Microscale and Ultrafast High Cycle Fatigue Testing

疲劳，特别是高周疲劳（HCF），是航空航天领域的一个非常重要的问题。提高对裂纹成核和生长机制的理解，通常将使设计人员能够消除设计中的一些保守性，从而减轻重量，同时保持严格要求的结构完整性。牛津大学最近的概念验证工作已经证明了微型疲劳测试的技术可行性，其中样品在弯曲中快速振动。高应力测试区域可以在几百微米到亚微米尺寸的范围内设计和切割。弯曲由~20 kHz的超声波振动驱动，这允许加速疲劳测试，其中可以在不到一分钟的时间内实现106个循环，并且可以达到千兆循环范围内的测试。通过激光微加工在较大的端部和通过聚焦离子束（FIB）在较小的端部的样品尺寸来实现。快速测试和小的物理尺寸的样品，使该方法非常适合于裂纹萌生的研究，包括初始的微塑性导致裂纹成核和生长的早期阶段，而裂纹是小的。此外，快速循环允许在非常低的裂纹生长速率下进行研究，从而可以探索接近裂纹生长速率阈值的条件，例如，对于具有有限裂纹尾流效应的小裂纹，是否存在真实的阈值Δ-K？迄今为止的应用主要集中在不锈钢和α基钛合金上。该项目将知识转移到镍基高温合金，并将方法扩展到高温。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。该计划的主要目标是：（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。这涉及调整超声波发生器的设计，以允许样品保持在一个小的管式炉。台架实验表明，这是可能的，在空气中的设计将适用于在环境/真空室内工作。(2)疲劳寿命数据作为应力幅值的函数（S-N曲线）将在室温和高温下确定多达三种合金。这将在~100微米的长度尺度上用激光微加工样品进行，以避免对强度的强烈尺寸效应。将使用断裂表面的SEM成像和整个量规部分的基于衍射的技术（高分辨率电子背散射衍射HR-EBSD和电子沟道对比成像ECCI）表征不合格样品。(3)根据已确定的基线疲劳寿命数据，将进行间歇性观察的附加试验，以研究失效前塑性和开裂的演变。这将涉及高分辨率数字图像相关（HR-DIC）的组合，以映射不可逆的累积塑性滑移，HR-EBSD和ECCI，以评估位错密度的演变和局部内应力。试件将被切割成针对特定的微观结构特征，例如在与底层树枝状微观结构相关的低角度晶界中生长。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”的组合主题一致。

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'.