QUBE: QUasi-Brittle fracture: a 3D Experimentally-validated approach

延性材料，如金属和合金，被广泛应用于工程结构中，无论是作为自身还是作为加固。在失败之前，它们通常可以承受大量的塑料损伤。工程师们非常了解金属失效的方式以及它们对破坏的容忍度，因此，设计高效且质量较小的结构时，可能会有明确的安全裕度或储备强度来应对极端事件。相比之下，玻璃和陶瓷等弹性脆性材料可以在没有事先警告的情况下失效，因此需要更大的安全裕度。准脆性材料是一类重要的结构材料。它们是具有一定损伤容忍度的脆性材料，包括混凝土、多颗粒石墨、陶瓷基复合材料、岩石等地质结构以及骨和骨替代物等生物医学材料。尽管它们的损伤容忍度远低于许多金属和合金，但与陶瓷和玻璃等脆性材料相比，它可能是相当重要的。但当工程师设计或评估准脆性材料时，这并不能很好地解释这一点，因为没有充分了解材料的微观结构或应力状态等因素对他们损伤容限的作用。准脆性材料通常被视为完全脆性的，很少或根本不考虑它们的损伤容忍度，因此评估纳入了非常重要的安全裕度，导致设计可能效率低下和不必要的笨重。即使包括了一些损伤容限的评估，微观结构也会随着材料的老化而发生变化，我们需要测量这种影响的方法，并预测它将对结构的安全造成什么影响。该项目旨在开发一种方法来预测由准脆性材料制成的结构和部件的性能并评估其完整性。这将扩大它们在工程应用中的应用机会，使设计更有效，安全性更有信心。准脆性是由材料的微观结构产生的一种特性。准脆性材料可以由非常易碎的部件(例如，多孔陶瓷)组成的网络连接而成。它表现出一种典型的“优雅”故障，因为部件在充分加载时会局部断裂，这赋予了它损坏容限。网络的强度和刚度以及连接形式的随机变化会影响到失效的“优美度”。这样的网络代表了材料微观结构的关键部分，为了理解准脆性断裂，我们需要构建适当描述微观结构的模型。有必要了解和定义在这些准脆性材料中控制小尺度和大尺度断裂的机制。这将使我们能够捕捉对微观结构差异和退化的敏感性，并产生适用于广泛的准脆性材料和应用的通用模型。使用现代三维显微镜方法，如X射线计算机层析成像，可以创建忠实于微观结构的三维模型。但这些模型太复杂了，不能简单地放大到相对于微观结构非常大的结构。目前还没有一台计算机能做到这一点。我们将开发能够在大范围的长度尺度上充分表示准脆性微结构的复杂性的建模方法，例如元胞自动机有限元。我们将使用先进的断层扫描和应变映射技术来观察损伤是如何发展的，并测试和改进我们的模型。然后，我们将利用这一点和我们所获得的理解来设计新的材料测试和表征方法，以便我们的方法可以用于从混凝土到先进核复合材料、骨替代生物材料和地质材料的广泛材料。

Ductile materials, like metals and alloys, are widely used in engineering structures either by themselves or as reinforcement. They usually can sustain a lot of plastic damage before failing. Engineers understand quite well the ways that metals fail and how tolerant they are to damage, so efficient and less massive structures may be designed with well-defined margins of safety or reserve strength to cope with extreme events. By comparison, elastic brittle materials such as glasses and ceramics can fail without prior warning, so much larger safety margins are needed.Quasi-brittle materials are an important class of structural materials. They are brittle materials with some tolerance to damage and include concrete, polygranular graphite, ceramic-matrix composites, geological structures like rocks and bio-medical materials such as bone and bone replacements. Although their damage tolerance is much less than many metals and alloys, it can be quite significant compared to brittle materials such as ceramics and glasses. But this is not accounted for very well when engineers design with, or assess, quasi-brittle materials, as there is not an adequate understanding of the role on their damage tolerance of factors such as the microstructure of the material or the state of stress. Quasi-brittle materials are usually treated as fully brittle, taking little or no account of their damage tolerance, so assessments incorporate very significant safety margins, leading to designs that may be inefficient and unnecessarily bulky. Even when some assessment of damage tolerance is included, the microstructure can change as the material ages over time, and we need ways to measure the effects of this and to predict what it will do to the safety of the structure. This project aims to develop a method to predict the performance and evaluate the integrity of structures and components made from quasi-brittle materials. This will extend opportunities for their use in engineering applications, enabling more efficient design with greater confidence in safety.Quasi-brittleness is a property that emerges from the material's microstructure. A quasi-brittle material can be made from a connected network of very brittle parts (for instance, a porous ceramic). It exhibits a characteristic "graceful" failure as parts break locally when loaded sufficiently, which gives it damage tolerance. The "gracefulness" of the failure is affected by the random variations of strength and stiffness of the network and the form of the connections. Such networks represent a key part of the microstructure of the material, and to understand quasi-brittle fracture we need to construct models that properly describe the microstructure. There is a need to understand and define the mechanisms that control the fracture at the small and the large scale within these quasi-brittle materials. This will allow us to capture sensitivity to microstructure differences and degradation, and to produce general models that are suitable for the wide range of quasi-brittle materials and applications.Three-dimensional models that are faithful to the microstructure can be created using modern 3D microscopy methods, such as X-ray computed tomography. But these models are far too complex to simply scale up to structures very large relative to the microstructure. There is no computer than can do this, yet. We will develop modelling methods that sufficiently represent the complexity of quasi-brittle microstructures over a wide range of length scales, such as cellular automata finite elements. We will use advanced tomography and strain mapping techniques to observe how damage develops and to test and refine our models. We will then use this and the understanding that we gain to design new material tests and characterisation methods so that our methods may be used in a wide range of materials, from concretes to advanced nuclear composites, bone replacement biomaterials and geological materials.