The behavior of the new AA2198-T8 high strength aluminum alloy during tensile and very high cycle fatigue (VHCF) tests is studied. The results of the tensile tests show that the alloy in this heat treatment state has excellent mechanical properties compared with the same alloy with other heat treatment conditions and the 2024 and 7075 conventional aluminum alloys. The VHCF tests show a fatigue life plateau with the fatigue life reaching 10⁹ cycles under a stress amplitude of 200 MPa, suggesting the existence of an endurance limit. Two different crack initiation modes are observed under scanning electron microscopy (SEM). Cracks tend to initiate from the interior of the specimen when fatigue life is in the VHCF regime. The interior crack initiation mode has a characteristic of a fish-eye, at the center of which is a white rough area (WRA) where the original micro-crack initiates. The thermo-mechanical process during ultrasonic fatigue is also studied. Quantitative analyses on micro-crack initiation and propagation show that the threshold stress intensity factor for stable micro-crack propagation is 1.66 MPa m^1/2, and the threshold stress intensity factor for unstable macro-crack propagation is 3.17 MPa m^1/2. Finally, analyses based on the Paris-Hertzberg law suggest that the micro-crack initiation stage likely consumes most of the fatigue life. The analyses do not apply to fatigue lives less than 10⁶ cycles. The prediction of fatigue life is confirmed by the analyses of the thermo-mechanical process.

研究了新型AA2198-T8高强度铝合金在拉伸和超高循环疲劳（VHCF）测试中的行为。拉伸试验的结果表明，与在其他热处理条件下的相同合金以及2024和7075常规铝合金相比，处于这种热处理状态的合金具有优异的机械性能。VHCF测试显示疲劳寿命达到10 ^9的平稳期在200 MPa的应力振幅下循环，表明存在耐力极限。在扫描电子显微镜（SEM）下观察到两种不同的裂纹萌生模式。当疲劳寿命处于VHCF范围时，裂纹往往会从试样内部开始。内部裂纹萌生模式具有鱼眼的特征，其中心是原始微裂纹萌生的白色粗糙区域（WRA）。还研究了超声疲劳过程中的热机械过程。对微裂纹萌生和扩展的定量分析表明，稳定的微裂纹扩展的阈值应力强度因子为1.66 MPa m ^1/2，不稳定的微裂纹扩展的阈值应力强度因子为3.17 MPa m ^1/2。最后，基于巴黎-赫兹伯格定律的分析表明，微裂纹萌生阶段可能会消耗大部分疲劳寿命。该分析不适用于少于10 ⁶个循环的疲劳寿命。疲劳寿命的预测通过热机械过程的分析得到证实。