Papers by Author: Bin Wu

Abstract: Most of imaging algorithms used in guided wave arrays share the same assumption that the signal scattered from defects will propagate uniformly in all directions. However, some recent researches on scattering properties of guided wave in defects demonstrate that the scattered signal is un-uniform even for a circular hole. Scattering properties of defects were studied and effects of sensors distribution strategies were presented in this paper. A 3D FE model was employed to study the sensor arrangement method and two basic imaging methods ellipse and hyperbola algorithm was used. Experiment was implemented in a 3mm aluminum plate with piezoelectric elements. Results show that for crack-like defects and through-hole, both ellipse and hyperbola imaging algorithms were valid. The number and position of transducers affect the accuracy of crack imaging. To detecting different kinds of defect the preferable array arrangement should be set according to the scattering characterization of defect.

Abstract: Truss structure is widely used in civil engineering applications for its advantages of easy transportation, convenient assembly and uniform loading. However, it is difficult to achieve real-time health monitoring because of connection diversity and complexity of truss structures. As a novel structural health monitoring technique, electro-mechanical impedance method could monitor the health state of one structure by measuring the spectra of impedance or admittance of the piezoelectric elements, which are bonded on the surface of this structure. This approach has the advantages of nonparametric model analysis, easy sensor installation and high local sensitivity, especially in sensitive frequency range. The damage information, which is tested and recorded by using electromechanical impedance method, could convert into intuitive results through neural network because of its good ability for nonlinear mapping. In this paper, a three-layer assembly truss structure was chosen as experimental object, piezoelectric elements were bonded on structure joints to measure structural impedance spectra, the change of these structural impedance spectra was tested and recorded under high frequency excitations when different truss bars were loosed, and then, one back-propagation (BP) neural network was built and trained by this damage information, which were treated as input samples. These results show that the sensitivity of impedance method is not the same to different frequency range and trained neural network could quickly identify loosen truss bars.

Abstract: An imaging system of ultrasonic detection is presented for non-destructive testing (NDT) of complex structures by using virtual instrument technology. The control devices for C-scan as well as for imaging system are designed and manufactured. In the coarse scan mode with the scan step of 1 mm, the system can quickly give an image display of a cross section of 80 mm (L) ×60 mm (D) by one measurement. In the refined scan model, the system can show a refine image of the coin. All experiments on coin, bulk metal, and other forms of specimen verify the efficiency of the proposed method. The experimental results are accurate and reliable.

Abstract: In this article, three dimensional Born inverse scattering method is modified to convenient form to reconstruct the shape of a three-dimensional flaw in an inhomogeneous cylindrical specimen. In this modified method, a measurement plane is restricted to the plane perpendicular to the axis of the inhomogeneous cylindrical specimen. Thus the cross-sectional image of the flaw can be obtained. By moving the measurement plane along the axis of the inhomogeneous cylindrical specimen, the cross-sectional image is obtained for each measurement plane. The three-dimensional flaw image is reconstructed by piling up the obtained cross-sectional images. Inhomogeneous cylindrical specimen with an eccentric circular cylindrical cavity model is prepared. The performance of the modified method to reconstruct the three-dimensional flaw is verified by using the experimentally measured waveforms. From the experimental research for the eccentric circular cylindrical defect, it is verified that the modified Born inversion works well for the volume type defect.

Abstract: It is well known that the Born approximation technique is highly effective under weak scattering conditions. In the paper, a cement paste cylindrical specimen with complex flaw is prepared and ultrasonic measurements are carried out by experimental means. The measurement area is restricted in the plane perpendicular to the axis of cylindrical specimen. The measured wave data are fed into the approximate technique formula—the inversion method and cross-sectional image is obtained. We find that good results have been obtained for strong scattering void flaws such as complex flaw in cement paste cylindrical specimen

Abstract: The objective of this paper is to predict the flaw response in an inhomogeneous solid medium. Multi-Gaussian Beam (MGB) model is used to represent the incident ultrasonic beam. The effect of ultrasonic attenuation and phase velocity dispersion due to grain scattering is included in the predictions. The effect of variation of mean diameter of the grains on the received voltage for different domain of interest is studied through the experimental results.

Abstract: Microwaves penetrate inside of low loss dielectric materials and they are sensitive to the presence of internal interfaces and non-uniformities. This allows microwave nondestructive inspection techniques to be utilized for inspecting dielectric in metallic substrate. This article simulated Microwave inspecting thickness and delamination in layered-dielectric in metallic substrate, using open-ended rectangular waveguide probe. Effective reflection coefficient of microwave is used in the detection and evaluation to thickness or delimination in the media. This paper optimized the detection frequency and standoff, which provide a reference for the experimental study.

Abstract: Ultrasonic methods are used in a wide variety of applications including medical studies, geological mapping, and nondestructive evaluation (NDE) tests. In the field of ultrasonic NDE, it is necessary to treat inverse problems of various types. The objective of this paper is to predict the flaw response in an inhomogeneous solid medium. A mathematical modelling of the testing situation is very valuable for a number of reasons. The modelling helps in developing physical intuition and in the interpretation of tests. Multi-Gaussian Beam (MGB) model is used to represent the incident ultrasonic beam. The effect of ultrasonic attenuation and phase velocity dispersion due to grain scattering is included in the predictions. The variation of received voltage is analyzed against the distance of the flaw from the transducer for different dimensions of a square cylinder void. The effect of variation of mean diameter of the grains on the received voltage for different domain of interest is also studied.

Abstract: In order to achieve active health monitoring of seven-wire steel strands, single magnetostrictive transducer with two-layer coil is developed and applied for the excitation and reception of ultrasonic longitudinal guided waves simultaneously. The transducer can be conveniently fixed at any single one position on the helical surface of these structures. The inner and outer layers of its coil are used for excitation and reception respectively with the help of same axisymmetric permanent magnets. Experimental results show that chosen ultrasonic longitudinal guided wave mode, L(0,1) at 140kHz, can be excited and received with high signal-to-noise ratio in steel strands by using the developed magnetostrictive transducer. Furthermore, to identify the performance of the transducer, the detection of an artificial defect is also conducted.

Abstract: This research develops a robust experimental procedure to monitor the evolution of early fatigue damage in AZ31 magnesium alloy with the acoustic nonlinearity parameter , and demons- trats its reliability by measuring the linear relationship between amplitudes of the second-harmonic waves and fundamental waves squared. Using this system, of two sets of specimens with different stress level is measured. The experimental results show that there is a significant increase in linked to fatigue degree in the early stages of fatigue life and reaches the maximum about 55%of fatigue life, when the stress level is ±60%of the yield stress, can characterize the early fatigue damage of magnesium alloy. However, when the stress level is ±70%of the yield stress, there is a regular fluctuation in linked to fatigue degree, this experimental results can’t be explained.