Origins of fracture and design of damage resistant materials

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In July 2010, a pipeline in Michigan operated by a Canadian pipeline company leaked and spilled more than three million litres of tar sands crude oil into Talmadge Creek resulting in the most expensive onshore cleanup in U.S. history (800 million US). Given that Canada's pipeline capacity is planned to double over the next decade, the issue of cracks in pipelines need to be seriously addressed. On June 17th , 2013, a container ship suffered a crack amidships about 370 km off the coast of Yemen and eventually broke in half and sank 10 days later even though the ship was built in 2008 using the most advanced materials and fabrication technologies. On June 2nd, 2013 a train derailed in Sudbury due to the catastrophic failure of a wheel-bearing. No injuries were reported but the collapse and derailment caused major damage. These are only a few recent examples showing how cracks in structures can have detrimental effects in terms of safety, the environment and the economy. There is therefore a need to better understand how cracks are formed in a structure and how they will affect its service life. It is now also necessary to design materials not only for strength but for fracture resistance which to date is a rather underexplored field of research. The long term goals of this proposal are to make a significant contribution to the understanding of materials fracture and to design materials with improved damage resistance. To address these long term goals, three short term objectives were identified in this proposal: 1) The mechanisms responsible for the origin of cracks will be identified in order to better predict fracture. A new combination of experimental techniques is proposed where ultrafast laser micromachining and small scale mechanical testing will be used to quantify the formation of cracks in metals. 2) A novel non-destructive method to close existing cracks in metallic parts based on local heating will be investigated to improve service life. 3) The microstructure of materials is generally optimized for strength without much consideration for when the material will break. We proposed to start designing materials not only for strength but also for fracture resistance. This will be done using microstructural gradients and hierarchy as these microstructures have shown to result in improved fracture properties in biological systems. The results of this work will impact both the scientific community and Canadian metal industries. The proposed research will provide the tools and information required to accurately model crack nucleation and fracture in metals. The proposed methodology, which relies on ultrafast laser machining of small samples, could also be used to better understand fracture in other materials including ceramics an polymers. The new crack closure technique presented in this proposal is expected to find immediate interest from pipeline, shipyard, aerospace, and nuclear industries due to the high costs of material failure for these industries. The work will also provide new strategies for fabricating damage resistant structures using gradients and hierarchy in which crack formation and propagation will be more difficult. Material designs with improved damage resistance will result in added value products that will benefit Canada's economy in the manufacturing sector. Finally, the variety of experimental and numerical tools used in this project going from ultrafast lasers to x-ray tomography to finite element simulations and the new concepts developed including crack repair and the design of damage resistant structures will prepare undergraduate and graduate students trained throughout this project for careers in industry, academia and at government laboratories.