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发生泄漏，将超过300万升的焦油砂原油泄漏到Talmadge Creek，导致美国历史上最昂贵的陆上清理费用（8亿美元）。鉴于加拿大的管道容量计划在未来十年翻一番，管道裂缝问题需要认真解决。2013年6月17日，一艘集装箱船在距离也门海岸约370公里处的船中部发生裂缝，最终断成两半并在10天后沉没，尽管该船是在2008年使用最先进的材料和制造技术建造的。2013年6月2日，一列火车在萨德伯里出轨，原因是车轮轴承发生灾难性故障。没有人员受伤的报道，但坍塌和脱轨造成了重大损失。这些只是最近的几个例子，表明结构中的裂缝如何在安全，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，而且还要考虑抗断裂性，这是迄今为止尚未充分探索的研究领域。本提案的长期目标是对材料断裂的理解做出重大贡献，并设计具有改进的抗损伤性的材料。为了实现这些长期目标，本提案确定了三个短期目标：1）将识别导致裂纹起源的机制，以便更好地预测断裂。提出了一种新的实验技术组合，其中超快激光微加工和小规模机械测试将用于量化金属中裂纹的形成。2）将研究一种基于局部加热来封闭金属零件中现有裂纹的新型非破坏性方法，以提高使用寿命。3)材料的微观结构通常针对强度进行优化，而不太考虑材料何时断裂。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。这项工作将利用显微结构梯度和层次结构来完成，因为这些显微结构已被证明会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。拟议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断裂的金属。所提出的方法依赖于小样品的超快激光加工，也可以用于更好地了解其他材料（包括陶瓷和聚合物）的断裂。本提案中提出的新裂纹闭合技术预计将立即引起管道、造船厂、航空航天和核工业的兴趣，因为这些工业的材料失效成本很高。这项工作还将提供新的策略，用于制造抗损伤结构，使用梯度和层次，其中裂纹的形成和扩展将更加困难。具有改进的抗损伤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，该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，再到有限元模拟，以及开发的新概念，包括裂缝修复和抗损伤结构的设计，将为整个项目中培训的本科生和研究生做好准备，以便在工业，学术界和政府实验室工作。

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.