Microscale and Ultrafast High Cycle Fatigue Testing

微型和超快高周疲劳测试

• 批准号: 2439042
• 金额: --
• 依托单位: University of Oxford
• 依托单位国家: 英国
• 项目类别: Studentship
• 财政年份: 2020
• 资助国家: 英国
• 起止时间: 2020 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=studentship-2439042
• 关键词: Microscale; Ultrafast; Cycle; Fatigue; Testing

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'.

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。提高对裂纹形核和扩展机制的理解通常会使设计人员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已经证明了微型疲劳测试的技术可行性，其中样品在弯曲时快速振动。高应力测试区域可以在几百微米到亚微米的范围内设计和切割。弯曲是由~ 20khz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个循环，并且可以实现千兆循环的测试。样品的尺寸是通过激光微加工在较大的一端和聚焦离子束（FIB）在较小的一端实现的。试样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起裂的研究，包括导致裂纹成核的初始微塑性和裂纹很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，从而可以探索接近裂纹扩展速率阈值的条件，例如，对于裂纹尾迹效应有限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α基钛合金上。该项目将把知识转移到镍基高温合金，并将方法扩展到高温。该计划旨在提供理解疲劳失效基本方面所需的新科学知识，并与重要和特定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。这包括调整声电极设计，使样品能够在一个小管式炉内保持。台式实验表明，这在空气中是可能的，该设计将适应在环境/真空室中工作。(2)在室温和高温下，将确定多达三种合金的疲劳寿命数据作为应力幅值（S-N曲线）的函数。这将在~100微米的长度尺度上用激光微加工样品来实现，以避免强烈的尺寸对强度的影响。失效样品将使用断裂面的SEM成像和基于衍射的技术（高分辨率电子反向散射衍射HR-EBSD和电子通道对比成像ECCI）在规范截面上进行表征。(3)在建立基线疲劳寿命数据的基础上，将进行额外的间歇性观察试验，以研究破坏前塑性和开裂的演变。这将涉及结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来评估位错密度演变和局部内应力。测试件将被切割成特定的微观结构特征，例如生长在与潜在枝晶微观结构相关的低角度晶界中。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“未来制造”的组合主题保持一致。