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

韧性材料，如金属和合金，被广泛用于工程结构中，无论是本身或作为增强。他们通常可以承受大量的塑料损坏之前失败。工程师们非常了解金属失效的方式以及它们对损坏的耐受性，因此，高效且体积较小的结构可以设计有明确的安全裕度或储备强度，以科普极端事件。相比之下，玻璃、陶瓷等弹性脆性材料可能在没有预先警告的情况下发生失效，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。它们是脆性材料，具有一定的损伤耐受性，包括混凝土，多颗粒石墨，陶瓷基复合材料，岩石等地质结构和生物医学材料，如骨和骨替代品。虽然它们的损伤容限比许多金属和合金小得多，但与陶瓷和玻璃等脆性材料相比，它可能非常重要。但是，当工程师设计或评估准脆性材料时，这一点并没有得到很好的考虑，因为没有充分了解材料的微观结构或应力状态等因素对其损伤容限的作用。准脆性材料通常被视为完全脆性，很少或根本不考虑其损伤容限，因此评估包含非常重要的安全裕度，导致设计可能效率低下且不必要的庞大。即使包括一些损伤容限评估，随着材料的老化，微观结构也会发生变化，我们需要方法来测量这种影响，并预测它对结构安全性的影响。该项目旨在开发一种方法来预测性能和评估由准脆性材料制成的结构和部件的完整性。这将扩大其在工程应用中的应用机会，从而实现更高效的设计，并提高安全性。准脆性是材料微观结构中出现的一种特性。准脆性材料可以由非常脆的部件（例如多孔陶瓷）的连接网络制成。它表现出一种特征性的“优雅”失效，因为当负载足够时，部件局部断裂，这使其具有损伤容限。破坏的“随机性”受网络强度和刚度的随机变化以及连接形式的影响。这种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。有必要了解和定义的机制，控制这些准脆性材料内的小规模和大规模的断裂。这将使我们能够捕捉到微观结构差异和退化的敏感性，并产生适用于广泛准脆性材料和应用的通用模型。可以使用现代3D显微镜方法（如X射线计算机断层扫描）创建忠实于微观结构的三维模型。但是这些模型太复杂了，不能简单地按比例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。我们将开发建模方法，充分代表了复杂的准脆性微观结构在很宽的长度尺度，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，并测试和完善我们的模型。然后，我们将利用这一点和我们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广泛的材料，从混凝土到先进的核复合材料，骨替代生物材料和地质材料。

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.