摘要
表面機械處理或應力改質技術中敲擊法是使用最廣泛的，其目的在於使材料表層產生壓縮殘留應力，用以消除機械元件或結構的應力侵蝕裂化，提高材料的負載強度及疲勞壽命。本研究利用有限元素軟體的分析，建立超音波空蝕敲擊系統，利用空蝕汽泡反覆崩裂時所產生的應力波，造成材料產生敲擊效應。本文針對圓柱型工件設計製作可產生軸對稱空蝕場的超音波發射器，藉由基因演算法和有限元素分析模型溝通運算進行最佳化，使圓環型發射器在特定頻率下，能在中心位置產生最大聲壓。本文將採用自製的壓電換能器驅動圓環型變幅桿，觀測其產生的空蝕場、空蝕氣泡敲擊訊號及其對圓柱型表面之敲擊效果。

Summary
Development of ultrasonic circular ring radiator
Author’s Name : Wei-Che Chen
Advisor’s Name : Yi-Chun Wang
Department & College: Department of Mechanical Engineering &
National Cheng-Kung University
SUMMARY
Peening is the most common means for mechanical surface treatment or surface stress improvement. Compressive residual stress is introduced during the peening process so that stress corrosion cracking induced by tensile residual stress is eliminated and the yield stress and tensile strength as well as the fatigue life of metallic materials are improved. In this study, finite element method is used for building ultrasonic cavitation peening (UCP) system. The violent collapses of the cavitation bubbles on the material surface producing stress waves which plastically deform the surface layer and induce compressive residual stress. This research is to develop an ultrasonic emitter that can produce axisymmetric cavitation fields for the peening of cylindrical specimen. We can optimize the geometry of the horn by coupling genetic algorithms with finite element analysis model, so that the ring-type emitter at the resonant frequency can produce the maximum sound pressure at the center position. In this study, the home-made piezoelectric transducer is used to drive ring-type horn. The cavitation field, signals of peening and the effect of peening on the Aluminium foil surface will be observed.

Keywords : Mechanical surface treatment, Ultrasound, Finite element method, Cavitation, Circular ring radiator

INTRODUCTION

This research presents a new high power ultrasonic (HPU) radiator, which consists of a transducer, an ultrasonic horn, and a metal circular ring. Both the transducer and horn in longitudinal vibrations are used to drive a metal circular ring in a radial–axial coupled vibration. This coupled vibration cannot only generate ultrasound in both the radial and axial directions, but also focus the ultrasound inside the circular ring. Except for the radial–axial coupled vibration mode, the third longitudinal harmonic vibration mode with relative large vibration amplitude is also detected, which can be used as another operation mode. Overall, the HPU with these two vibration modes should have good potential to be applied in liquid processing, such as sonochemistry, ultrasonic cleaning, and mechanical surface treatment. All of these applications present the ultrasonic vibration systems with higher requirements: high power, high efficiency and longterm stable work. Therefore, as the core component of the power ultrasonic system, HPU vibration system has already become a hotspot in international researches and many scholars and engineers have been doing a lot of works in this field. Up to now, a number of types of high power composite ultrasonic transducer has been developed and used in different fields. The stepped-plate directional
HPU transducers, designed by the Spanish scholars, have already been commercialized and applied industrially. Both the resonance and radiated sound characteristics of the tubular focused sonochemistry reactor have been investigated by some Chinese scholars.

MATERIALS AND METHODS

Peening is the most common means for mechanical surface treatment or surface stress improvement. Compressive residual stress is introduced during the peening process so that stress corrosion cracking induced by tensile residual stress is eliminated and the yield stress and tensile strength as well as the fatigue life of metallic materials are improved. In this study, finite element method is used for building ultrasonic cavitation peening (UCP) system. The violent collapses of the cavitation bubbles on the material surface producing stress waves which plastically deform the surface layer and induce compressive residual stress. This research is to develop an ultrasonic emitter that can produce axisymmetric cavitation fields for the peening of cylindrical specimen. We can optimize the geometry of the horn by coupling genetic algorithms with finite element analysis model, so that the ring-type emitter at the resonant frequency can produce the maximum sound pressure at the center position.
RESULTS AND DISCUSSION

This research is to develop an ultrasonic emitter that can produce axisymmetric cavitation fields for the peening of cylindrical specimen. We can optimize the geometry of the horn by coupling genetic algorithms with finite element analysis model, so that the ring-type emitter at the resonant frequency can produce the maximum sound pressure at the center position. In this study, the home-made piezoelectric transducer is used to drive ring-type horn. The cavitation field, signals of peening and the effect of peening on the Aluminium foil surface will be observed.

CONCLUSION
Immersed-type ultrasonic emitter contain two main parts, namely piezoelectric transducer, horn, and reaction cell. Ultrasonic wave is generated by the piezoelectric transducer and transmits to the liquid in the cell through the horn. Cavitation bubbles are then generated by ultrasound. For a general commercialized immersed-type ultrasonic system, cavitation bubbles occur very near the horn tip, causing dispersion and dissipation of the ultrasound, poor ultrasonic efficiency, severe cavitation erosion of the horn, contamination of the reaction mixture, and degeneration of the system performance. To solve this problem, this study employs a finite element analysis incorporated with a genetic algorithm to optimize a 30-kHz ultrasonic ultrasonic emitter. The most important finding of this study is that, through proper analysis and design, the location of the maximum ultrasonic pressure can be moved from the vicinity of the horn tip to the interior of the cell and a large field of cavitation bubbles can be generated using a very small ultrasonic power density.

參考文獻
[1] 賴建宇,高強度超音波與氣泡空蝕場應用於奈米粉體製備與養分萃取,國立成功大學機械工程研究所碩士論文,(2005)
[2] K.S. Suslick, The chemical effects of ultrasound Scientific American, Vol.2, pp.80-86,(1989)
[3] W.T. Richards and A.L. Loomis, The chemical effects of high frequency sound waves l. A Preliminary survey, J. Am. Chem. Soc, Vol.49, No 12, pp.3068-3100,(1927)
[4] T.G. Leighton, Bubble population phenomena in acoustic cavitation, Ultrasonics Sonochemistry, Vol.2,(1994)
[5] S. Curtis, E.R. delos Rios, C.A. Rodopoulos, A. Levers, Analysis of the effects of controlled shot peening on fatigue damage of high strength aluminum alloys, International Journal of Fatigue, Vol.25, No 1, 25(1),
pp.59-66,(2003)
[6] J.E. Masse, G. Barreau, Laser generation of stress waves in metal, Surface ＆ Coatings Technology, Vol.70, No.2, pp.231-234 (1995)
[7] P. Peyre, X. Scherpereel, L. Berthe, C. Carboni, R. Fabbro, G. Beranger, C. Lemaitre, Surface modifications induced in 316L steel by laser peening and shot-peening, Materials Science and Engineering, Vol.280, No.2, pp.294-302,(2000)
[8] C.S. Montross, T. Wei, L. Ye, G. Clark, Y.W. Mai, Laser shock processing and its effects on microstructure and properties of metal alloys: a review, International Journal of Fatigue, Vol.24, No.10, pp.1021-
1036,(2002)
[9] P. Peyre, A. Sollier, I. Chaieb, L. Berthe, E. Bartnicki, C. Braham, R. Fabbro, FEM simulation of residual stresses induced by laser peening, European Physical Journal-Applied Physics, Vol.23, No.2, pp.83-88,(2003)
[10] E.S. Statnikov, O.V. Korolkov, V.N. Vityazev, Physics and mechanism of ultrasonic impact, Ultrasonics, Sup.44, pp.533-538,(2006)
[11] D. Odhiambo, H. Soyama, Cavitation shotless peening for improvement of fatigue strength of carbonized steel, International Journal of Fatigue, Vol.25, No.9-11, pp.1217-1222,(2003)
[12] H. Soyama, J.D. Park, M. Saka, Use of cavitating jet for introducing compressive residual stress, Journal of Manufacturing Science And Engineering-Transactions of The ASME, Vol.122, No.1, pp.83-89,(2000)
[13] M. Turski, S. Clitheroe, A.D. Evans, C. Rodopoulos, D.J. Hughes, P.J. Withers, Engineering the residual stress state and microstructure of stainless steel with mechanical surface treatments, Applied Physics A, Vol.99, pp.549-556,(2010)
[14] T.G. Leighton, What is ultrasound? , Progress in Biophysics and Molecular Biology, Vol.93, pp.3-83,(2007)
[15] L.E. Kinsler, A.R. Frey, A.B. Coppens, and J.V. Sander, Fundamentals of Acoustic, 3rd edition, John Wiley & Sons,(1982)
[16] B. Toukoniitty, J.P. Mikkola, D.Yu. Murzin,and T. Salmi, Utilization of electromagnetic and acoustic irradiation in enhancing heterogeneous catalytic reactions, Vol.279 ,pp.1-22,(2005)
[17] T.J. Mason and J.P. Lorimer, Applied Sonochemistry, The Uses of Power Ultrasound in Chemistry and Processing, Wiley-VCH,(2002)
[18] Li HL, H. JH, H. Chan, Finite element analysis on piezoelectric ring transformer, IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, Vol.51(10), pp.1247-1254,(2004)
[19] X. Long, L. Shuyu , H. Wenxu, Optimization design of high power ultrasonic circular ring radiator in coupled vibration, Ultrasonics Vol.51, pp.815–823,(2011)
[20] G. Hunter, M. Lucas, I. Watson, R. Parton b, A radial mode ultrasonic horn for the inactivation of Escherichia coli K12, Ultrasonics Sonochemistry, Vol.15, pp.101–109,(2008)
[21] S.W. Dahnke and F.J. Keil, Modeling of linear pressure fields in sonochemical reactors considering an inhomogeneous density distribution of cavitation bubbles, Chemical Engineering Science, Vol.54, pp. 2865-2872,(1999)
[22] A. Moussatov, C. Granger, B. Dubus, Ultrasonic cavitation in thin liquid layers, Ultrasonics Sonochemistry, Vol.12, pp.415–422,(2005)
[23] 徐鈺翔, 40-kHz 浸水式聲化學反應器共振空蝕模態之分析與實驗,國立成功大學機械工程研究所碩士論文(2011)
[24] 施智荃, 超音波空蝕敲擊技術之研究,國立成功大學機械工程研究所碩士論文 (2012)