Microscale and Ultrafast High Cycle Fatigue Testing

疲劳，特别是高周疲劳(HCF)，对航空航天等行业来说是一个非常重要的问题。提高对裂纹形核和扩展机制的了解，通常将使设计者能够消除设计中的一些保守性，从而在保持关键要求的结构完整性的同时减轻重量。牛津大学最近的概念验证工作证明了微型疲劳测试的技术可行性，在这种测试中，样品在弯曲时快速振动。高应力测试区的设计和切割范围可以在几百微米到亚微米的范围内。弯曲是由~20 kHz的超声波振动驱动的，这使得加速疲劳测试可以在一分钟内完成106次循环，并且可以进行千兆周的测试。样品尺寸较大端采用激光微细加工，较小端采用聚焦离子束(FIB)。样品的快速测试和较小的物理尺寸使得该方法非常适合于裂纹萌生的研究，包括导致裂纹形核的初始微塑性和裂纹小时的早期扩展阶段。此外，快速循环允许在非常低的裂纹扩展速率下进行研究，以便探索接近裂纹扩展速率阈值的条件，例如，对于具有有限裂纹尾迹效应的小裂纹，是否存在真正的阈值Delta-K？迄今为止，应用主要集中在不锈钢和阿尔法基钛合金上。该项目将向镍基高温合金传授知识，并将方法推广到高温。该计划旨在提供了解疲劳失效的基本方面所需的新科学知识，并与重要和具体的行业需求保持一致。该方案的主要目标是：(1)第一个目标是将测试能力从室温扩展到高温，并控制测试环境。这涉及到调整声波电极的设计，使样品可以保存在一个小管式炉中。台式实验表明，这在空气中是可能的，该设计将被调整为在环境/真空室中工作。(2)将确定多达三种合金在室温和高温下的疲劳寿命数据作为应力幅值的函数(S-N曲线)。这将在~100微米长度的激光微机械加工样品中进行，以避免对强度产生强烈的尺寸效应。失效样品将使用断口的扫描电子显微镜成像和基于衍射的技术(高分辨率电子背散射衍射HR-EBSD和电子沟道对比成像ECCI)进行表征。(3)利用建立的基线疲劳寿命数据，将进行额外的试验和断续观察，以研究失效前的塑性和裂纹的演变。这将涉及到组合高分辨率数字图像相关(HR-DIC)来绘制不可逆累积塑性滑移，以及HR-EBSD和ECCI来评估位错密度演化和局部内应力。试件将被切割成特定的微结构特征，例如与底层树枝状微结构相关的低角度晶界生长。该项目与劳斯莱斯合作，研究领域与材料工程-金属和合金以及“工程”和“制造未来”的投资组合主题保持一致。

Fatigue, and in particular high cycle fatigue (HCF), is a very significant issue for the aerospace sector amongst most others. Improved understanding of crack nucleation and growth mechanisms will generally enable designers to remove some of the conservatism in design and thus reduce weight while maintaining the critically required structural integrity. Recent proof of concept work at Oxford has demonstrated the technical feasibility of miniature fatigue testing where the sample is rapidly vibrated in bending. The highly stressed test regions can be designed and cut in the range of a few hundred micrometres across down to sub-micron dimensions. The bending is driven by ultrasonic vibration at ~20 kHz which allows accelerated fatigue testing in which 106 cycles can be achieved in a little under a minute, and test out into the giga-cycle regime are attainable. The sample sizes are achieved by laser micro-machining at the larger end and by focused ion beam (FIB) at the smaller end.The rapid testing and small physical size of the sample makes the method well-suited to studies of crack initiation, including initial incipient micro-plasticity leading to crack nucleation and the early stages of growth while the crack is small. Additionally, the rapid cycling allows studies at very low crack growth rates so that conditions close to the crack growth rate threshold can be explored, for example is there a real threshold delta-K for small cracks with limited crack wake effect?Applications to date have concentrated on stainless steel and alpha based titanium alloys. This project will transfer knowledge to nickel-based superalloys and extend methodologies to elevated temperatures. The programme is set out to deliver new scientific knowledge required for fundamental aspect of understanding fatigue failure and is aligned with important and specific industry needs. The main goals of the programme are:(1) The first aim is to extend testing capabilities from room temperature to elevated temperatures and with control of the test environment. This involves adapting the sonotrode design to allow the sample to be held within a small tube furnace. Bench top experiments show this is possible in air the design will be adapted to work within an environmental/vacuum chamber.(2) Fatigue life data as a function of stress amplitude (S-N curves) will be determined for up to three alloys at room and elevated temperatures. This will be pursued with laser micro-machined samples at the ~100 micrometer length scale to avoid strong size effects on strength. Failed samples will be characterised using SEM imaging of fracture surfaces, and diffraction based techniques (high resolution electron back scatter diffraction HR-EBSD, and electron channelling contrast imaging ECCI) across the gauge section.(3) With base-line fatigue life data established additional tests with intermittent observations will be made so as to investigate the evolution of plasticity and cracking before failure. This will involve combinations of high resolution digital image correlation (HR-DIC) to map irreversible accumulated plastic slip, and HR-EBSD and ECCI to evaluate dislocation density evolution and local internal stresses. Test pieces will be cut to target specific microstructural features such as grown in low angled grain boundaries associated with underlying dendritic microstructure.The project is in collaboration with Rolls Royce and the research area aligns with Materials Engineering - Metals and Alloys and portfolio themes of both 'Engineering' and 'Manufacturing the Future'.