Online Nondestructive Testing Research Articles

Simulation on Far Field Eddy Current Detection on Condenser Tube Groups in Fossil Power Plants (I)

Condenser tube tubes are widely applied in condenser in thermal power plants. Leakage often occurs in condenser tube groups due to the quality of condenser media or high serving stress. Condenser tube leakage not only brings economic losses to power stations, but even influences the safety operation of the power generation units. This paper simulates and experimentally studies the ferromagnetic heat exchanger tubes based on the detection mechanism of far field eddy current. The results show that the position of the far field is with the increase of the wall thickness and the inner diameter of the tubes, and the detection sensitivity of the far field is high for the same size defect, but the difference of the magnetic flux intensity varies equally when the defects with the same size are located at the inner and outer walls. This is consistent with the experimental results of real tube defect detection and meets the principle of far field eddy current detection. It is concluded that far field eddy current testing technology can better determine and identify ferromagnetic tube defects, and has a good application prospect for online nondestructive testing and safety performance evaluation of service tubes

On-line Monitoring by Acoustic Emission of Butt Joint Structure on the Composite Fuselage Frame

Acoustic emission (AE) technique is an online and dynamic non-destructive testing technology by diagnosing sounds, applicable to damage identification of all sorts of materials and structures. In order to fulfill the requirement of on-line monitoring for butt joint structure on the composite fuselage frame in static loading, Acoustic Emission (AE) is employed in this article. The experiment research is focused on examining the damage evolution process of the specimen subjected to mechanical tests by constantly monitoring the AE and strain responses. Combined with strain results, the recorded acoustic signals are analyzed using AE characteristic parameters such as AE amplitude and AE accumulative energy to identity five different damage stages and conclude the corresponding damage style during the entire test. The outcomes of this article can be available for supplementation and improvement of the current on-line non-destructive testing research for composite structures.

Detection of bruised potatoes using hyperspectral imaging technique based on discrete wavelet transform

In the view of difficulties in bruise detection of potatoes, a texture recognition technique based on hyperspectral imaging and discrete wavelet transform was presented and discussed in this paper. Firstly, characteristic bands of intact and damaged potato samples based on the principal component analysis (PCA) were obtained. Then, the PCA method was used again to generate the principal component images that make it possible to distinguish the bruised potatoes from the healthy ones. In order to enhance the texture characteristics of the damaged samples, histogram equalization, mean filter and gradient method were utilized to preprocess the reference image. The paper achieved the best image damage recognition effects with the method of histogram equalization. Next, the preprocessed images were discomposed by the paper to obtain sub-band images using a two-level discrete wavelet transform (DWT), generating gray level co-occurrence matrix (GLCM) from sub-band images and texture feature data. Finally, the optimal parameters of gray level co-occurrence matrix were found out with the method of grid search and AdaBoost-Fisher algorithm to reduce the dimension of texture data generated under each parameter. The following results were obtained experimentally: the highest damage identification rate was 99.82%, the data dimensions were reduced from 56 to 4, the picture gotten from WT was reduced from 7 to 4, and the optimal parameter combination was obtained when the prediction effect was best after dimensionality reduction. The results show that the hyperspectral image texture recognition technology based on DWT-GLCM can accurately identify slightly damaged potato samples, providing a reference for potato on-line non-destructive testing.

Spectral diagnosis of wire arc additive manufacturing of Al alloys

Online nondestructive testing for quality control is a critical direction for research in additive manufacturing in the future. In this study, for the first time, optical emission spectroscopy was employed to probe the arc characteristics in the wire arc additive manufacturing (WAAM) of an Al alloy and to detect its structural features. The arc characteristics, such as spectral intensity, electron density, and electron temperature, were calculated based on the atomic emission spectral lines. The resulting structural features of the deposited layers, namely the forming width, composition, grain size, and porosity defects, were analyzed, and a correlation between the arc characteristics and the structural features was proposed. The arc cathode size, which changed with the number of deposited layers, controlled the arc energy distribution. Hence, the forming width had an approximately linear relation with the spectral intensity of Mg (a constituent of the alloy used for the wire feed) and the electron density. The porosity in the alloy was observed to be caused by H, which was a dominant pollutant in the process. Furthermore, the correlation between the porosity and H spectral intensity was observed to be approximately linear. However, no significant correlation between the grain size and the spectrum was noticeable. The results from this study establish the applicability of spectral diagnosis of the forming size and the porosity in WAAM.

Analysis of the Mathematical Model of Open Coaxial Measuring Microwave Converters

The open coaxial sensor (OCS) enables online non-destructive testing of a wide range of objects. To solve the electrodynamics problem in the OCS design, this paper puts forward a mathematical model for an OCS with a sample located outside the sensor, and performs the theoretical calibration of the OCS in cylindrical form. The proposed method can be applied to OCSs with different forms of working area. The findings help to develop algorithms and programs for qualitative analysis on OCS features, and to compute the parameters and functions of OCS design. In addition, the analysis method can identify the basic and side effects of the qualitative level, and facilitate the construction of actual OCS designs.

Enhancing drying efficiency and product quality using advanced pretreatments and analytical tools—An overview

ABSTRACTDry food has the advantages of a convenient storage, long shelf life, and so on, which is widely consumed at present. And there is increased awareness of quality attributes of dehydrated foods such as color, texture, flavor, and nutrient content. In this article, we review several potential pretreatment technologies and analytical tools developed in recent years, which can be used to improve drying efficiency and rapid nondestructive detection. High-pressure processing and ultrasonic treatment can disinfect the wet feedstock before drying. Smart drying with online nondestructive testing using advanced analytical tools such as electronic nose, NMR spectra can help improve product quality in food drying. Each technique has its advantages in the field of food drying. Cost-effectiveness of these modern analytical tools will likely improve with more widespread utilization in industrial practice.

Design of a Comprehensive Pressure Vessel Experimental Device

The equipment used in stress- and pressure-related non-destructive testing (NDT) experiments tends to have single functionality and a low degree of automation. There are also safety risks inherent in existing pressure testing methods. This paper develops a remote online NDT comprehensive experimental device. It uses a high degree of automation to remotely monitor changes in a pressure vessel’s prefabricated defects during pressure tests. Safety is guaranteed, as the operator is located remotely. The experimental pressure vessel employs a split structure connected by a quick-release system using flange-type U-slot bolt holes. Pressure vessel defects can be prefabricated in different forms, and the vessels can be disassembled and changed easily. This process reduces the experiment materials needed, lowers costs, and saves experimental time. The device can be used for various experiments, such as acoustic emission monitoring of the pressure-bearing and load-holding capacities of pressure vessels, measurement of pressurized vessel strain, and pressure vessel NDT (magnetic powder inspection, ultrasonic flaw detection, magnetic flux leakage testing, magnetic memory testing). Operators can obtain experimental data and conduct analyses simply by controlling the computer. The device can enrich the content of undergraduate experiments and solve the single-functionality problem of existing experimental equipment. With this device, the acoustic emission online monitoring experiment of pressure vessel and the stress measurement of pressure vessel are carried out. Which verify the diversity of function and reliability of the test.

Friction stir welding of high-strength aerospace aluminum alloy and application in rocket tank manufacturing

Friction stir welding (FSW) has been widely adopted in aerospace industry for fabricating high-strength aluminum alloy structures, such as large volume fuel tanks, due to its exceptional advantages including low distortion, less defects and high mechanical properties of the joint. This article systematically reviews the key technical issues in producing large capacity aluminum alloy fuel tanks by using FSW, including tool design, FSW process optimization, nondestructive testing (NDT) techniques and defect repairing techniques, etc. To fulfill the requirements of Chinese aerospace industry, constant-force FSW, retractable tool FSW, lock joint FSW, on-line NDT and solid-state equal-strength FSW techniques, as well as a complete set of aerospace aluminum FSW equipment, have been successfully developed. All these techniques have been engineered and validated in rocket tanks, which enormously improved the fabrication ability of Chinese aerospace industry.

Online nondestructive testing for fine steel wire rope in electromagnetic interference environment

The diameter of fine steel wire rope (FSWR) is generally a few millimeters. Its magnetic flux leakage (MFL) signal is weak, and the number of magnetic sensors installed for defect detection is limited because of the small diameter. In FSWR production workshops, different kinds of machinery work together, deteriorating the power quality and making the spatial electromagnetic environment complex; the weak MFL is thus interfered with further. It is difficult to carry out online nondestructive testing (NDT) of FSWR in the process of manufacturing. In this paper we present a novel MFL method for FSWR NDT in a strong electromagnetic interference environment. We use a three-dimensional finite element method (FEM) to analyze the MFL signals. A simplified magnetic circuit is presented to excite the FSWR; the circuit comprises two half-sized radial magnetizing ring NdFeB magnets, and because there is no need for a magnetic yoke, the device is simple and light. A single Hall sensor is used to measure the flux leakage field. A stable performance power system is designed for the NDT power supply, which is not only resistant to voltage sags, but also has very low output noise. To enhance the signal-to-noise ratio (SNR) of the MFL defects signal, a signal conditioning and processing circuit are designed to enhance the detectability of signals in MFL data. The novel and small FSWR NDT system realizes on-line testing in an environment of strong electromagnetic interference, and for the experiment with a 1.5-mm-diameter wire rope twisted by 19 wires, the minimum damage of a pit on half of a wire can be identified.

A feasibility study on different NDT techniques used for testing bond quality in cold roll bonded Al-Sn alloy/steel bimetal strips

This paper presents non-destructive testing (NDT) results for the detection of bond defects in aluminium-tin (Al-Sn) alloy/steel bimetal strips. Among all types of bimetal strip that are used in the automotive industry for plain journal engine bearings, Al-Sn alloys cold roll bonded (CRB) onto steel backing is the most common type. The difficulty to evaluate the metallurgical bond between the two dissimilar metals is a major industrial concern, which comprises the risk that bearings fail in the field. Considering the harsh performance requirements, 100% online non-destructive testing would be desirable to significantly reduce the business risk. Nowadays bimetal strip manufacturers still rely on destructive testing through different peel-off tests. This work offers the results from four independent NDT studies, using active thermography, shearography, ultrasound and guided wave electromagnetic acoustic transducers (EMATs) and samples with different artificially implanted defects, to explore the feasibility to qualitatively indicate the occurrence of bond defects. A destructive peel off test was used to correlate the NDT results with known bond quality. The studies were done under laboratory conditions, and in case of ultrasound also online under production conditions. During the ultrasound online test, the requirements that a NDT technique has to fulfil for online inspection of Al-Sn alloy/steel bimetal strip were established. For active thermography, shearography and guided wave EMAT techniques, it was theoretically analysed, if the laboratory test results could be transferred to testing under production conditions. As a result, guided waves using EMATs, among the four tested methods, are best suited for online inspection of Al-Sn alloy/steel bimetal strip. This research was carried out in collaboration with MAHLE Engine Systems UK Ltd., an Al-Sn alloy/steel bimetal strip manufacturer for the automotive industry.

Three Point Bending Fatigue of Carbon Fiber Fabric Reinforced Polyphenylensulfide in the Very High Cycle Fatigue Regime

Carbon fiber reinforced polymers (CFRP) are getting more and more important for structural components for automotive and aerospace applications. These components are subjected to 1011and more loading cycles during their time in service. Therefore the VHCF behavior and the corresponding failure mechanisms have to be well understood. To obtain a comprehensive knowledge about the fatigue behavior and failure mechanisms of CFRP in the VHCF regime, a new Ultrasonic Testing Facility (UTF) for cyclic 3-point bending at 20 kHz has been developed at WKK. This UTF with combined online nondestructive testing via laser vibrometry and IRthermography enables VHCF experiments up to 109cycles in twelve days without overcritical heating up to the glass transition temperature of the polymer. The chosen material in this research project is the commercially available and aircraft qualified carbon fiber fabric reinforced polyphenylensulfide (CF-PPS). To determine the fatigue characteristics of CF-PPS load increase tests and based on these results constant amplitude tests up to 109cycles have been carried out. Light optical and SEM microscopy have been performed in defined fatigue states or finally after reaching 109cycles with shear stress amplitudes of at least 44% of the monotonic ultimate shear strength. The induced fatigue damage of CF-PPS in the VHCF regime was studied in detail.

Fatigue testing of CFRP in the Very High Cycle Fatigue (VHCF) regime at ultrasonic frequencies

Carbon fiber reinforced polymers (CFRP) are more and more used for high performance applications such as aircraft structures. These structural components are subjected with more than 108 cycles during their lifetime. At present the fatigue behavior of CFRP and the corresponding failure mechanisms are not sufficiently investigated in the VHCF regime. To obtain comprehensive knowledge about the fatigue behavior as well as the failure mechanisms of CFRP in the Very High Cycle Fatigue (VHCF) regime, a new Ultrasonic Testing Facility (UTF) for cyclic three point bending of CFRP at 20 kHz has been developed at the Institute of Materials Science and Engineering (WKK), University of Kaiserslautern. This UTF with combined online nondestructive testing via laser vibrometry and IR-thermography enables VHCF experiments up to 109 cycles in twelve days. All experiments are carried out at a stress ratio between 0.29 and 0.49. To keep the specimen temperature below the glass transition temperature of the CFRP the cyclic bending load is divided in pulse pause sequences. The specimen material used in this investigation is a commercially available carbon fiber fabric reinforced polyphenylensulfide (CF-PPS). For the accurate calibration of the loads as well as for the visualization of the strain distribution and the oscillation mode, measurements with 3D-scanning-laser-vibrometry have been carried out. Furthermore light optical micrographs plus SEM investigations have been performed upon completion and in defined interceptions of the experiments. Based on the results of the microscopic investigations the failure mechanisms of CF-PPS for the VHCF are described.

Line-field parallel spectral domain optical coherence tomography and its application in defect inspection

For the needs of online nondestructive testing method in glass industry, we have presented a large-range line-field parallel spectral domain optical coherence tomography system. Based on fast area scan CMOS camera, the whole cross-sectional image can be acquired by a single shot. Depth-resolved image at different lateral positions can be acquired simultaneously, without the lateral scanning mechanism. The axial resolution is 17.9 μm, the lateral resolutions in parallel direction and scanning direction are 55.7 μm and 24.8 μm, respectively. The system measurement range is 32 mm in lateral direction and 6 mm in axial direction. At 1 mm axial position, the system sensitivity can reach 62 dB at a rate of 128 000 A-scan/s. By using the present system, application in glass defect inspection has been investigated.

Inspection method of cable-stayed bridge using magnetic flux leakage detection: principle, sensor design, and signal processing

A nondestructive testing technique based on magnetic flux leakage is presented to inspect automatically the stay cables with large diameters of a cable-stayed bridge. Using the proposed inspection method, an online nondestructive testing (NDT) modular sensor is developed. The wreath-like sensor is composed of several sensor units that embrace the cable at equal angles. Each sensor unit consists of two permanent magnets and a hall sensor to detect the magnetic flux density. The modular sensor can be installed conveniently on cables with various diameters by increasing the number of sensor units and adjusting the relative distances between adjacent sensor units. Results of the experiments performed on a man-made cable with faults prove that the proposed sensor can inspect the status signals of the inner wires of the cables. To filter the interfering signals, three processing algorithms are discussed, including the moving average method, improved detrending algorithm, and signal processing based on a digital filter. Results show that the developed NDT sensor carried by a cable inspection robot can move along the cable and monitor the state of the stay cables.

Detection of gas pipe wall thickness based on electromagnetic flux leakage

Based on electromagnetic flux leakage (EMFL), a nondestructive testing (NDT) technique for the detection of gas pipe wall thickness is presented, and its principle and feasibility is evaluated by means of equivalent magnetic circuit analysis and finite element analysis. An online NDT device adopting this technique is developed, and its structure and working principle are introduced in detail. This device is composed of a detector array with 32 pipe wall thickness sensors that employ a Hall element as the element for sensing the magnetic flux density, and it can be adapted to pipe diameters from O400mm to O650 mm. On the basis of the experimental investigation for this device, the influences of some factors on thickness measurement, namely the excitation current, excitation coil turns, gap distance, concentrator of the magnetic field, magnetization time, and number of sensors in the detector array, are revealed and the optimal excitation voltage for the sensors of the detector array is selected. The measuring calibration is given to establish the relationship between the pipe wall thickness and the output voltage of the sensors. The results show that the proposed EMFL for measuring the wall thickness of ferromagnetic pipe is feasible, the technical parameters of the sensor are important for improvement of measurement precision and resolution, and the developed device has precision, resolution, and a linear output curve. Carried by the developed gas pipeline inspection robot through a universal joint, this NDT device can move inside the gas pipeline and monitor the state of the pipe wall.

An introduction to wavelet transforms: a tutorial approach

Non-destructive testing (NDT) and condition monitoring techniques are among the most rapidly developing engineering disciplines. In the past decade methods have been sought to enable real-time or on-line analysis of degradation in structures ( 1 - 3 ) . Methods developed in the 1980s based on modal analysis ( 4 ) have resurfaced, partly due to new developments in signal processing. Many of the on-line NDT techniques currently under research utilise wavelet transforms ( 5 - 9 ) to decompose spatial signals into the time/frequency domain. The aim of this paper therefore, is to provide a friendly tutorial to the two most commonly used forms of the wavelet transform. It is hoped that this article will provide interested readers with sufficient knowledge of the subject to allow productive usage of the various software packages offering wavelet technologies. Being aware of the building knowledge provided both by literature and the internet, a list of useful references and world-wide-web links has been provided at the end of the paper.

Non-destructive testing of the quality of naturally white food products

Results of research on the determination of a method and a system for on-line non-destructive testing of the quality of naturally white materials are presented. Taking into account analysis of the color properties of the material, two methods are proposed: the first is based on calculation of color indices (whiteness, yellowness, and blue reflectance), and the second is based on comparison of the measured reflectance spectra in appropriately selected wavelength ranges. Practical realization of the second sensing method is described using an optical-fiber color sensor. The testing methods presented can be applied for non-destructive on-line inspection of various color products in different industries.

Peace will bring pests

Carbon fiber reinforced polymers (CFRP) are more and more used for high performance applications such as aircraft structures. These structural components are subjected with more than 108 cycles during their lifetime. At present the fatigue behavior of CFRP and the corresponding failure mechanisms are not sufficiently investigated in the VHCF regime.To obtain comprehensive knowledge about the fatigue behavior as well as the failure mechanisms of CFRP in the Very High Cycle Fatigue (VHCF) regime, a new Ultrasonic Testing Facility (UTF) for cyclic three point bending of CFRP at 20 kHz has been developed at the Institute of Materials Science and Engineering (WKK), University of Kaiserslautern. This UTF with combined online nondestructive testing via laser vibrometry and IR-thermography enables VHCF experiments up to 109 cycles in twelve days. All experiments are carried out at a stress ratio between 0.29 and 0.49. To keep the specimen temperature below the glass transition temperature of the CFRP the cyclic bending load is divided in pulse pause sequences. The specimen material used in this investigation is a commercially available carbon fiber fabric reinforced polyphenylensulfide (CF-PPS). For the accurate calibration of the loads as well as for the visualization of the strain distribution and the oscillation mode, measurements with 3D-scanning-laser-vibrometry have been carried out. Furthermore light optical micrographs plus SEM investigations have been performed upon completion and in defined interceptions of the experiments. Based on the results of the microscopic investigations the failure mechanisms of CF-PPS for the VHCF are described.