The irregularity of peak and valley stress values of loading histories is the main impediment for accurate fatigue-based structural health monitoring. Since the available fatigue material data (e.g., S-N curves) have been derived by cyclic loading tests, and the existing damage accumulation models can be implemented on structures under cyclic loading, cycle-counting algorithms for the transformation of a spectrum loading into an equivalent cyclic one are necessary for deterministic fatigue damage estimation and life prediction. The existing four cycle-counting algorithms have an inherent disadvantage; the sequence of the derived cycles is unknown, and the loading sequence effect on the fatigue damage estimation and life prediction cannot be accounted for (linear damage summation). The present work proposes a new damage estimation algorithm containing two modules: a new cycle-counting algorithm and a new damage summation algorithm. Unlike the existing methods, the proposed algorithm takes into account the loading sequence effect and has a solid physical base because it simulates the nonlinear damage accumulation with a multi-linear damage summation procedure. With the aid of a new concept of inserting fictitious loading cycles in the irregular spectrum, the proposed cycle counting module counts loading cycles that correspond to loading loops with an algorithm that is easily applicable in engineering practice. According to the author's knowledge, nonlinear damage accumulation estimation in structures under an irregular loading spectrum is carried out for the first time. The effect of the loading sequence to high-low two-stage random loading, low-high two-stage random loading, decreasing amplitude multi-stage random loading, increasing amplitude multi-stage random loading, and alternating amplitude random loading is examined and discussed.

中文翻译：

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非线性、基于疲劳的结构健康监测的确定性算法

加载历史的峰值和谷值应力值的不规则性是基于疲劳的精确结构健康监测的主要障碍。由于可用的疲劳材料数据（例如，SN 曲线）是通过循环载荷测试得出的，并且现有的损伤累积模型可以在循环载荷下的结构上实现，循环计数算法用于将谱加载转换为等效循环一个是确定性疲劳损伤估计和寿命预测所必需的。现有的四种循环计数算法有一个固有的缺点；导出循环的顺序是未知的，加载顺序对疲劳损伤估计和寿命预测的影响无法解释（线性损伤总和）。目前的工作提出了一种新的损伤估计算法，它包含两个模块：一个新的循环计数算法和一个新的损伤求和算法。与现有方法不同，该算法考虑了加载顺序效应，并且具有坚实的物理基础，因为它通过多线性损伤求和过程模拟非线性损伤累积。借助在不规则频谱中插入虚构加载循环的新概念，所提出的循环计数模块使用易于在工程实践中应用的算法计算与加载循环相对应的加载循环。据笔者所知，首次开展了不规则载荷谱下结构的非线性损伤累积估计。