敲擊法是使用最廣泛的表面機械處理及應力改質技術。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，且提高材料的負載強度及疲勞壽命。液膜超音波空蝕敲擊系統能輸出較浸水式超音波空蝕敲擊系統還高的功率且僅需較低的輸入能量即能產生空蝕場，因此非常適合運用在工業上。以往的浸水式超音波空蝕敲擊系統需要準備水槽用來在水中產生空蝕效果，因此在使用水槽的過程中，會造成空間上的佔用以及實驗操作上的不方便性。本研究利用COMSOL有限元素軟體及最佳化基因演算法分析，來設計超音波換能器的最佳幾何尺寸及建立液膜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，來使材料表面上產生敲擊效應。並就不同液膜厚度及不同的單位面積敲擊時間，來進行空蝕敲擊的效果研究，並將敲擊結果送往工研院進行表面微結構顯微分析(SEM)與表面微硬度測量(Vicker's Hardness Tester)。分析結果顯示，改變液膜厚度及單位面積敲擊時間，確實會影響表面處理的效果。在觀察不同液膜厚度下之空蝕場型態時，發現有三種不同的空蝕場型態，並隨著液膜厚度的變化而改變。並利用壓力感測薄膜(PVDF，聚偏氟乙烯)放置在液膜下，且取得空蝕汽泡在破裂時所產生的敲擊訊號，並經過快速傅立葉轉換(FFT，Fast Fourier Transform)後，分析其頻譜訊號。以及利用COMSOL模擬分析無因次液膜寬度與放大因子之間的關係，並且發現無因次液膜寬度為1時，此時無因次液膜寬度再增加，放大因子便不在增加而逐漸趨近於平緩，反之無因次液膜寬度再減少，則放大因子會明顯的減少。

Peening is the most common technology for mechanical surface treatment and surface stress improvement. The main purpose of technology is to induce compressive residual stress on material surface. This technology not only can eliminate stress corrosion cracking which is induced by tensile residual stress but also can improve the fatigue life and yield stress as well as tensile strength. Ultrasonic cavitation peening system (UCP) in thin liquid layer generate stronger power than immersion-type ultrasonic cavitation peening system and generate cavitation with lower input power which is attractive for industrial applications. For a general immersion-type ultrasonic cavitation peening system have to prepare water tank. That causes the experiment very inconvenience. This study employs COMSOL finite element method software associated with a genetic algorithm to optimize ultrasonic transducer and build ultrasonic cavitation peening system in thin liquid layer. The result of ultrasonic cavitation in thin liquid layer peening applied on stainless steel test piece specimen has done scanning electron microscope analysis and Vicker's hardness tester analysis. These analyses show that different thickness of liquid layers and different peening time per unit area are really effect of mechanical surface treatment. This study observe cavitation types with different thickness of liquid layers and discover three different cavitation types with different thin liquid layers. Moreover, this study use the pressure sensor (PVDF) to obtain the signal of peening which caused by bubbles collapses and use fast fourier transform (FFT) to analyze the spectrum. Furthermore, this study use the COMSOL software to analyze the variation of amplification factor with radius of liquid layers. If the radius of liquid layers is equal to one, then increasing the radius of liquid layers dose not make the amplification factor increase sharply.

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