Hongjuan Yan

Properties of copper film characterized by scanning acoustic microscope

The scanning acoustic microscope is used to detect the properties of film or coating materials. The ultrasonic wave propagates in the materials with thickness h, sound velocity c, acoustic impedance Z2 between medium with acoustic impedance Z1. The echoes from different interfaces overlap and interfere. The interference phenomena are observed in spectrum of echoes. The spectrum has periodic maximums at integral multiples of base frequency f0. When thickness h is known, speed c of the specimen can be calculated by the relation of fn = nc/2h. According to the principle, the properties of copper films such as thickness, acoustic impendence and elastic modulus are detected by scanning acoustic microscopy. The experimental results are accorded with the actual properties of specimens.

Study on the Thick-Walled Pipe Ultrasonic Signal Enhancement of Modified S-Transform and Singular Value Decomposition

When detecting the ultrasonic flaw of thick-walled pipe, the flaw echo signals are often interrupted by scanning system frequency and background noise. In particular when the thick-walled pipe defect is small, echo signal amplitude is often drowned in noise signal and affects the extraction of defect signal and the position determination accuracy. This paper presents the modified S-transform domain singular value decomposition method for the analysis of ultrasonic flaw echo signals. By changing the scale rule of Gaussian window functions with S-transform to improve the time-frequency resolution. And the paper tries to decompose the singular value decomposition of time-frequency matrix after the S-transform to determine the singular entropy of effective echo signal and realize the adaptive filter. Experiments show that, using this method can not only remove high frequency noise but also remove the low frequency noise and improve the signal-to-noise ratio of echo signal.

Properties of GH4169 Superalloy Characterized by Nonlinear Ultrasonic Waves

The nonlinear wave motion equation is solved by the perturbation method. The nonlinear ultrasonic coefficients β and δ are related to the fundamental and harmonic amplitudes. The nonlinear ultrasonic testing system is used to detect received signals during tensile testing and bending fatigue testing of GH4169 superalloy. The results show that the curves of nonlinear ultrasonic parameters as a function of tensile stress or fatigue life are approximately saddle. There are two stages in relationship curves of relative nonlinear coefficients β′ and δ′ versus stress and fatigue life. The relative nonlinear coefficients β′ and δ′ increase with tensile stress when tensile stress is lower than 65.8% of the yield strength, and they decrease with tensile stress when tensile stress is higher than 65.8% of the yield strength. The nonlinear coefficients have the extreme values at 53.3% of fatigue life. For the second order relative nonlinear coefficient β′, there is good agreement between the experimental data and the comprehensive model. For the third order relative nonlinear coefficient δ′, however, the experiment data does not accord with the theoretical model.

Defect detection of curved thin shell based on ultrasonic transducer array

The curved spherical thin shell structures are widely used in the practical industrial applications, because of the defects in it, a lot of major accidents occurred and caused significant economic losses. So monitoring and quantifying the defects in curved plate structure are studied here. On the basis of the fundamental theory of the guide wave that propagating in the curved thin shell, and by solving and analysing the guide waves dispersion, the guide waves inspiring method has been obtained from the wave–mode conversion. The ellipse localisation imaging algorithm is applied to identify the defects orientation and localisation accurately. Wavelet transform is used to obtain the defects location parameters. A series of experiments for detecting the artificial defect at the specimen have shown that the defects direction and location can be detected effectively.

Thickness of coating tested nondestructively based on Welch frequency

A method for ultrasonic thickness measurement of coatings by Welch power spectrum is described. The multiple reflections of ultrasonic waves in coating layer lead to a series of maximum energy occurred in the Welch power spectrum which can be implemented to evaluate the thickness of coatings. A great of experiments verified it is effective and feasible to test nondestructively coating thickness on stainless steel and ceramics used as the substrate by the low-frequency immersion focused ultrasonic wave. The relative errors between the evaluated and actual thickness are estimated less than 6%.The results has proven the feasibility of thickness of coating tested nondestructively based on Welch frequency analysis.