Shu-yi Zhang

Application of adaptive time-frequency decomposition in ultrasonic NDE of highly-scattering materials

In the paper, adaptive time-frequency decomposition by basis pursuit (BP) is utilized to improve ultrasonic flaw detection in highly-scattering materials as an alternative to the Wavelet Transform technique. The detection of ultrasonic pulses using the BP is described. Computer simulation was performed to verify the signal detection improvements for an ultrasonic wave embodied in white noise, and numerical results show good detection even for signal-noise ratio (SNR) of -18 dB. The improvement in detection is experimentally verified using cast steel samples with artificial flaws.

Optimal frequency-to-bandwidth ratio of wavelet in ultrasonic non-destructive evaluation

The wavelet transform is the most recent technique for processing signals with time-varying spectra. The frequency-to-bandwidth ratio of wavelet is an important factor for getting success in the practical applications of the technique. In this paper, a theoretical model is presented to estimate the optimal frequency-to-bandwidth of the Gaussian wavelet in ultrasonic non-destructive evaluation. Then the experimental verification is performed, and the experimental results confirm the availability of the theoretical model.

Thermal diffusivity of composites determined by photoacoustic piezoelectric technique

The thermal diffusivity of fiber-reinforced composites (Al2O3x/SAE321) has been studied by a photoacoustic piezoelectric technique based on a simplified thermoelastic theory. It is shown that this method is easy to perform and especially effective in determining the average thermal characters of materials with microinhomogeneous structures.

Coupling between thermoacoustic resonance pipes and piezoelectric loudspeakers studied by equivalent circuit method

In order to miniaturize thermoacoustic refrigerators, piezoelectric loudspeakers are used as acoustic sources for operating at high resonance frequencies. An integrated equivalent circuit model of a thermoacoustic resonance pipe driven by a piezoelectric loudspeaker is presented to investigate the coupling conditions between the piezoelectric loudspeaker and thermoacoustic resonance pipe. Using the equivalent circuit model the optimized coupling conditions can be obtained, with which the highest electroacoustic transfer efficiency and the largest acoustic output power of a piezoelectric acoustic source can be achieved. The theoretical simulations are taken to optimize the structures and the operating frequencies of the thermoacoustic systems.

Parametric and cascaded parametric interactions in a quasiperiodic optical superlattice

Quasi-phase-matching optical parametric and cascaded parametric processes in a two-component quasiperiodic superlattice were studied in theory and experiment. This letter demonstrates how to obtain red at 666 nm and blue at 443 nm simultaneously from the superlattice using a 532 nm laser as a pump through these two processes mentioned above. The result confirms that some nonlinear frequency conversion processes occurring in a high-dimension χ(2) nonlinear photonic crystal may be efficiently achieved in such a one-dimension quasiperiodic optical superlattice.

Acoustic anisotropy of piezoelectric PbB4O7 crystals studied by laser ultrasonics

Acoustic anisotropic properties of PbB4O7 crystals are investigated and discussed. The slowness curves of three bulk waves as well as the surface waves for the plates with different cut directions are calculated by considering piezoelectric effects. The theoretical calculations are in agreement with experimental results determined by laser ultrasonic technique. On the other hand, the excitation efficiency of surface waves of PbB4O7 crystals is obtained by calculating the change of the velocities of acoustic surface waves get from the plates bare and deposited by a thin metal film. The results of this paper can be used for selecting PbB4O7 crystal orientations for designing the related devices.

Extraction of electromechanical coupling coefficient of piezoelectric thin films deposited on substrates

Abstract A method to extract the electromechanical couplig coefficient kt2 of the thickness extensional mode of a piezoelectric thin film deposited on a substrate is presented, in which a two-layer structure is as a composite resonator. Based on the method proposed by Wang et al. (IEEE Trans. UFFC 46 (1999) 1327), we present two alternative approximate equations of kt2, then kt2 can be calculated directly from the effective electromechanical coupling factor keff2 of two special modes of the composite resonator. The new method is unnecessary to know the thickness of the piezoelectric film, which is very difficult to accuracy measure for a high frequency composite resonator or transducer. The accuracy and validity of this method are checked by numerical simulation on two typical samples. The results of numerical simulation show that the proposed approximate method is available for a real piezoelectric thin film, if the mechanical loss in the film is less than 5% and the mechanical loss in the substrate is less than 0.8%.

Mapping an underground rock mass by anisotropic acoustical transmission tomography

This paper briefly describes an acoustical transmission traveltime tomography method to simultaneously account for the anisotropy and heterogeneity of media. This method is applied to real data gathered at the rock wall of an underground plant in southwest China. The anisotropic tomograms reveal the rock wall features, which coincide well with the site observations, and provide very useful information for the evaluation and reinforcement of the rock wall.

Thermal property of biological tissues characterized by piezoelectric photoacoustic technique

A photoacoustic piezoelectric method based on a simplified thermoelastic theory is employed to determine thermal diffusivities of biological tissues. The thermal diffusivities of porcine tissues with different preparation conditions, including fresh, dry and specially prepared conditions, are characterized. Comparing the experimental evaluated diffusivities of the tissues in three conditions with each other, it can be seen that the diffusivities of the fresh tissues are the biggest and the diffusivities of the specially prepared tissues are bigger than that of the dry ones generally. The results show that the piezoelectric photoacoustic method is especially effective for determining macro-effective (average) thermal diffusivities of biological materials with micro-inhomogeneity and easy to be performed, which can provide useful information for researching thermal characters of biological tissues.

Photoacoustic spectral studies on lanthanide amino acid complexes

Several kinds of lanthanide complexes with glycine, alanine, phenylalanine, and tryptophan were synthesized and their photoacoustic (PA) spectra were measured. For the complexes of weakly fluorescent lanthanide ions with amino acids, the PA spectra reflect the influences of the ligands on the energy levels of lanthanide ions, whereas for the complexes of fluorescent lanthanide ions with amino acids, the PA spectra can be used to study the energy transfer from aromatic amino acids to lanthanide ions. At last, separating the overlapping peaks of lanthanide complex with tryptophan using the PA phase resolved method is introduced.