结构系统疲劳裂纹形成寿命的可靠性分析

Reliability Analysis of Fatigue Life of a Structural System

以Miner线性累积损伤理论为依据,在全量加载方法的基础上提出了在给定外载和使用寿命条件下计算结构系统疲劳寿命可靠度的阶段分枝—约界法。采用此方法对典型结构系统在对称循环外载作用下的疲劳寿命可靠度进行了数值计算,由此得出一些有益的建议和结论。

It has been verified by previous research that fatigue failure is the main failure mode of structural systems. Explorations on reliability analysis method of fatigue life belong to the frontier of the field of fatigue research. In most cases, important components such as stressed boxes of wing are designed as fail-safe multi-path force-transfer structures based on Damage Tolerant Design Regula- tions. Such a type of structure is multi-redundant and has a number of possible failure modes. A large memory and storage capacity and much computer running time are needed to discover all possible failure modes and compute the total failure probability. This is an unsatisfactory feature of existing methods. Another more serious unsatisfactory feature is that some important failure modes may still escape discovery. This paper succeeds in elimi- nating the above-mentioned two shortcomings. The authors success depends on the reasoning that, in fact, correlation exists among different failure modes. Thus the total failure probability of a structural system can be accurately estimated by considering just the significant failure modes. The main contributions of this paper is the proposal of a method for enumerating the significant fatiguc failure modes of a structural system and analyzing the failure evolving history. The proposed method is called by the authors Stage-Fatigue-Life Branch-and-Bound Method. The proposed method has the following three features: (1) Stage-Fatigue-Lives are taken as bounding parameters. Using these bounding parameters and employing one-order and multi-order searching method, the authors are completely confident that no significant failure modes can escape discovery. A rigorous proof that all significant failure modes can be discovered is given in the paper. (2) The algorithm proposed by the authors for Stage-Fatigue-Life Branch-and-Bound Method is completely automatic. In contrast, algorithms proposed by previous authors depend on empirical parameters and / or selection of parameters by trial and error. To the authors best knowledge, the fatigue life reliability of a large scale engi- neering structure has not yet been analyzed and computed. The authors fully automatic algorithm make such analysis and computation feasible. (3) The authors proposed algorithm is rigorously convergent. A good efficiency in convergence is obtained by employing adjustable branching bounds. In analyzing and computing the reliability of a structural system, it is evident that the reliability of a component must be first considered. While Miner's linear accumulative damgae rule can be used in computing component accumulative damage and reliability, the authors believe something better can be done. Another contribution of this paper is to propose something better. The authors intro- duce the concept of normalized damage factor and establish a rational randomized accumulative damage rule. The authors rule eliminates the shortcoming of statistical inconsistency that exists between previous accumulative damage algorithms and fatigue failure criterion. It is shown that an approximate value of system failure probability can be estimated by means of the failure probability of the main significant failure mode. This is believed to be useful in preliminary design of aircraft.