Bispectra-based impact acoustic non-destructive evaluation

This paper presents a novel cost-effective non-destructive evaluation (NDE) method for detecting defects in tiled-walls of high-rise buildings. The method is based on impact acoustic technique which identifies the bonding quality of tiles by analyzing the acoustic vibration signals generated by tapping a structure with an impacting device. However, the basic impact acoustic method, which uses a single impacting device, can only provide a single-point inspection and will take time to inspect a large high-rise building. To speedup the evaluation process, impacting the structure simultaneously with multiple impacting devices are being developed to increase the coverage area of the inspection. However, the sound signals generated by multiple impacting devices are more complicated than those from a single impacting device. Through the analysis of the raw sound data, this paper proposes a regional integrated bispectra method to extract the feature vector and a threshold method for classifying the bonding quality of tiles for multiple impacting devices. The experimental results confirm the validity of the proposed method.     

Structural characterization of thermal building insulation materials using terahertz spectroscopy and terahertz pulsed imaging

Terahertz (THz) spectroscopy and imaging is used to analyse different types of thermal building insulation materials. First, the absorption coefficients of polymer foams are calculated, showing an inverse relationship with thermal conductivity. In addition, manufacturing imperfections and internal structures within the foams are clearly visualized with THz amplitude imaging. Secondly, different fibre orientations within aerogel composite blanket are recognized in THz spectra and amplitude images. Lastly, the phase transition of microencapsulated phase-change material is indicated by occurrence of spectral peaks in THz spectrum. The selected method is promising for a non-destructive testing of the thermal insulation material properties and structure.     

Reverberation reduction in ultrasonic images via predictive deconvolution

Reverberations are usually observed in ultrasonic nondestructive testing (NDT) results and medical images. They can blur the echoes from the targets, as a result, lack the accuracy of material examinations as well as clinical diagnoses. The reverberation can be recognized or suppressed via some known techniques. However, the repetitive period and reverberation waveform may vary and differ from their primary reflections. Under such situation, the known techniques will not work efficiently.In order to improve these situations, a predictive deconvolution technique is proposed to suppress reverberations in ultrasonic images. Because the appropriate prediction lag (α) and filter length (N) in the predictive deconvolution can be specified, the reverberations with specified periodicities between α and α+N−1 are able to be suppressed, and the length of the desired output wavelet can be controlled as α. In addition, a better output of predictive deconvolution can be obtained if (1) the prediction lag is slightly less than the repetitive period of the reverberation, and (2) the prediction filter length is greater than the length of the source echo. Experiments show that reverberations with different repetitive periods in B-scans are greatly reduced via predictive deconvolution, though the waveform of the source echo changes slightly when propagates in the specimen. Once buried in reverberations, now the images of the holes located at different depths, with different sizes, can be easily observed.     

High-frequency ultrasonic testing of young cement-based materials using the ‘prism technique’

A new technique for testing young cement paste and mortar is presented. It is based on a method, which uses unconventional prism-shaped specimens. It involves the precise measurement at fixed intervals of 24 h of the time-of-flight of both compressional and shear waves using high-frequency ultrasound. A water tank was designed that allows for easy coupling and handling of the specimens. Both waves are generated by mode conversion using the same transducer. The echoes from all the specimens were strong, and no signal processing was required. Thus, reliable and repeatable results were achieved using a very basic apparatus. Second-order elastic constants were evaluated, and correlation was made with the porosity and age of mortar. A new standard based on this method is proposed which would help in the making of prediction models for evaluating the strength of cement-based materials.     

无损检测仪器设备内部校准的有效性

为确保无损检测数据准确和结果可靠,应以量值溯源为基础,对无损检测仪器设备实施内部校准时,应保证内部校准的有效性。     

NDT techniques for railroad wheel and gauge corner inspection

The development of non-destructive techniques (NDT) techniques for the in-service inspection of railroad wheels and gauge corners was the main activity of the NDT division VIII. 4 at BAM over the last 2 years. For such different components, two different inspection techniques were fundamentally chosen in order to fulfil the end-user requirements. Firstly the inspection of the wheels—rim and disk—should be carried out without dismantling the wheels and using ultrasonic techniques. On the other hand, the inspection of the railroad track surface at a train speed of about 70 km/h should be guaranteed using eddy current techniques. The above-mentioned tasks were a challenge for the lab staff. The accessibility for the wheel inspection was limited due to several impassable barriers such as sand tubes, etc. Eddy current application focused mainly on the detection of head check defects occurring at the gauge corner of the rail. Investigations carried out also showed, that other types of surface defects (e.g. Belgrospis, wheel burns, short-pitch corrugations, etc.) could easily be detected. Some aspects of the inspection system as well as an overview of test results are presented in the current contribution.     

Diagnosis of austenitic steel valves with the magneto-optical method

Samples selected from damaged austenitic steel valves of diesel engines have been examined using the magneto-optical (MO) method. The method uses a magnetic excitation normal to the material surface and enables the assessment of two conditions of the originally non-magnetic austenitic steel, namely an integral one, when the coercivity H_c is measured, and a local one that concentrates on the detection and assessment of individual defects. The results have shown that the operational load increases H_c of these materials and a defect of the fatigue crack type has been detected in the position of the valve failure.     

Application of ultrasonic method in asphalt concrete testing for fatigue life estimation

This paper presents the possibilities of applying ultrasonic methods in asphalt concrete specimen testing in order to estimate fatigue life using ultrasonic characteristics of specimens. The procedures and results of testing cylinder shaped samples of both ultrasonic analysis and fatigue life testing are described. The ultrasonic method was first used in order to obtain the seismic characteristics of specimens. Then, the specimens were tested for fatigue lives using the repeated-loading indirect tensile test equipment (SDU-Asphalt Tester). To be able to model the fatigue lives, in addition to conventional fatigue model parameters new parameters from this ultrasonic analysis were taken into consideration. These additional parameters are acquired from the seismic measurements. Hence, only by examining the ultrasonic method, it will be possible to predict the fatigue lives of the specimens non-destructively.     

中国无损检测标准化与国际标准化工作

介绍全国无损检测标准化技术委员会的工作任务、组织机构、工作程序以及我国无损检测标准化的发展历程和与国际标准化接轨的工作。     

Difference in the detection limits of flaws in the depths of multi-layered and continuous aluminum plates using low-frequency eddy current testing

We examined the difference in the detection limits of flaws in the depths of multi-layered and continuous aluminum plates using low-frequency eddy current testing. The detection limits were measured by using a magneto-resistive sensor. Comparing the frequency of an applied magnetic field, the detection limit at 50 Hz is deeper than that at 1 kHz. Comparing the sample structure, the detection limit in the multi-layered samples is deeper than that in the continuous samples. These results are likely due to the differences in the skin depth and conductivity of the sample.     

Anisotropic damage evolution in a 0°/90° laminated ceramic-matrix composite

Anisotropic damage evolution in a 0°/90° laminated Nicalon^™ SiC fiber-reinforced calcium aluminosilicate (CAS) glass–ceramic composite during uniaxial tensile deformation has been investigated using a variety of non-invasive characterization techniques. The elastic constant reduction in the three principal directions was measured from in situ laser-generated ultrasonic velocity measurements in various sound propagation directions. They indicate that, in addition to a large drop in elastic stiffness in the loading direction, the constants characterizing the nominal elastic stiffness transverse to the loading direction were also degraded. Surface replicas taken intermittently during loading revealed that transverse softening of the elastic stiffness was associated with fiber/matrix interface damage mainly in the 0° plies, while the large softening of the elastic stiffness in the loading direction was the result of multiple matrix cracking in both the 0° and 90° plies. While the ultrasonic data allowed a detailed characterization of the anisotropic damage evolution in this laminate, acoustic emission measurements and surface replica data identified the crack initiation stress in the 90° plies and correlated it to macroscopically observable deviations of the stress–strain curve from linear elastic behavior. These matrix cracks were found to have initiated preferentially in the weak 90° plies near the 90°/0° ply boundaries.     

Development of SQUID-based non-destructive evaluation system for detecting fatigue induced transformation of δ-ferrite to non-magnetic phases

A SQUID based NDE set up based on a precision XY scanner developed and used to evaluate the transformation of δ-ferrite to non-magnetic phases during fatigue in weldment samples has been described. The SQUID based NDE system comprises of a SQUID insert housed in a liquid helium cryostat, precision XY scanner for the sample movement and the data acquisition module to acquire the SQUID output signal with respect to the sample coordinates for flat plate as well as cylindrical samples. The system has been used to detect δ-ferrite content of virgin and fatigue tested austenitic stainless steel weldments.