Papers by Author: Cun Fu He

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: 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: The oxygen transmission rate (OTR) of SiOx coated polyethylene terephthalate (PET) and biaxially oriented polypropylene (BOPP) affected by fine defects is discussed in this paper. With an ultrasonic AFM (UAFM), which is an advantageous to distinguishing tiny defects on/ in the deposited films, it is found that the OTR of the coated films is relevant to the morphology scanned by UAFM. Herein SiOx layers with a thickness in the order of nano-scale were fabricated in 13.56 MHz-radio frequency (RF) -plasma-enhanced chemical vapor deposition (PECVD). The monomer for the coating fabrication is hexamethyldisiloxane (HMDSO). Fourier transform inferred (FTIR) spectra of the deposited coating with a strong peak at 1062 cm^-1, corresponding to Si-O-Si stretching vibration, confirm the formation of SiOx coatings through PECVD. The higher OTR value of SiOx coated PET is consistence with defects on film surface and in the subsurface of coatings through UAFM. It obtains that the OTR value of the defect free SiOx coated film was reduced by ca. 89% compared with the defect existence SiOx coated PET.

Abstract: A structure of capacitance sensor used for an automatic car-wiper system is presented. It aims to develop a low-cost sensor with high sensitivity and robustness. Firstly, two kinds of sensing technique based on optical and piezoelectric sensors for an automatic car-wiper system are introduced respectively. Then, three types of capacitance sensors of different shapes are designed. The parameters which affecting the sensor signal strength and measurement sensitivity are discussed. Thirdly, the electric field distributions of the sensors are simulated, and the fringe capacitance of the sensors are measured and analyzed. Finally, the optimum excitation frequency of designed sensor is determined, and the sensor noise is discussed in this paper. The experimental results indicate the validity and importance of the capacitance-based sensing technique which can be used for liquid detection on windshield of vehicles.

Abstract: The nano-thickness SiOx deposited on polyethylene terephthalate (PET) film and biaxially oriented polypropylene (BOPP) were fabricated by plasma enhanced chemical vapor deposition (PECVD) in a radio frequency (13.56 MHz) glow discharge. The nano-coatings were characterized by using Fourier transform infrared spectroscopy (FTIR), atomic force microscopy (AFM) and ultrasonic atomic force microscopy (UAFM). With AFM and UAFM, the topography and ultrasonic amplitude images were obtained. In particular, the UAFM images reveal the subsurface defects in the coating. The tensile property, contact angle and OTR of the PET present and absent of the SiOx coating were investigated experimentally respectively. The results can show that the SiOx coating can improved the barrier and the tensile strength.

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.