A Holistic Method for Fatigue Durability and Reliability Evaluations of Engineering Machinery and Structures

现代损伤容限方法涉及工程对象的疲劳耐久性需要两个阶段的分析，即疲劳裂纹萌生和扩展寿命的估计。这种方法需要使用两种不同的理论和任意定义的疲劳裂纹萌生期。该方法是根据2007 - 2012年赠款范围内开展的研究活动制定的，消除了其中一些不一致之处。然而，该方法的基本要素之一是材料特性Rho *，假定为裂纹尖端的有效半径或可被视为裂纹的最小材料缺陷的尺寸。该材料常数目前是由疲劳裂纹扩展试验数据间接确定的。作为申请的2013 - 2018年发现赠款的一部分，建议对选定材料进行系统的微观结构研究，目的是在不了解任何初步疲劳数据的情况下确定Rho * 常数。我们自己的最新研究表明，可以根据断裂韧性、KIC和理论强度来评估Rho * 常数。基于UniGrow模型和相关软件的疲劳裂纹扩展的最新研究表明，仅基于疲劳裂纹扩展分析来估计总疲劳寿命是可能的。因此，有必要开发一个模型，用于模拟裂纹的尺寸与Rho * 参数可比的疲劳扩展。如果建议的研究活动得到支持，将开发一种通用的疲劳分析方法。然而，确定性分析是不够的，当需要进行可靠性和安全性决策，同时估计剩余疲劳寿命的工程对象或应用损伤容限的概念时，因为所有的输入数据实际上是分散的。因此，建议联合收割机结合确定性疲劳裂纹扩展模型与蒙特卡罗模拟方法，并运行多个疲劳裂纹扩展分析的基础上，根据其规定的概率分布采样的输入数据。如果成功，分析将提供各种工程对象的安全疲劳寿命的信息。

Contemporary damage tolerance methods concerned with fatigue durability of engineering objects require two-stage analysis, i.e. the estimation of fatigue crack initiation and propagation lives. Such approaches require using two different theories and an arbitrary definition of the fatigue crack initiation period. The methodology, developed as a result of research activities carried out within the current Grant 2007-2012, eliminates some of those inconsistencies. However, one of the basic elements of the approach is the material property Rho* assumed to be the effective radius of the crack tip or the size of the smallest material imperfection which could be consider as a crack. This material constant is at present determined indirectly from experimental fatigue crack growth data. It is proposed, as a part of the requested Discovery Grant 2013-2018, to undertake systematic microstructural studies of selected materials, with the aim of determining the Rho* constant without the knowledge of any preliminary fatigue data. Our own up-to-date studies have shown that the Rho* constant can be evaluated based on the fracture toughness, KIC, and the theoretical strength. The up-to-date studies of the fatigue crack growth based on the UniGrow model and associated software indicate that it is possible to estimate the total fatigue life based solely on the fatigue crack growth analysis. Therefore it is necessary to develop a model for simulating fatigue growth of cracks comparable in size with the Rho* parameter. If the proposed research activities are supported, a universal method of fatigue analysis will be developed. However, the deterministic analyses are not sufficient when reliability and safety decisions need to be carried out while estimating remaining fatigue lives of engineering objects or when applying the Damage Tolerance concept, because all input data are in reality scattered. Therefore, it is proposed to combine the deterministic Fatigue Crack Growth model with the Monte Carlo simulation method and to run multiple fatigue crack growth analyses based on the input data sampled according to their prescribed probability distributions. When successful the analysis would provide information on safe fatigue lives of various engineering objects.