Fatigue Crack Growth in Complex Residual Stress Fields due to Surface Treatment and Foreign Object Damage under Simulated Flight Cycles

在航空发动机关键部件结构完整性评估中，损伤容限方法经常出现高周疲劳和低周疲劳的联合作用。低周期与起飞和降落之间的时间一致，而高周期是飞行空气动力诱导振动的结果。对航空发动机部件进行高载荷和低载荷复合载荷工况下的疲劳完整性评估，以便在实际工况下进行模拟是十分必要的。朴茨茅斯团队在HCF和LCF复合载荷条件下的疲劳裂纹扩展研究方面一直处于领先地位，与罗尔斯·罗伊斯公司、QinetiQ公司和美国空军密切合作了30多年。近年来，外界物体损伤（FOD）已被确定为限制涡轮叶片寿命的主要因素之一。在起飞和降落过程中，由于吸入小的硬颗粒而产生的冲击可以达到500米/秒的速度，并对航空发动机的机翼造成严重损坏。据估计，飞机发动机行业每年因残障造成的损失达40亿美元。由于弹丸冲击引起的疲劳强度降低已经专门研究了HCF加载条件。首次在Portsmouth进行了FOD下HCF和LCF联合对裂纹扩展影响的研究（GR/R79258，最终报告提交）。在残余应力及其对裂纹驱动力影响的评估方面取得了重大进展。其中，FOD形成的火山口根部附近存在拉伸残余应力；这些拉伸应力的深度可达0.2 mm以上。与单独加载HCF或LCF相比，在HCF和LCF联合加载块的情况下，局部应力比升高，优先从这些位置促进早期裂纹扩展，导致裂纹扩展速率显著增加。对于翼型，只有在计算裂纹驱动力时考虑残馀应力时，才能揭示裂纹的加速扩展。机翼的前缘特别容易受到FOD的影响。近年来，激光冲击强化（LSP）或低塑性抛光（LPB）等表面处理的引入显著提高了材料的疲劳强度和抗裂纹扩展能力。这种处理的目的是在风扇叶片的前缘产生显著的压残余应力，这样叶片的关键区域变得相当具有损伤容忍度，以避免灾难性故障。通常，使用LSP或LPB可以实现1-2 mm的压缩残余应力深度，而传统的喷丸强化只能达到~0.2 mm。如果要充分利用LSP/LSB来提高断裂关键部件的抗疲劳能力，特别是在发生FOD的情况下，对由于FOD而导致的疲劳损伤过程的基本理解是至关重要的。本研究旨在建立模拟飞行循环下疲劳裂纹发生和早期扩展的预测模型。该模型将考虑由于表面处理和FOD引起的残余应力的影响。动态冲击将建模，残余应力的稳定和松弛将使用有限元分析进行研究，并通过x射线衍射方法进行验证。将评估表面处理的有效性，并使用模拟飞行试验数据验证模型。如果要准确地模拟由残裂引起的初始裂纹和早期裂纹扩展，这些研究是至关重要的，这样就可以为航空发动机工业提供预测工具，以预测残裂影响的疲劳完整性。该工作有助于涡轮叶片等航空发动机关键部件的安全设计和寿命管理。

Damage tolerance approach has been employed in the assessment of structural integrity of critical aeroengine components, where a conjoint action of high cycle fatigue (HCF) and low cycle fatigue (LCF) often occurs. The low cycles are identified with the period between takeoff and landing, while high cycles are a result of inflight aerodynamically induced vibrations. It is imperative that fatigue integrity assessment of aero engine components is carried out under the combined HCF and LCF loading conditions to simulate in service loading conditions. The Portsmouth team has been one of the leading exponents in the study of fatigue crack growth under combined HCF and LCF loading conditions, in close collaboration with Rolls-Royce, QinetiQ and USAir Force for over 30 years. In recent years, foreign object damage (FOD) has been identified as one of the main life limiting factors for turbine blades. Impacts due to small hard particle ingestion during takeoff and landing can reach velocities up to 500 m/s and cause severe damage to aerofoils in aero engines. Damage due to FOD is estimated at 4 billion US dollars annually for the aeroengine industry. Reduction in fatigue strength due to projectile impacts has been studied exclusively for HCF loading conditions. The first study on the effects of combined HCF and LCF under FOD on crack growth was carried out at Portsmouth (GR/R79258, final report submitted). Significant progress has been made in the assessment of residual stresses and their effect on crack driving force. Specifically, tensile residual stresses were found in the vicinity of the crater root made by FOD; and the depth of these tensile stresses can reach to more than 0.2 mm. Upon application of a combined HCF and LCF loading block, the local stress ratios were elevated which prompted early crack growth preferentially from these sites, resulting in significantly increased crack growth rates, as compared with either HCF or LCF loading alone. For aerofoils, the accelerated crack growth was only revealed when the residual stresses due to FOD were considered in the calculation of crack driving force. The leading edge of aerofoils is particularly susceptible to FOD. In recent years, the introduction of surface treatments, such as laser shock peening (LSP) or low plasticity burnishing (LPB) have significantly improved the fatigue strength and crack growth resistance. Such treatments aim at producing significant compressive residual stresses along the leading edge of fan blades, such that the critical region of the blades becomes considerably more damage tolerant to avoid catastrophic failures. Typically, the depths of the compressive residual stresses can be achieved using LSP or LPB are 1-2 mm, as opposed to ~0.2 mm by conventional shot peening. A fundamental understanding of fatigue damage process due to FOD in the presence of LSP/LSB is vital, if they are to be utilized to the full potential in enhancing fatigue resistance of fracture critical components, particularly in the event of FOD.The proposed research aims at developing a predictive model for fatigue crack onset and early growth under simulated flight cycles. The model will take into account of the effect of residual stresses due to surface treatment as well as FOD. Dynamic impact will be modelled and the stabilised and the relaxation of residual stresses will be studied using the finite element analysis and validated by the X-ray diffraction method. The effectiveness of the surface treatment will be evaluated and the model will be validated using simulated inflight test data. Such studies are critical if onset and early crack growth due to FOD is to be modelled accurately, so that predictive tools may be made available to aeroengine industry for FOD-affected fatigue integrity. The work contributes to the safe design and life management of critical aeroengine components such as turbine blades.