应变疲劳可靠性理论与方法的新进展

介绍概率应变疲劳试验法、评价模型和损伤随机演化机制方面的新进展。突破经典极大似然法在参考试验载荷之外的单试样限制，提出新的广义极大似然疲劳试验法。新发现随机循环应力—应变响应现象并建立其概率模型。新发展概率疲劳应变一寿命模型。概率模型参数由称为广义极大似然法的方法确定，在整体数据层面综合考虑数据分散性规律与样本量对概率评价的影响。考虑随机应力—应变关系反映“随机应变载荷”、随机应变—寿命关系揭示材料的“应变强度”，提出随机应变载荷—强度干涉恒幅可靠性模型，与Keciguliu的递推法结合可进行变幅和应变载荷谱下的可靠性分析，形成了应变疲劳可靠性理论和方法的新体系。进一步根据有效短裂纹准则，揭示疲劳性能随机及演化性的本质原因源于主导有效短裂纹萌生区域及其裂尖区域微观结构扩展条件的差异及演化性；随机疲劳裂纹扩展、应变—寿命和应力—应变关系是3个关联的随机疲劳关系；疲劳损伤是一个由初始混沌状态，到独立无关的随机状态，然后到史相关随机状态的演化过程。

NEW ADVANCES OF THE STRAIN-BASED FATIGUE RELIABLITY THEORY AND METHODS

New advances are introduced in fatigue test method, assessment curves, and fatigue damage mechanism dealing the strain-based fatigue reliability theories and methods. From saving cost, a so-called general maximum likelihood test method is newly presented with a few of specimens to obtain the data for assessing the material random fatigue properties. The random cyclic stress-strain responses are found and its probabilistic model is established. At the same time, a new probabilistic model of random cyclic strain-life relations is also developed. These probabilistic models should be appropriately estimated by a so-called general maximum likelihood method in which both probabilistic effects of the scattered regularities of test data and the sampling size are taken into account on a sense of entire test data. From considerations of the random stress-strain relations deriving into a random cyclic strain load and the random strain-life relations reflecting the material cyclic strain strength, a cyclic strain load-strength interference model is derived to perform the reliability analysis under a constant cyclic straining. Combining the Kecocioglu's recurrence formula, the reliability analysis under conditions of variable cyclic strain amplitudes and spectrum can be made. These indicate that a new systematic reliability theory has been established. Further, a so-called effectively short fatigue crack criterion is applied to represent the fatigue damage mechanism. It is revealed that the intrinsic cause of random fatigue properties is the difference of the microstructural conditions in the initial zone and then the zones ahead of the dominant effectively fatigue crack, which result in the specimen failure. The three random relations of fatigue crack growth rates, cyclic stress/strain-lives, and cyclic stress-strains are relatively random relations. The fatigue damage is involving a process involving from an initial chaotic state, to an independent random state, and then to historically related random state.