TDR 2006 Third International Symposium and Workshop on Time Domain Reflectometry

翻译后摘要：许多人工材料，如混凝土，但也有其他人，其特征在于较大尺寸的夹杂物（骨料）嵌入在一个更细粒度的矩阵。 嵌入可以是完全的，即包围较大的晶粒，或者较细的基质可以仅填充彼此接触的较大夹杂物之间的间隙。 类似的情况存在于许多天然岩石和冻土中。有证据表明，夹杂物可以强化和削弱材料，即随着夹杂物的加入，晶粒更细的材料获得强度，反之亦然。 夹杂物的影响可以通过微观连续或非连续方法进行实验研究和分析预测：在微观连续方法中，晶粒被认为是可变形基体中的刚性夹杂物。相反，即较软的夹杂物也是可能的，但这将不在本研究中处理。 通过应用均匀化方案，可以考虑裂纹密度对弹性性能的影响，并最终对复合材料的破坏载荷。 然而，由于这涉及体积平均，其涂抹了裂纹周围的应力强度，连续体方法不能捕获裂纹扩展和合并，导致裂纹网络。 在不连续体方法中，人们特别考虑裂纹的行为。 当裂纹扩展时，它们合并并最终形成彻底的破坏面。 相反，扩展的裂纹将遇到较大的晶粒，进一步的扩展可能会停止。为了从这两种方法的相对优势中获益，并消除其缺点，将它们结合起来，以开发一个预测模型，这是准确的和计算有效的。 这种组合的优点是，微观连续方法是能够捕获的特征属性的未开裂的矩阵与刚性夹杂物，这样的属性，然后可以使用在不连续的方法。 这项研究将包括实验和理论工作。 将对含有夹杂物和预制裂纹的材料进行实验，以研究相关的不连续行为。 其他实验使用纳米压痕将同时进行，以获得信息，可用于约束裂纹扩展准则。 理论工作将使用实验结果来产生相结合的微观力学不连续的方法，反过来，将通过与实验的比较进行验证。 这项研究将产生科学，实际工程和教育影响。 这项工作的科学效益和影响是对材料行为的更好的基本理解。 具体来说，这项研究将提高我们对夹杂物和裂纹扩展和合并的影响的理解，这在许多天然和人造材料中具有极其重要的意义。- 这项工作对执业专业的影响涉及何时采用连续性方法和何时采用非连续性方法以及界定两者之间的界限的问题。 这个问题与结构和材料的理性分析和设计一样古老。 这项研究，虽然不意味着达到这个规模依赖的问题的最终解决方案，将提供机械为基础的分析工具，以选择适当的方法。 事实上，它将更进一步，产生一种结合连续-不连续的方法。 明显的材料/结构，这将在实践中有重要的影响是岩石和许多土壤以及混凝土-教育将受益于这样一个事实，即研究将由两名研究生（博士）学生进行，并像往常一样，也将包括本科生。 学生不仅将积极参与研究工作，而且还将撰写论文和进行口头报告。

Abstract:Many artificial materials such as concrete, but also others, are characterized by larger size inclusions (the aggregate) embedded in a finer grained matrix. The embedment may be complete, i.e. surrounding the larger grains, or the finer matrix may only fill the interstices between larger inclusions which are in contact with each other. Similar conditions exist in many natural rocks and frozen sands. Evidence indicates that inclusions can both strengthen and weaken a material, i.e. the finer grained material gaining strength as inclusions are added or vice versa. The effect of inclusions can be experimentally investigated and analytically predicted through either a micro-continuum or a discontinuum approach: In the micro-continuum approach, grains are considered as rigid inclusions in a deformable matrix. The opposite, i.e. softer inclusions are also possible but this will not be dealt with in this research. By application of a homogenization scheme, the effects of the crack density on the elastic properties, and eventually on the failure load of the composite can be taken into account. However, since this involves volume averaging, which smears out the stress intensity around cracks, the continuum approach cannot capture crack propagation and coalescence, leading to a crack network. In the discontinuum approach, one specifically considers the behavior of cracks. As cracks propagate, they coalesce and eventually form thoroughgoing failure surfaces. Conversely, propagating cracks will encounter the larger grains and further propagation might be stopped.To benefit from the relative advantages and to eliminate the disadvantages of both approaches, they will be combined to develop a predictive model which is both accurate and computationally effective. The advantages of such combination are that the micro-continuum approach is able to capture the characteristic properties of the uncracked matrix with rigid inclusions; such properties can then be used in the discontinuum approach. The research will consist of experimental and theoretical work. Experiments will be conducted on material containing inclusions and preformed cracks to study the associated discontinuum behavior. Other experiments using nano-indentation will be conducted simultaneously to obtain information which can be used to constrain the crack propagation criterion. The theoretical work will use the experimental results to produce the combined micromechanics-discontinuum approach which, in turn, will be validated through comparison with experiments. This research will have scientific, practical engineering and educational impacts. The scientific benefits and impacts of this work are a better fundamental understanding of material behavior. Specifically, the research will improve our understanding of the effects of inclusions and of crack propagation and coalescence, which are of utmost significance in many natural and artificial materials. -The impact of this work on the practicing profession relates to the question as to when to apply continuum approaches and when discontinuum approaches and to define the boundary between the two. This question is as old as rational analysis and design of structures and materials. This research, while not implying to reach a final solution to this scale dependent problem, will provide mechanically based analytical tools to select the appropriate approach. As a matter of fact, it will go a step further and produce a combined continuum-discontinuum approach. Obvious materials/structures where this will have important implications in practice are rocks and many soils as well as concrete - Education will benefit from the fact that the research will be conducted by two graduate (doctoral) students and, as usual, will also include undergraduate students. The students will not only be strongly involved in the research work but also in writing papers and making oral presentations.