EAGER: Submicron Fracture Toughness Measurements for Cement Paste using Focused Ion Beam (FIB) and Nanoindentation

这一探索性研究（EAGER）早期概念基金的研究目标是通过聚焦离子束（FIB）研磨样品结构的纳米压痕实验，测量水泥浆体在亚微米尺度上的断裂韧性。对水泥和混凝土材料的断裂和塑性破坏进行建模通常需要引入微裂缝的概念，微裂缝会在更大的尺度上扩展并最终聚并导致材料破坏。这些裂缝的小尺度和它们周围的体积给测量和建模带来了许多挑战。需要新的实验技术来定量评估控制微裂纹和最终材料破坏的小尺度准脆性破坏机制。研究任务将从开发实验方法开始，利用FIB铣削制造样品微观结构。微柱、缺口梁和楔形几何将被尝试作为替代方案。接下来，这些样品结构将使用纳米压痕设备进行测试，以施加载荷并测量位移。这些数据将被分析，以提供强度和断裂韧性特性。最后，将收集样本数据，建立测量性能的初步统计模型，重点是测量材料性能的能力，这些性能在每个阶段都是可重复的，但在各个阶段之间是可区分的。水泥生产产生的二氧化碳排放量占全球总排放量的3- 5%，但如果混凝土设计得更能抵抗准脆性破坏，这一排放量可以显著减少。混凝土的损伤和破坏通常通过引用水泥浆体中微裂缝的存在来模拟，但这些微裂缝的特性和导致微裂缝的材料行为尚未得到很好的理解。极端荷载条件下的混凝土损伤或耐久性问题（包括碱-硅反应、冻融循环、钢筋腐蚀和延迟钙矾石形成）可能导致微裂缝，如果了解微裂缝的起源，就可以理解和预防这些损伤。关于失效行为的基本起源的新知识将使改进和新材料的合理设计成为可能，这将导致更有效地利用自然资源和更可持续和有弹性的结构。该项目的研究代表了实现这一总体目标的重要的第一步，并将使未来的研究能够更广泛地表征水泥和混凝土在小长度尺度上的破坏行为。所开发的技术将适用于广泛的其他材料，包括沉积岩，如页岩和碳酸盐，以及生物材料，如骨骼，牙齿和贝壳。

The research objective of this Early-concept Grant for Exploratory Research (EAGER) award is to measure the fracture toughness of cement paste at submicron length scales using nanoindentation experiments with Focused Ion Beam (FIB) milled sample structures. Modeling fracture and plastic failure of cement and concrete materials often involves introducing the concept of microcracks, which propagate and eventually coalesce to cause material failure at larger scales. The small scale of these cracks and the volumes around them pose many challenges for measurements and modeling. New experimental techniques are required to quantitatively assess the quasi-brittle failure mechanisms at small scales that govern microcracking and eventually material failure. Research tasks will begin with development of experimental methods to create sample micro-structures using FIB milling. Micropillars, notched beams, and wedge-splitting geometries are will be tried as alternatives. Next, these sample structures will be tested using nanoindentation equipment to apply loads and measure displacements. This data will be analyzed to deliver strength and fracture toughness properties. Finally, sample data will be collected to build a preliminary statistical model of measured properties, focusing on the ability to measure material properties that are repeatable within each phase but distinguishable between phases. The production of cement generates between 3-5 percent of total carbon dioxide emissions worldwide, but this could be significantly reduced if concrete is engineered to be more resistant to quasi-brittle failure. Damage and failure of concrete is often modeled by invoking the existence of microcracks in cement paste, but the properties of these microcracks and the material behaviors that lead to microcracks are not well understood. Concrete damage from extreme loading conditions or from durability issues including alkali-silica reaction, freeze-thaw cycles, reinforcement corrosion, and delayed ettringite formation that lead to microcracking may be understood and prevented if the origins of microcracking are understood. New knowledge about the fundamental origins of failure behaviors of will enable rational design of modified and new materials that will lead to more efficient use of natural resources and more sustainable and resilient structures. The research in this project represents an important first step towards this overall goal, and will enable future research to more broadly characterize the failure behavior of cement and concrete at small length scales. The techniques developed will be applicable to a wide range of other materials, including sedimentary rocks such as shale and carbonates, and biological materials such as bones, teeth, and shells.