Mixed-Mode Fatigue Crack Studies

航空航天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，并提高燃油效率。为了做到这一点，材料必须尽可能有效地使用，因此充分了解它们在负载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元件的尺寸。在许多情况下，这些数据来自对破碎样品的简化测试，可能非常保守。这可能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生成数据的实验技术，这些数据可用于模拟实际的、轻量级的、安全关键的组件。这类部件的例子是机翼蒙皮板，其带有一系列加强筋和孔，呈现出复杂的加载问题，其中任何裂缝都要承受几个方向的载荷，从而改变其生长方向。两种实验技术将被研究：热弹性应力分析（TSA）和数字图像相关（DIC）。在TSA中，测量了结构在循环荷载作用下的温度变化。这些温度变化是由施加的载荷引起的，其大小与结构表面上的主应力之和成正比。另一方面，DIC使用高分辨率数码相机在三维空间中跟踪表面特征。对图像进行分析以确定由于加载引起的相对位移。这两种技术都可以用来确定裂纹在金属或复合材料结构中同时向多个方向加载的扩展机制。计划在北美使用TSA和DIC方法花三个月的时间研究金属和复合材料在双轴载荷下的裂纹尖端应力场。这项工作将用于提高对不同载荷量级、加载模式和塑性裂纹尖端行为之间关系的理解。另一个重要的产出将是建立未来的合作研究项目。此次行程的大部分时间将在美国密歇根州立大学的复合材料汽车研究中心（CVRC）进行。还收到了访问加拿大渥太华航空航天研究所（IAR）结构和材料性能实验室的邀请。CVRC已经建立了一套全面的实验室设施，用于测试材料和组件，拥有一套最先进的光学实验力学设备。IAR是加拿大国家研究委员会的一部分，该委员会是加拿大政府的研究和发展组织，在实验力学方面拥有广泛的研究设施，包括DIC和TSA，并在一系列航空航天结构中得到应用。这两家世界领先的研究机构都有潜力在关键的跨职能和工业相关学科中发展一流的研究伙伴关系。

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.