Fatigue analyses of adhesively bonded composite laminates were conducted to investigate discrepancies in experimentally measured rates of crack propagation in double cantilever beam specimens subjected to increasing or decreasing load histories. A cohesive fatigue damage model that accounts for the quasi-static traction-separation response and bridging was used in a finite element model of the test to identify transient effects caused by the evolution of the process zone in the adhesive, as well as the long-range, low-intensity bridging induced by the knit supporting carrier inside the adhesive. In addition to the quasi-static cohesive properties, the fatigue model requires two parameters that were obtained by fitting the rates of propagation at two experimental data points. The results indicate that the analyses accurately reproduce transient and steady-state rates of propagation observed in the experimental results, as well as the threshold of propagation. Transient rates of propagation consistently exceed those in steady state, which underscores the need to understand transients and account for them in fatigue characterization. The results also highlight that a Paris curve for steady-state propagation can be obtained most quickly using a decreasing load function, but that this curve is an unconservative bound to the rates of propagation. A lateral shift in the Paris curve is proposed for a conservative characterization that removes the effect of bridging and other R-curve effects.

中文翻译：

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J 控制的粘合剂疲劳表征中的传播速率瞬态

对粘合复合材料层压板进行疲劳分析，以研究在载荷历史增加或减少的情况下，双悬臂梁样本中实验测量的裂纹扩展速率的差异。在测试的有限元模型中使用了考虑了准静态牵引分离响应和桥接的内聚疲劳损伤模型，以识别由粘合剂中的工艺区域的演变以及长期影响引起的瞬态效应。范围内，由粘合剂内的针织支撑载体引起的低强度桥接。除了准静态内聚特性之外，疲劳模型还需要两个参数，这些参数是通过拟合两个实验数据点的传播速率而获得的。结果表明，分析准确地再现了实验结果中观察到的瞬态和稳态传播速率以及传播阈值。瞬态传播速率始终超过稳态传播速率，这强调了了解瞬态并在疲劳表征中考虑瞬态的必要性。结果还强调，使用递减载荷函数可以最快地获得稳态传播的巴黎曲线，但该曲线对于传播速率来说是一个不保守的界限。巴黎曲线的横向移动被提议用于保守的表征，消除桥接效应和其他 R 曲线效应。