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An assessment of a strain-life approach for fatigue crack growth

机译：对疲劳裂纹扩展的应变寿命方法的评估

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Purpose - Fatigue crack growth models based on elastic-plastic stress-strain histories at the crack tip region and strain-life damage models have been proposed. The UniGrow model fits this particular class of fatigue crack propagation models. The residual stresses developed at the crack tip play a central role in these models, since they are applied to assess the actual crack driving force. This paper aims to assess the performance of the UniGrow model based on available experimental constant amplitude crack propagation data, derived for several metallic materials from representative Portuguese bridges. It also aims to discuss key issues in fatigue crack growth prediction, using the UniGrow model, in particular the residual stress computation and the suitability of fatigue damage rules. Design/methodology/approach - The UniGrow model is assessed using data derived by the authors for materials from Portuguese riveted metallic bridges. Strain-life data, from fatigue tests on smooth specimens, are used to propose a convenient fatigue damage model. Predicted crack growth rates are compared with experimental crack propagation data obtained by authors using fatigue tests on compact tension specimens. Since the UniGrow model is a residual stress-based propagation model, elastoplastic finite element analysis is proposed for comparison with the analytical approach implemented in the original UniGrow model. Findings - The use of the Smith-Watson-Topper damage parameter overestimates the stress R-ratio effects on crack propagation rates, mainly if the material shows crack propagation rates with small to moderate sensitivity to stress R-ratio, which is the case of the materials under investigation in this paper. Alternatively, the application of the Coffin-Manson damage law leads to consistent fatigue crack growth predictions for the investigated range of positive stress R-ratios. The stress R-ratios effects may be solely attributed to the residual stresses. Their estimation, using an analytical approach, may lead to inconsistent results, which is demonstrated by an alternative elastoplastic finite element analysis. Originality/value - Contributions for more accurate predictions of fatigue crack propagation rates, for several stress ratios, using a strain-based approach is proposed. This approach is valuable since it may be used to reduce the time consuming and costly fatigue crack propagation tests. Furthermore, the proposed approach shows potential for an unified crack initiation and propagation approach.
机译：目的-提出了基于裂纹尖端区域的弹塑性应力-应变历史的疲劳裂纹扩展模型和应变寿命损伤模型。 UniGrow模型适合此类特定的疲劳裂纹扩展模型。裂纹尖端处产生的残余应力在这些模型中起着核心作用，因为它们被用于评估实际的裂纹驱动力。本文旨在根据可得的实验性恒定振幅裂纹扩展数据评估UniGrow模型的性能，该数据是从代表葡萄牙桥梁的几种金属材料中得出的。它还旨在使用UniGrow模型讨论疲劳裂纹扩展预测中的关键问题，尤其是残余应力计算和疲劳损伤规则的适用性。设计/方法/方法-UniGrow模型是使用作者从葡萄牙铆接金属桥材料获得的数据进行评估的。通过对光滑试样进行疲劳测试得到的应变寿命数据，可以提出一种方便的疲劳损伤模型。将预测的裂纹扩展速率与作者在紧凑的拉伸试样上使用疲劳测试获得的实验裂纹扩展数据进行比较。由于UniGrow模型是基于残余应力的传播模型，因此提出了弹塑性有限元分析，以与原始UniGrow模型中实施的分析方法进行比较。研究结果-使用Smith-Watson-Topper损伤参数会高估应力R-比率对裂纹扩展速率的影响，主要是如果材料显示的裂纹扩展速率对应力R-比率的敏感性小到中度的话。本文正在调查的材料。另外，对棺材-曼森损伤定律的应用可以对研究的正应力R比率范围产生一致的疲劳裂纹增长预测。应力R-比率效应可以完全归因于残余应力。使用分析方法进行的估计可能会导致结果不一致，这可以通过其他弹塑性有限元分析来证明。原创性/价值-提出了使用基于应变的方法对几种应力比率下的疲劳裂纹扩展速率进行更准确预测的贡献。这种方法很有价值，因为它可用于减少耗时且昂贵的疲劳裂纹扩展测试。此外，所提出的方法显示了统一的裂纹萌生和扩展方法的潜力。

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