Titanium 64 cold dwell fatigue loading - effect of stress concentrations and fatigue crack growth rates

在循环载荷下，最大拉伸载荷在给定的持续时间(例如2分钟)内保持不变，可能会出现冷驻留疲劳现象。这种现象被称为冷疲劳，因为它在室温下很普遍，在120摄氏度时仍然很明显，但在200摄氏度以上就会消失。人们已经发现，当某些钛合金(如Ti64)中存在强烈的晶体织构不均匀时，这种加载方式会导致疲劳寿命大幅下降。冷驻留疲劳被发现是最近发生的几起航空发动机故障的原因，这些发动机发生了钛-前风扇毂断裂。因此，飞机制造商的当务之急是更好地了解这种现象可能对其他机身部件构成的风险，这些部件可能具有不同的微观结构和加载制度。特别是，迫切需要了解驻留疲劳是否会导致紧固件周围的疲劳寿命下降，从而产生局部应力集中但更复杂的局部应力场，并在裂纹扩展过程中。这是因为机身的设计考虑了“损伤容限”，即假定存在预先存在的裂纹，并根据裂纹扩展速率计算寿命。这两个问题以前都没有在居留借记的范围内得到充分的处理。因此，该项目的目的将是使用先进的材料测试和表征技术以及建模来了解具有不同织构的Ti64材料对应力集中周围的损伤积累以及在驻留疲劳载荷条件下的裂纹扩展的敏感性。这将涉及运行高保真的现场循环加载测试，同时使用来自不同类型纹理异质性的模型材料，使用数字图像相关技术来测量微观结构中局部发生的塑性。这些实验的结果将使用Lightform研究小组(https://lightform.org.uk).)开发的建模工具与晶体塑性模拟进行比较

The phenomenon of cold dwell fatigue can occur under cyclic loading where the maximum tensile load is maintained for a given duration (e.g. 2 minutes). The phenomenon is called "cold" dwell fatigue as it is prevalent at room temperature and remains pronounced at 120C but fades out above 200C. It has been discovered that this loading regime can lead to a large knock-down in fatigue life in certain titanium alloys, such as Ti64, when there is strong crystallographic texture heterogeneity in the material. Cold dwell fatigue was found to be the cause of several relatively recent failures of aero engines, which experienced Ti-64 front fan-hub ruptures. It is therefore a high priority for aircraft manufactures to better understand the risks this phenomenon might pose to other airframe components, which can have different microstructures and loading regimes. In particular, there is an urgent need to understand if dwell fatigue can lead to a fatigue life debit around fasteners which create a local stress concentration but a more complex local stress field, and during growth of cracks. This is because airframes are designed for 'damage tolerance', where a pre-existing crack is assumed to exist and life is calculated on the basis of the crack growth rate. Both of these topics have not been previously sufficiently addressed in the context of a dwell debit. The aim of this project will therefore be to use advanced materials testing and characterization techniques and modelling to understand the sensitivity of Ti64 materials with heterogeneous textures to damage accumulation around stress concentrations and during crack propagation under dwell fatigue loading conditions. This will involve running high fidelity in-situ cyclic loading tests, while employing a digital image correlation technique to measure the plasticity that occurs locally within the microstructure, using model materials that have been sourced with different types of texture heterogeneity. The results of such experiments will be compared to crystal plasticity simulations, using modelling tools being developed in the LightForm research group (https://lightform.org.uk).