Mixed-Mode Fatigue Crack Studies

航空航天工业正在努力设计更轻的结构，以提供更高的有效载荷、更低的碳排放和更高的燃料效率。为了做到这一点，必须尽可能有效地利用材料，因此充分了解它们在载荷下的行为是至关重要的。传统的工程设计使用实验室数据来确定结构构件的尺寸。在许多情况下，这些数据来自对破裂样本的简化测试，可能非常保守。这可能会导致过度设计的组件重量超过最佳设计。这项工作建议开发能够产生数据的实验技术，这些数据可以用来模拟实际的、轻型的、安全关键的组件。这类部件的例子是机翼蒙皮板，它们的加筋板和孔洞构成了一个复杂的加载问题，其中任何裂纹都受到多个方向的载荷，从而改变了它们的扩展方向。将研究两种实验技术：热弹性应力分析(TSA)和数字图像相关(DIC)。在TSA中，测量的是结构在循环荷载作用下经历的温度变化。这些温度变化是由施加的载荷引起的，其大小与结构表面的主应力总和成正比。另一方面，DIC使用高分辨率数码相机来跟踪三维表面特征。对图像进行分析，以确定由于加载而产生的相对位移。这两种技术都可以用来确定在多个方向上同时加载的金属或复合材料结构中的裂纹扩展机制。建议在北美用三个月的时间使用TSA和DIC方法来研究金属和复合材料在双向加载下的裂纹尖端应力场。这项工作将有助于更好地理解不同载荷大小、加载模式和塑性裂纹尖端行为之间的关系。另一个关键成果将是建立未来的合作研究项目。行程的大部分时间将在美国密歇根州立大学的混合动力汽车研究中心(CVRC)度过。还收到了访问加拿大渥太华航空航天研究所结构和材料性能实验室的邀请。CVRC建立了一系列全面的测试材料和部件的实验室设施，拥有一套最先进的光学实验力学设备。IAR是加拿大政府研发机构加拿大国家研究理事会的一部分，在实验力学方面拥有广泛的研究设施，包括对DIC和TSA的兴趣，并在一系列航空航天结构中应用。这两家世界领先的研究机构都提供了在关键的跨职能和行业相关学科发展一流研究伙伴关系的潜力。

The aerospace industry is striving to design lighter structures to give higher payloads, lower carbon emissions, and improved fuel efficiency. In order to do this, materials must be used as efficiently as possible, and so it is essential that their behaviour under load is fully understood. Traditional engineering design uses laboratory data to determine the dimensions of structural elements. In many cases these data are from simplified testing of cracked samples and can be very conservative. This can lead to over-engineered components which weigh more than the optimum design.The work proposes to develop experimental techniques capable of generating data that can be used to model actual, lightweight, safety-critical components. Examples of such components are wing skin panels, which, with their array of stiffeners and holes, present a complex loading problem, where any cracks are subjected to loads in several directions thereby altering their direction of growth.Two experimental techniques will be studied: Thermoelastic Stress Analysis (TSA) and Digital Image Correlation (DIC). In TSA, temperature changes experienced by a structure under cyclic loading are measured. These changes in temperature are caused by the applied loads and their magnitude is proportional to the sum of the principal stresses on the surface of the structure. DIC, on the other hand, uses a high resolution digital camera to track surface features in three dimensions. The images are analysed to determine the relative displacements due to loading. Both these techniques can be used to determine the mechanisms of crack propagation through a metallic or composite structure loaded simultaneously in more than one direction.It is proposed to spend three months in North America using the TSA and DIC methodologies to investigate crack tip stress fields under biaxial loads in both metallic and composite materials. This work will be used to improve understanding of the relationship between different load magnitudes, loading modes, and plastic crack tip behaviour. Another key output will be the establishment of future collaborative research projects. The majority of the trip will be spent at the Composite Vehicle Research Centre (CVRC) at Michigan State University, USA. An invitation has also been received to visit the Structures and Materials Performance Laboratory at the Institute for Aerospace Research (IAR) in Ottawa, Canada. The CVRC has established a comprehensive array of laboratory facilities for testing materials and components, with a suite of state-of-the-art optical experimental mechanics equipment. The IAR is part of the National Research Council Canada, the Canadian government's organisation for research and development and has extensive research facilities in experimental mechanics, including interests in DIC and TSA, with applications in a range of aerospace structures. Both these world-leading research institutions offer the potential to develop first class research partnerships in key cross-functional, and industrially relevant disciplines.