双剪连接件及双耳连接耳片疲劳寿命估算的逐次累计求和算法

为了研究复杂连接件疲劳失效机理和估算其裂纹形成和扩展寿命，在双剪连接件和双耳连接耳片疲劳试验的基础上，首先，通过扫描电子显微镜（SEM）分析，研究了其破坏模式和机理，并利用断口定量反推技术判读了疲劳裂纹形成与扩展寿命。然后，根据应力严重系数法，建立了复杂连接件疲劳性能S-N-L（疲劳应力-寿命-应力严重系数）曲面；利用该曲面，发展了复杂连接件疲劳裂纹形成寿命估算算法；基于断裂力学理论，推导出裂纹扩展长度与扩展角度公式，建立了疲劳裂纹扩展寿命估算的累计求和算法。最后，运用寿命估算方法，估算了双剪连接件的疲劳裂纹形成寿命、双剪连接件和双耳连接耳片的裂纹扩展寿命，预测结果与断口判读结果吻合良好。本文寿命估算方法为复杂连接件疲劳定寿提供了理论依据。 

Cycle-by-cycle accumulation algorithm for predicting fatigue lives of double-lap and double-lug joints

This paper aims to investigate the failure mechanics of complex joints and evaluate the crack initiate and growth lifetimes of complex joints. Fatigue tests were respectively carried out on double-lap and double-lug joints, failure modes and mechanisms were investigated through scanning electron microscope (SEM) analysis, and fatigue lifetime for crack initiation and growth was determined from fractographic quantitatively interpreted data by using reverse inference method. From the stress severity factor method, fatigue S-N-L (stress-life-stress severity factor) surface model was developed to characterize fatigue characteristics of complex joints for predicting fatigue crack initiation life. Based on fracture mechanics, the formulations were crafted to predict the length and angle of crack growth, and the cycle-by-cycle accumulation algorithm was presented to assess the lifetime of crack growth of joint. Finally, the proposed model and algorithm were used to respectively simulate the lifetime of fatigue crack initiation and propagation for double-lap and double-lug joints, which demonstrates the good agreement between the prediction results and the fracture interpretation results. The proposed model and algorithm provide theoretical basis for determining fatigue lifetime of complex joints.