The trend towards virtual testing and digital-twin assisted management means that the accurate and reliable simulation of fatigue crack propagation behaviour is more important than ever. Reliable but conservative approaches to support this are in widespread use in the aerospace industry. Nevertheless, the conservatism comes at a significant cost in terms of reduced structural life and an increased ongoing inspection requirement and, as such leads to questions about the economic burden of these approaches. Recent comparisons between blind predictions and test results revealed the extent of the issue for cracking in aluminium alloy 7075-T7351 coupons with configuration and loading representative of military transport aircraft wing skins. The current models were generally conservative by a factor of two or more in terms of crack propagation life. This suggested that there was significant scope to improve the modelling to better reflect all the complex contributing factors. The current work has investigated the issue of changes in the crack front constraint as the crack progresses from a state of high constraint (close to plane strain) to a lower constraint (approaching plane stress). This issue was investigated both experimentally and with the development of an improved analytical model. A test program was conducted on several specimens, loaded under constant-amplitude, constant-amplitude with spike-overloads and a variable amplitude spectrum. Crack-opening stress levels were measured at key points in the tests and the results were used to develop and evaluate improved modelling approaches. The improved model was generally able to predict crack growth within about ± 30 % of that demonstrated along with the correct form of the crack growth, which is a significant advance and will lead to reduced costs and increased safety.

中文翻译：

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尖峰过载和飞机谱载荷下铝合金 7075-T7351 疲劳裂纹扩展的模拟


虚拟测试和数字孪生辅助管理的趋势意味着准确可靠的疲劳裂纹扩展行为仿真比以往任何时候都更加重要。支持这一点的可靠但保守的方法在航空航天工业中得到广泛使用。然而，保守主义以结构寿命缩短和持续检查要求增加为代价，因此导致了对这些方法的经济负担的质疑。最近对盲法预测和测试结果的比较揭示了铝合金 7075-T7351 试样开裂的程度，其配置和载荷代表军用运输机机翼蒙皮。目前的模型在裂纹扩展寿命方面通常保守两倍或更多。这表明，为了更好地反映所有复杂的影响因素，还有很大的改进空间。目前的工作研究了当裂纹从高约束状态（接近平面应变）发展到较低约束（接近平面应力）时裂纹前沿约束的变化问题。通过实验和改进的分析模型的开发对这个问题进行了研究。对几个试样进行了测试程序，这些试样在恒定振幅、具有尖峰过载的恒定振幅和可变振幅谱下加载。在测试的关键点测量裂纹张开应力水平，并将结果用于开发和评估改进的建模方法。 改进的模型通常能够预测裂纹扩展±约为 30% 以及裂纹扩展的正确形式，这是一项重大进步，将降低成本并提高安全性。