Abstract

In this paper, we present the different characterization techniques used to measure the mechanical properties of silica aerogels. The mechanical behavior of aerogels is generally described in terms of elastic and fragile materials (such as glasses or ceramics) but also in terms of plastic media in compression testing. Because of these very different mechanical behaviors, several types of characterization techniques are proposed in the literature. We first describe the dynamic characterization techniques such as ultrasounds, Brillouin scattering, dynamic mechanical analysis (DMA) to measure the elastic properties: Young’s modulus (E), shear modulus (G), Poisson ratio (υ) but also attenuation and internal friction. Thanks to "static" techniques such as three-point bending, uniaxial compression, compression we also access to the elastic modulus (E) and to the rupture strength (σ). The experimental results show that the values of the elastic and fracture moduli measured are several orders of magnitude lower than those of a material without porosity are. With regard to the brittleness characteristics, Weibull's analysis is used to show the statistical nature of the fracture resistance. We also present the SENB (Single Edge Notched Beam) technique to characterize toughness (K_IC) and the stress corrosion mechanisms, which are studied in ambient conditions and temperature by the double cleavage drilled compression experiment (DCDC). In the last part of the paper, we show how, during the isostatic compression test, aerogels behave like plastic materials. The data allow calculating the bulk modulus (K), the amplitude of the plastic deformation and the yield strength (σ_el), which is the boundary between the elastic and plastic domains. These different techniques allow understanding which parameters influence the overall mechanical behavior of aerogels, such as pore volume, but also pore size, internal connectivity and silanol bounds content. It is shown that the pore size plays a very important role; pores can be considered as flaws in the terms of fracture mechanics.

中文翻译：

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表征气凝胶力学性能的技术

摘要

在本文中，我们介绍了用于测量二氧化硅气凝胶力学性能的不同表征技术。气凝胶的机械性能通常用弹性和易碎的材料（例如玻璃或陶瓷）来描述，但也要用压缩试验中的塑料介质来描述。由于这些非常不同的机械行为，在文献中提出了几种类型的表征技术。我们首先描述动态表征技术，例如超声波，布里渊散射，动态力学分析（DMA）以测量弹性性能：杨氏模量（E），剪切模量（G），泊松比（υ））以及衰减和内部摩擦。得益于“静态”技术，例如三点弯曲，单轴压缩，压缩，我们还获得了弹性模量（E）和断裂强度（σ）。实验结果表明，所测得的弹性模量和断裂模量比没有孔隙的材料的模量低几个数量级。关于脆性特征，使用威布尔的分析来显示抗断裂性的统计性质。我们还提出了SENB（单边切口梁）技术来表征韧性（K _IC）和应力腐蚀机理，这些应力腐蚀机理是通过双裂隙钻孔压缩实验（DCDC）在环境条件和温度下研究的。在本文的最后一部分，我们展示了在等静压测试期间气凝胶如何表现得像塑料一样。该数据允许计算体积模量（ķ），塑性变形的幅度和屈服强度（σ _EL），这是弹性域和塑性域之间的边界。这些不同的技术可让您了解哪些参数会影响气凝胶的整体机械性能，例如孔体积，但也会影响孔径，内部连通性和硅烷​​醇键合含量。结果表明，孔径起着非常重要的作用。就断裂力学而言，孔隙可被视为缺陷。