Origins of fracture and design of damage resistant materials

2010年7月，一家加拿大管道公司在密歇根州运营的一条管道泄漏并向塔尔梅奇溪泄漏了300多万升焦油砂原油，导致美国历史上最昂贵的陆上清理工作（8亿美元）。考虑到加拿大的管道容量计划在未来十年翻一番，管道裂缝的问题需要认真解决。2013年6月17日，一艘集装箱船在距也门海岸约370公里处船体中部出现裂缝，最终断成两半，10天后沉没，尽管这艘船是2008年建造的，使用了最先进的材料和制造技术。2013年6月2日，一列火车在萨德伯里脱轨，原因是车轮轴承的灾难性故障。没有人员伤亡报告，但坍塌和脱轨造成了重大损失。这些只是最近的几个例子，显示了结构裂缝如何在安全、环境和经济方面产生有害影响。因此，有必要更好地了解裂缝是如何在结构中形成的，以及它们将如何影响其使用寿命。现在设计材料不仅要考虑强度，还要考虑抗断裂性，这是一个迄今为止尚未充分开发的研究领域。本提案的长期目标是为理解材料断裂和设计具有更好的抗损伤性的材料做出重大贡献。为了实现这些长期目标，本提案确定了三个短期目标：1)确定裂纹起源的机制，以便更好地预测断裂。提出了一种将超快激光微加工和小尺度力学测试相结合的实验技术，用于量化金属裂纹的形成。2)研究一种基于局部加热的新型非破坏性方法来关闭金属零件现有裂纹，以提高其使用寿命。3)材料的微观结构一般都是为了强度而优化，没有过多考虑材料何时会断裂。我们建议开始设计材料，不仅要考虑强度，还要考虑抗断裂性。这将通过微观结构梯度和层次来实现，因为这些微观结构已经证明可以改善生物系统的断裂性能。这项工作的结果将影响科学界和加拿大金属工业。提出的研究将提供所需的工具和信息，以准确地模拟裂纹成核和金属断裂。所提出的方法依赖于小样本的超快激光加工，也可用于更好地理解包括陶瓷和聚合物在内的其他材料的断裂。由于管道、造船厂、航空航天和核工业的材料失效成本高，本提案中提出的新裂纹关闭技术有望立即引起这些行业的兴趣。这项工作还将为使用梯度和层次结构制造抗损伤结构提供新的策略，在梯度和层次结构中，裂纹的形成和扩展将更加困难。具有更好的抗损坏性的材料设计将产生附加值产品，这将有利于加拿大制造业的经济。最后，在这个项目中使用的各种实验和数值工具，从超快激光到x射线断层扫描到有限元模拟，以及开发的新概念，包括裂缝修复和抗损伤结构的设计，将为在这个项目中接受培训的本科生和研究生在工业、学术界和政府实验室的职业生涯做好准备。

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.