasing fundamental wave amplitudes to calculate the different values of the nonlinearity parameters. The results of the simulations verify the analytical results, when taking into account assumptions and approximations of the analytical solution procedure. Furthermore, these numerical finite element results provide further insights into the intricacies of the setup, including the need to avoid interaction with the diffracted waves. On the basis of these numerical results, a recommendation for the measurement position and angle is given. Finally, the nonlinearity parameters of two similar specimens with different levels of nonlinearity are experimentally measured with the proposed measurement setup, and the results show the potential of the single-sided determination of a change in acoustic nonlinearity using reflected bulk waves.

Effect of Spectral Reshaping on Frequency Modulated Thermal Wave Imaging for Non-destructive Testing and Evaluation of Steel Material

Abstract: Infrared thermographic techniques show their potential usage for non-destructive testing and evaluation of various materials due to their inherent capabilities such as whole field, non-contact, qualitative and quantitative to detect surface and sub-surface anomalies. This contribution introduces a novel data analysis scheme by spectral reshaping of linear frequency modulated temperature profile captured over a mild steel specimen. Time and frequency domain based processing methods are adopted on the generated temporal thermal data to reveal the hidden defects. Obtained results show the potential capabilities of spectral reshaping based on Gaussian windowed chirp with enhanced resolution and sensitivity for sub-surface defect detection.

Magnetic Flux Leakage Testing of Reverse Side Wall-Thinning by Using Very Low Strength Magnetization

Abstract: This study focused on the very low-field region of the initial magnetization curve and developed a method to inspect and evaluate reverse side wall-thinning on a ferromagnetic test object by using very low strength magnetized magnetic flux leakage testing. The principle of the method was derived based on the magnetic circuit theory, and a parameter named as differential magnetic-leaking reluctivity was found to be a linear function of the remaining thickness of a ferromagnetic steel plate with thinning on the reverse side. This approach was verified by electromagnetic numerical simulation, and highly sensitive magneto-impedance sensor utilized measurements. Preliminary measurements demonstrated that it is possible to measure and estimate the thicknesses of ferromagnetic plates using very low-level magnetized magnetic flux leakage testing, suggesting the applicability of using this approach to inspect and evaluate over a sufficiently large area reverse side thinning.

Research on the Detection Model and Method for Evaluating Spot Welding Quality Based on Ultrasonic A-Scan Analysis

Abstract: To establish a system which can judge the spot welding quality automatically and effectively, the mathematical and acoustic simulation models of the spot welds detection through ultrasonic A-scan are established. The following results can be obtained: (1) The quantitative relationship between nugget diameter and detecting oscillogram is deduced and verified by simulation and experiments, and then the nugget diameter mathematic model is developed. (2) The attenuation characteristic of ultrasonic waves in spot welds is studied. It shows that ultrasonic attenuation is caused by two reasons: one is absorptive and scattering effect determined by base metal, the other is the multiple reflection and transmission effect caused by the sheet-structure. And their contribution varies in different welds (no weld, undersize weld and good weld). (3) The assessment software of spot welding quality is developed, the weld nugget diameter, attenuation coefficient and spot welding quality can be obtained within a short time. In addition, the whole detecting process and system is designed. The research results of this paper indicate the feasibility of applying this spot welding quality detecting method to the automatic production line.

The Aircraft Skin Crack Inspection Based on Different-Source Sensors and Support Vector Machines

Abstract: Currently, most aircraft skin crack inspections are done by naked-eye vision and video instrumentation, but a single vision inspection can only provide partial information and incomplete description of the crack characteristics. A multi-class classification method for skin crack inspection based on multi-class support vector machines and data fusion strategy is proposed in this paper. A mobile platform for aircraft skin inspection carries the image sensor and ultrasonic sensor which are used to collect the signals of the skin inspection in the information fusion system and the sample features from the two different data sources are centralized to construct a feature input space for the skin crack classification. Then a multi-class support vector machine classifier is trained by using some samples of the data and the parameters of support vector machine (SVM) are optimized by using Genetic Algorithm (GA). The other samples of the data are used to forecast the skin crack. The experimental results showed that the method of the different-source information fusion can improve the aircraft skin crack distinguishing degree and accuracy compared with a single sensor and the GA–SVM method can be a more accurate model to recognize the skin crack. The ultrasonic sensor provides some additional information which a single image sensor and eyes can’t find. The recognition accuracy has been effectively improved by using different-source sensor information fusion which overcomes the low resolution and the impact of human factors.

Towards the Establishment of a Continuous Nondestructive Monitoring Technique for Fresh Concrete

Abstract: Despite the fact that concrete is the most widely used construction material, conventional techniques for the estimation of the mechanical properties are only applicable to the hardened material. Moreover, concerning the fresh concrete examination, the already existing techniques provide only qualitative information, are not representative over the full time period of curing, setting and hardening or have limited accuracy and repeatability. In this study, the acoustic emission (AE) technique is employed in order to investigate the activity within fresh concrete as early as a few minutes after mixing. Several processes like settlement, segregation, formation and migration of air bubbles, as well as formation of hydrates produce AEs which so far have not been properly examined in literature. The first and foremost goal is to check the capacity of the new AE setup to record emissions earlier than previous studies (just after casting). The next goal concerns the possibility to characterize the different types of emissions relatively to their original source mechanism while the final goal would be the investigation of possible correlations between the early age acoustic activity and the final mechanical properties. For this reason, an AE setup is applied utilizing the favorable wave transmission properties of the metal casting molds, which act as a sort of waveguide. After validating its sensitivity, several laboratory tests are performed in concrete with varying water and aggregate content. It is found that the rate of AE activity follows trends similar to the typical hydration curves known from literature and is also indicative of the mix parameters. Finally, it should be mentioned that it is the first study that exploits the acoustic activity as early as the moment of mixing while apart from the number of emissions which is usually examined, the monitoring includes waveform parameters that show strong characterization potential.

An Intelligent Fault Diagnosis of Induction Motors in an Arbitrary Noisy Environment

Abstract: In this paper, an intelligent fault diagnosis system based on instantaneous power spectrum analysis is proposed. The instantaneous noise variations and sensor off-sets are considered to be one of the common factors that yield erroneous fault tracking in an online condition monitoring and fault diagnosis system. The developed system has the capability to detect bearing inner race defects at incipient stages with in an arbitrary noise conditions. An adaptive threshold has been designed to deal with line current noise ambiguities for decision-making on the existence of small fault signatures. The performance of the developed system has been analyzed theoretically and experimentally on a custom designed test rig under various loading conditions of the motor.

Background Thermal Compensation by Filtering for Contrast Enhancement in Active Thermography

Abstract: A new contrast enhancement technique, called Background Thermal Compensation by Filtering (BTCF), is presented for application in active thermography inspection, based on median filtering with a decimated kernel to decouple the background thermal information from defective areas, without depending on any heat propagation model or the pre-selection of a healthy region. Furthermore, the selection of a reference thermogram is not needed unless a normalized version of the method is carried out. After decoupling the background thermal distribution, the enhanced contrast is obtained by applying the unsharp filtering strategy to only reveal the heterogeneities inside the material under analysis. Results obtained from CFRP slab inspection show that BTCF can reduce the residuary non-uniformity heating by 70 % and improves SNR values up to 15 dB when negative intensity values are truncated.

Stress Response of Magnetic Barkhausen Noise in Submarine Hull Steel: A Comparative Study

Abstract: The development of magnetic Barkhausen noise methods for rapid detection of residual stress concentrations has implications for integrity assessment of submarine pressure hulls. However, the stress-response of Barkhausen noise in submarine hull steel, HY-80, is complicated by the influence of the material’s martensitic microstructure. The present work sheds light on the stress-dependent behavior of Barkhausen noise in HY-80 by comparing its signal characteristics with those of more common ferrite/pearlite steels. HY-80 and various ferrite/pearlite steel plates were uni-axially stressed up to and beyond the level for plastic deformation. Barkhausen noise measurements, performed using the same sensor under reproducible flux-controlled magnetization conditions, facilitated a direct comparison of material responses. Results showed that with the application of tensile stress, the Barkhausen noise signal of ferrite/pearlite steels linearly increased, reached a peak value and saturated in the elastic region. By contrast, HY-80 demonstrated a linear increase with tensile stress characterized by a transition from a lower to a seven times higher rate of change for stresses above 200 MPa up to its yield point. The different stress-response of HY-80 was attributed to its martensitic microstructure, which modifies the response of the domain structure under tensile stress conditions.

Analyzing Deformation and Damage of VT1-0 Titanium in Different Structural States by Using Infrared Thermography

Abstract: This study is devoted to the comparison of strength characteristics in the process of deformation and damage of axially strained VT1-0 titanium in the ultrafine-grained (UFG) and coarse-grained (CG) states. The temperature distributions on the surface of titanium specimens were recorded by means of IR thermography. The VT1-0 titanium in UFG state formed by applying severe plastic deformation is characterized by twice the yield stress and strength limit but half the deformation limit compared to CG titanium. The fracture of CG titanium is accompanied by local powerful generation of heat, while, in UFG titanium, the damage nuclei are less intense and more evenly distributed over the fracture cross-section. The titanium in UFG state, being deformed, utilizes structural channels of energy absorption more efficiently than in the CG state by involving the whole deformed volume in the fracturing process.