Techniques for characterizing the mechanical properties of aerogels

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 (KIC) 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 (σel), 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.