超高性能混凝土的尺寸效应及断裂特性

Fracture in random quasibrittle media: I. Discrete mesoscale simulations of load capacity and fracture process zone

Numerical simulations of concrete fracture performed with a probabilistic mesoscale discrete model are presented. The model represents a substantial part of material randomness by assigning random locations to the largest aggregates. The remaining part of randomness is introduced by causing material parameters to fluctuate randomly via a homogeneous random field. An extensive numerical study performed with the model considers prisms loaded in uniaxial tension with both fixed and rotating platens, and also beams with and without a notch loaded in three point bending. The results show the nontri…

Brittle fracture studied by ultra-high speed synchrotron X-ray diffraction imaging

Ever since the very first human-made knapped tools, the control of fracture propagation in brittle materials has been a vector of technological development. Nowadays, a broad range of applications relies on crack propagation control, from the mitigation of damages, e.g., from impacts in glass screens or windshields, to industrial processes harnessing fracture to achieve clean cuts over large distances. Yet, studying the fracture in real time is a challenging task, since cracks can propagate up to a few km/s in materials that are often opaque. Here, we report on the in situ investigation of cra…

超高性能混凝土早期性能分析

Analysis of the Behavior of Ultra High Performance Concrete at Early Age

超高性能混凝土（UHPC）是具有高水平性能的水泥基复合材料，其特征在于高抗压强度、高抗拉强度和优异的耐久性，通过低水胶比、优化骨料粒度分布、热活化和纤维增强来实现。在过去的几十年里，越来越多的超高性能混凝土被开发出来并应用于实践。因此，对能够描述和预测相关老化现象以辅助设计和规划的计算模型的需求正在增加。本文介绍了早期的实验表征，以及随后的模拟结果的一个典型的UHPC矩阵。执行和模拟测试包括无侧限压缩，分裂（巴西），和三点弯曲测试。的计算框架制定耦合湿热化学（HTC）理论和一个全面的中尺度离散模型与制定老化功能。HTC组件允许考虑不同温度和相对湿度的各种类型的固化条件，并预测混凝土老化的水平。力学部分，格子离散颗粒模型（LDPM），允许在主要异质性的长度尺度上模拟混凝土的破坏行为。老化功能涉及的中尺度LDPM的力学性能的老化程度，在这项工作中定义为在一定的年龄和其渐近值的准静态弹性模量之间的比率。所获得的结果提供了见解在UHPC早期老化机制和计算模型老化UHPC结构的分析。

动态断裂失稳与经典裂纹理论的失效

Instability in dynamic fracture and the failure of the classical theory of cracks

裂纹是材料破坏的主要载体，在脆性材料中倾向于加速到高速度。在三维空间中，裂纹一般在其音速极限速度的40%左右发生微分支不稳定性。最近的实验表明，在足够薄的系统中，裂纹前所未有地加速到接近其极限速度而没有微分支，直到它们经历振荡不稳定性。尽管动态断裂不稳定性具有根本的重要性，并且与凝聚态物理和材料科学中的其他不稳定性有明显的相似之处，但人们对它们的了解仍然很少。经典的裂纹理论并没有描述它们，经典的裂纹理论假设在受压材料内部线性弹性是有效的，并使用一个无关的局部对称性准则来预测裂纹路径。在这里，我们开发了一个二维动态脆性断裂模型，能够预测任意路径的超高速裂纹的弹性非线性的存在下，没有额外的标准。我们表明，通过大量的计算，裂纹进行动态振荡不稳定性控制的小尺度弹性非线性裂纹尖端附近。这种不稳定性发生在超高临界速度之上，并且具有与断裂能与弹性模量的比率成比例地增加的固有波长，与实验定量一致。这个比例出现作为一个基本的缩放长度假设发挥不了作用，在经典理论的裂纹，但在这里强烈影响裂纹动力学。这些结果为解决材料失效中其他长期存在的难题铺平了道路。

细观尺度建模：约束下的劈裂压杆试验对混凝土的影响：内部损伤和应变率效应

Mesoscale Modelling of Confined Split-Hopkinson Pressure Bar Tests on Concrete: Effects of Internal Damage and Strain Rates

本文通过细观尺度有限元建模研究了约束劈裂压杆试验中加载速率、内摩擦和围压对混凝土动态增加因子的影响，发现加载速率和围压能提高DIF，但内摩擦会减弱应变率效应。

一种用于岩土材料弹塑性变形与断裂分析的消除几何表面非物理应力和虚拟屈服的改进近场动力学框架

An improved peridynamic framework to eliminate unphysical stress and fictitious yield at geometry surface for geomaterials elastoplastic deformation and fracture analysis

针对传统非普通态基近场动力学模拟岩土材料弹塑性行为时的表面效应与虚拟屈服问题，提出结合非局部微分算子散度公式、牵引力边界条件和载荷平衡校正的改进框架，经验证可有效消除数值误差，实现可靠的变形与破坏模拟。

深度学习用于定量动态碎裂分析

Deep Learning for Quantitative Dynamic Fragmentation Analysis

本研究开发了基于图像的卷积神经网络，用于超高速光学成像下不透明脆性材料碎裂行为的定量时间分辨测量，扩展了U-net模型，通过监督学习训练二元、3类和5类模型，以量化防护应用中结构材料的失效过程。

Reactive Two-Step Additive Manufacturing of Ultra-high Temperature Carbide Ceramics

Ultra-high-temperature ceramics (UHTCs) are candidate structural materials for applications that require resiliency to extreme temperature (>2000°C), high mechanical loads, or aggressive oxidizing environments. Processing UHTC transition metal carbides as standalone materials using additive manufacturing (AM) methods has not been fully realized due to their extremely slow atomic diffusivities that impede sintering and large volume changes during indirect AM that can induce defect structures. In this work, a two-step, reactive AM approach was studied for the formation of the ultra-high temperat…

Ultra-uniform Nanocrystalline Materials via Two-Step Sintering

Nanocrystalline metals and ceramics with <100 nm grain sizes and superior properties (e.g., mechanical strength, hardness, fracture toughness and stored dielectric energy) are of great interest. Much has been discussed about achieving nano grains, but little is known about maintaining grain-size uniformity that is critical for material reliability. An especially intriguing question is whether it is possible to achieve a size distribution narrower than what Hillert[1] theoretically predicted for normal grain growth, a possibility suggested, for growth with a higher growth exponent, by the gener…

界面Fe3O4纳米颗粒对超快高温烧结致密化织构氧化铝微观结构与力学性能的影响

Effect of interfacial Fe3O4 nanoparticles on the microstructure and mechanical properties of textured alumina densified by ultrafast high-temperature sintering

本文研究Fe3O4涂层对织构氧化铝在超快高温烧结下微观结构和力学性能的影响，发现Fe3O4诱发晶体学缺陷并促进塑性流动，使抗弯强度和断裂韧性分别提升至约287 MPa和7 MPa.m0.5。

紧束缚和分子动力学相结合的方法模拟金属在塑性区的行为

Combining Tight-binding and Molecular dynamics Methods to Model the Behaviour of Metals in the Plastic Regime

金属的超精密加工、纳米线在拉伸应力下的断裂以及纳米级材料的断裂是技术上重要的工艺的例子，这些工艺在实验上进行研究是极其困难和昂贵的。我们描述了一个多尺度的方法，用于模拟这样的系统，其中的积极活跃的区域建模使用一个强大的紧束缚计划在海军研究实验室（NRL-TB）和系统的其余部分与分子动力学治疗。我们提出了一个计算机代码实现的方法，面向非平衡，跨尺度紧束缚和分子动力学模拟。除了介绍的方法和实施，我们讨论了初步的物理结果，并讨论其有效性。

Design and performance of an ultra-high vacuum spin-polarized scanning tunneling microscope operating at 30 mK and in a vector magnetic field

We describe the design and performance of a scanning tunneling microscope (STM) which operates at a base temperature of 30 mK in a vector magnetic field. The cryogenics is based on an ultra-high vacuum (UHV) top-loading wet dilution refrigerator that contains a vector magnet allowing for fields up to 9 T perpendicular and 4 T parallel to the sample. The STM is placed in a multi-chamber UHV system, which allows in-situ preparation and exchange of samples and tips. The entire system rests on a 150-ton concrete block suspended by pneumatic isolators, which is housed in an acoustically isolated an…