表面應力改質主要目的是藉由各種直接、間接的機械或加熱方式，改變材料表面應力狀況以抑制或緩和材料破裂劣化現象。超音波促發空蝕震波可應用於具高張應力及破裂敏感度高之表面。空蝕氣泡在金屬表面潰滅時可產生高強度震波及高速微流，直接衝擊金屬表面造成具壓應力之塑性變形層。本硏究主要是利用超音震波進行表面應力改質技術開發，於純水中針對SS304不銹鋼材料表面以超音波變幅桿進行20 kHz震波處理。測量超音波處理後對材料表面粗糙度、硬度及微結構特性之影響，同時也進行殘留應力層深變化量測及裂縫抑制效應的驗證。實驗結果顯示超音震波表面應力改質技術對於金屬表面粗糙度只有 輕微的影響，表面所產生的壓應力影響層深度可達150μm以上。應力腐蝕測試結果顯示，在嚴重腐蝕環境中也具備顯著抑制破裂傾向之功效。

The objective of stress improvement is to alter the stress state of processed surface through direct or indirect thermo-mechanical methods to prevent or mitigate material cracking susceptibilities. Ultrasound induced cavitation impact can be utilized to reduce cracking propensity of surfaces in the presence of high tensile stresses. Cavitation bubbles upon collapse can generate high tensile shock waves and high velocity micro jet streams which will directly impinge on the metal surface and thus create a compressively plastic deformation layers. This study is to examine the effectiveness of ultrasonic stress improvement technology (USIT). Ultrasonic cavitation treatment of SS304 stainless steel with horn type probes at 20 kHz under pure water. The cavitation impact on surface is measured in terms of surface roughness, hardness and microstructural characteristics. In addition, the in-depth residual stress distribution and crack mitigation effect are also evaluated. Test results indicate controlled ultrasonic stress improvement treatment has only a minor effect on surface roughness. The extent of compressive stress can reach a depth above 150 μm underneath the surface. Stress corrosion cracking tests show effective mitigation of environmental assisted cracking of susceptible stainless steel in highly corrosive conditions.