Mixed-Mode Fatigue Crack Studies

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.

The main aim of proposed visit is that the collaborative experiments with the team at Michigan State University (MSU) and discussions at the National Research Council (NRC), Institute for Aerospace Research in Ottawa, Canada, will result in exciting plans for future experimental fracture mechanics research. The envisaged outcomes will be in the development and exploitation of: 1. additional understanding of the relationship between different load magnitudes, loading modes, and plastic crack-tip behaviour; 2. novel methods to determine accurately fracture parameters from TSA data recorded from fatigue cracks under biaxial mixed-mode loading; 3. improved understanding of the fracture mechanisms of materials under mixed-mode loading. The main non-academic beneficiaries of this research will be designers and manufacturers of any component which may fail under fatigue loading. Examples of such fatigue problems are in complex, safety-critical aerospace components such as wing skin panels, which, with their array of stiffeners and holes, present a complex mixed-mode loading problem where cracks change their growth direction. With greater understanding of failure mechanisms and thus improved structural integrity, the transportation industry will be able to design less conservative, and hence more efficient, structures in order to meet weight reduction targets, and consequently give improvements in fossil fuel use. This, in turn, will contribute to meeting the UK government targets on reduction of greenhouse gases. Other structures prone to fatigue failure are wind turbines, thus this research will impact on the production of renewable energy. Using aircraft manufacture as an example, the general public will benefit from a cleaner greener environment. With weight reduction comes noise reduction and this will benefit those living under flight paths. Additional beneficiaries will be the designers and manufacturers of new materials for complex components. For example with the use of composites, including toughened polymers, steadily moving towards primary structure applications, particularly in the aerospace industry, there is an urgent and vital need to understand and improve the toughness and damage resistance of such materials. The future collaborative research projects which should follow from this proposal could include work on advancing the understanding of the failure mechanisms in new toughened resins, thus assisting the design of novel matrices with enhanced fracture resistance properties. The maximum benefit from the initial experiments at MSU will be gained through the collaboration agreements for future research with researchers at MSU and NRC. It is anticipated that these plans will also include other collaborators in the aerospace field such as Airbus and Cytec Engineered Materials Ltd. Advances in fundamental understanding from the experiments at MSU and future collaborations will be disseminated through the traditional routes of archived publication in peer reviewed journals and contributions to conferences. Dr Tomlinson and her proposed collaborators at MSU and NRC have a wide range of contacts with industrial end users in the aerospace, automotive, military and marine sectors, and it is proposed to exploit these existing relationships for steering the direction of the future research.