The effect of inclusions on crack initiation and propagation in gigacycle fatigue was investigated experimentally and analytically in six high strength low alloy steels. Fatigue testing was performed at very high numbers of cycles through ultrasonic fatigue tests at 20kHz. Inclusions at subsurface are common sites for fatigue crack nucleation in these alloys when cycles to failure was 107 cycles. A significant change in the slope of the S-N curve was observed accompanying the transition from surface to subsurface crack initiation. A deterministic model has been developed to predict the total fatigue life, i.e. crack initiation life and crack propagation life, from the measured inclusion sizes. The predicted fatigue strength agreed reasonably well with the experimental results. It is a tendency that smaller inclusions are associated with longer fatigue life. The results demonstrated that the portions of life attributed to subsurface crack initiation between 107 and 109 cycles are>99%. Science Ltd. All rights reserved.

The objective of this paper is to determine the very long fatigue life of ferrous alloys up to 1

Corrosion-fatigue is one of the damage mechanisms affecting the structural integrity of the aging aircraft. Simple analytical and probabilistic models for predicting pitting corrosion and corrosion-fatigue life in aluminum alloy 2024/T3 used in aircraft structures is presented. The developedmodel includes the effect of the interaction of cyclic load and the corrosive environment on the fatigue life, and considers all stages of the corrosion-fatigue process. The probabilistic model investigated considers the uncertainties in initial pit size, corrosion pitting current, and material properties due to scatter found in the experimental data.The analytical model shows very good quantitative agreement with the existing experimental data and analytical models over a wide range of parameters. The results predicted that the crack initiation life varies approximately 10-40% as compared to crack propagation life in corrosion-fatigue problems.

通过20kHz压电超声加速疲劳试验机对多种金属材料进行测试，实验地证明了超高周疲劳裂纹往往从试件内部萌生的机制。试件在超过107周应力循环后继续发生破坏。研究还发现，材料超高周疲劳的S-N曲线可以表现出另两种有别于传统规范的107极限型，即持续下降型和阶梯下降型。还总结了近年来超声疲劳的研究进展，结合作者多年国外的研究工作，着重从加速超声疲劳试验设备的发展，多种材料及条件下的实验研究结果及其重要结论作了概述与讨论。

利用20kHz压电超声疲劳试验技术，对六种抗拉强度高于1500MPa的低合金钢进行测试。结果表明，这些材料并不存在传统规范中所谓的“疲劳极限”，它们在超过107，甚至109应力循环后仍然继续发生破坏，而且其S-N曲线表现出“阶梯”型特征。研究还发现，这些材料都存在一个分界应力幅值区间（或应力循环数区间,大约在106到107 的平台内），裂纹萌生由试件表面向其内部转换。实验证明了十亿周级超高周疲劳裂纹由试件内部萌生的机理。