Che-Hua Yang

An investigation on wedge waves and the interaction with a defect using a quantitative laser ultrasound visualization system

Wedge waves (WW) are guided acoustic waves propagating along wedge tips with their energy tightly confined near the wedge tip. Antisymmetric flexural (ASF) modes are WWs with their particle motion antisymmetric about the mid-plane bi-sectioning the apex angle. Machine tool blades exhibit typical wedge-shaped tips where defects are very likely to exist and call for nondestructive characterization. Due to the exhibited wedge shape, ASF modes are good candidate for probing the defects along the wedge tip. A quantitative laser ultrasound visualization system (QLUVS) is employed to investigate the interaction behaviors of ASF mode propagating along a damaged wedge tip. The QLUVS uses a pulsed laser generate acoustic waves which are then detected with a piezoelectric transducer. By scanning the generating laser together with the aid of reciprocal theorem, dynamic behaviors of ASF modes encountering the defect can be reconstructed.

P4G-6 A New Method for the Inspection of Tool Wear Based on the Dispersion of ASF Modes

Discovered by Lagasse in early 1970s through a numerical study, wedge waves are guided acoustic waves propagating along the tip of a wedge, with energy tightly confined near the apex. Like Lamb waves, wedge waves with displacement field anti-symmetric about the mid-apex-plane are called anti-symmetric flexural (ASF) modes. Without apex truncation, the ASF modes are non-dispersive. However, with apex truncation, the ASF modes are dispersive and the dispersion relation is highly sensitive to the apex truncation. In this research, a laser ultrasound technique (LUT) with a three- dimensional scanning is employed for a non-contact measurement of the ASF dispersion which is related to the tool wear. The basic measurement principle is the dispersion behavior of ASF mode influenced by the wear condition of the machine tool. Figure 1 shows a micrograph of the tool tip before it is used for machining. The tool is then arranged for a sequence of machining/LUT measurement procedure. The ASF dispersion curve is strongly influenced by the machining time. The measured dispersion slopes show as good measure for the quantitative wear of the tools. The current result suggests that dispersion of the ASF modes propagating along the tool tip could be an effective measure for the wear condition. The current technique is continuously been developed to meet the criteria of serving as an on-line wear characterizing technique.

Acoustic streaming induced by anti-symmetrical flexural modes near a wedge tip

Acoustic streaming (AS) is a steady fluid flow driven by the absorption of high amplitude ultrasonic vibrations due to the nonlinear effect. With potential applications in micro-pumping, the AS-related behaviors remain as interesting topics to the acoustic community. Anti-symmetric flexural (ASF) modes are wedge waves with their particle motion anti-symmetric about the apex mid-plane. With the energy tightly confined near the wedge-tip, ASF mode has relatively high acoustic amplitude which suggest its as a good candidate for the investigation of AS. In this study, the AS behaviors induced by ASF modes are investigated by computational fluid dynamics (CFD). The ASF induced three-dimensional AS flow field are presented through the CFD analysis.

Laser ultrasound technique for material characterization of thermal sprayed nickel aluminum coatings in elevated temperature environment

Thermal spraying processing usually use nickel-aluminum alloy system as major powder due to its strong adhesion to substrates. The contents of powder material and the processing parameters used in the spraying process cause material properties of coating exhibiting a wide variation. It is difficult to investigate mechanical properties of coating layer in nondestructive way. This research focuses on characterizing mechanical properties of thermal spraying coatings at high temperature environment up to 295° C in nondestructive way. A laser ultrasound technique (LUT) is used for the measurements of dispersion spectra of guided waves. Theoretical model for surface waves propagating along a multi-layered structure with coating and substrate is used to model the sprayed coatings. An inversion algorithm based on Shuffled Complex Evolution (SCE-UA) is used to extract mechanical properties from the measured dispersion spectra cooperating with theoretical model. In the results, surface wave dispersion spectra are m...

Material characterization of thermal sprayed nickel aluminum coatings using a laser ultrasound technique

Thermal spraying processing usually use nickel-aluminum alloy system as major powder due to its strong adhesion to substrates. The contents of powder material and the processing parameters used in the spraying process cause material properties of coating exhibiting a wide variation. It is difficult to investigate mechanical properties of coating layer in nondestructive way. This research aims at nondestructive characterization of thermal spraying coatings. A laser-generation/laser-detection laser ultrasound technique (LUT) is used for the measurements of dispersion spectra of guided waves. Theoretical model for surface waves propagating along a multilayered structure with coating and substrate is used to model the sprayed coatings. An inversion algorithm based on Shuffled Complex Evolution (SCE-UA) is used to extract mechanical properties from the measured dispersion spectra cooperating with theoretical model. In the results, three sets of surface wave dispersion spectra are measured for three different thickness coatings. The difference thickness of coatings is reacted to dispersion spectrum of guided waves. This method is potentially useful to characterize the mechanical properties of thermal spraying coating in a nondestructive way.

Modeling guided waves propagating in bones with a multilayered model

This study focuses on modeling guided waves propagating in bones with a multilayered model to simulate the phenomenon of osteoporosis. Double layer bone phantoms with a layer of water simulating soft tissue are used to discuss the dispersion relation of guided waves. The theoretical model is based on recursive asymptotic stiffness matrix method. A laser ultrasound technique is used to measure the experiment data. This model is used to predict dispersion spectra associated with the thinning of cortical and increasing porosity in the progression of osteoporosis. Results of this study will help to improve the accuracy for osteoporosis diagnostics using ultrasonic technique based on guided waves.

A study in wedge waves with applications in delay-line

Wedge waves are guided acoustic waves propagating along the tip of a wedge. Wedge waves has large motion amplitude and low energy attenuation when tip with small truncation. The difficulty of traditional probe measurement is hard to avoid noise from coupling agent and to raise spatial resolution by contact area. Before using wedge delay line transducer need to know the effect if wedge exist defect in tip. This study using laser ultrasound technique (LUT) and finite element method (FEM) is used to characterize the ASF modes propagation along wedge tips with defects. Defect parameter of depth are discussed regarding to their influences on the reflection coefficient (RC) and transmission coefficient (TC). Also this study discover wedge wave has mode conversion (MC) at tip with defect.

A full-field mechanical property mapping reconstruction algorithm with quantitative laser ultrasound visualization system

This research employs a quantitative laser uItrasound visualization system (QLUVS) for the full-field mechanical property mapping in plate-Iike structures. The QLUVS has the advantage of fast, full-field and quantitative inspection. The QLUVS uses a pulsed laser generate acoustic waves with fast scanning mechanism to reach two dimensional scanning goal and then detected with a piezoelectric longitudinal transducer. By utilizing QLUVS, the spatial and temporal information of guided wave can be obtained with further signal processing, the velocity map can be extracted. Finally the analytical model and inversion algorithm will be integrated into QLUVS for the full-field mechanical property mapping purpose.

Material characterization for Zircaloy claddings in elevated temperatures using a laser ultrasound technique

Material properties of Zircaloy cladding can degrade substantially and threaten the fuel integrity while in nuclear reactor service at elevated temperature. Other than radiation exposure, precipitated hydrides are also shown to be responsible for hydrogen embrittlement of the claddings. A procedure corporate with an experimental technique is used to investigate the effects of high temperature on the dispersion spectra of guided waves. A laser ultrasound technique (LUT) is used to measure the dispersions of guided waves propagating along the axial direction of the cladding tubes at different temperature environment. It is shown that the LUT is able to measure the dispersion curve of Zircaloy in the elevated temperature environment. The dispersion spectra shift towards the direction of lower frequency and lower velocity while the temperature increase. With the inversion procedure, material properties such as elastic modulus is successfully characterized in various elevated temperature. The Youngs modulus is found to decrease linearly as the temperature increasing. This research is focuses on the characterization of material properties, elastic moduli in particular, at elevated temperatures for Zircaloy cladding tubes of various hydrogen concentrations (H.C.).

Guided waves propagating in plate with temperature gradients

This research is focused on the theoretical modeling and experimental measurements for the dispersion relations of guided waves propagating in different temperature gradient of plates. A theoretical model based on a recursive asymptotic stiffness matrix method (RASM) with recursion computation algorism is used to provide numerical calculations for the dispersion relations. A laser ultrasound technique is used to measure the dispersion relations. For all the experiments, the measured dispersion curves show good agreement with the theoretical calculation, indicating the reliabilities in the measurement and modeling. This study is useful to thermometry in different temperature gradient environment in a non-contact and non-destructive ways.