Crack path instabilities in DCDC experiments in the low speed regime

We studied the low speed fracture regime (10⁻⁴-10⁻⁹ m s⁻¹) in different glassy materials (soda-lime glass, glass ceramics) with variable but controlled length scale of heterogeneity. The chosen mechanical system enabled us to work in pure mode I (tensile) and at a fixed load on double cleavage drilled compression specimen. The internal residual stresses of studied samples were carefully relaxed by appropriate thermal treatment. By means of optical and atomic force microscopy techniques fracture surfaces have been examined. We have shown for the first time that the crack front line underwent an out-of-plane oscillating behavior as a result of a reproducible sequence of instabilities. The wavelength of such a phenomenon is in the micrometer range and its amplitude in the nanometer range. These features were observed for different glassy materials providing that a typical length scale characterizing internal heterogeneities was lower than a threshold limit estimated to few nanometers. This effect is the first clear experimental evidence of crack path instabilities in the low speed regime in a uniaxial loading experiment. This phenomenon has been interpreted by referring to the stability criterion for a straight crack propagation as presented by Adda-Bedia et al. [Phys. Rev. Lett. 76 (1996) 1497].     

Mechanical Properties of Gel-Derived Materials

The mechanical behaviour of xerogels and aerogels is generally described in terms of brittle and elastic materials, like glasses or ceramics. The main difference compared to silica glass is the order of magnitude of the elastic and rupture moduli which are 10⁴ times lower. However, if this analogy is pertinent when gels are under a tension stress (bending test) they exhibit a more complicated response when the structure is submitted to a compressive stress. The network is linearly elastic under small strains, then exhibits yield followed by densification and plastic hardening. As a consequence of the plastic shrinkage it is possible to densify and stiffen the gel at room temperature. These opposite behaviours (elastic and plastic) are surprisingly related to the same two kinds of gel features: the silanol content and the pore volume. Both elastic modulus and plastic shrinkage depend strongly on the volume fraction of pores and on the condensation reaction between silanols. On the mechanical point of view (rupture modulus and toughness), it is shown that pores and silanols play also an important role. Pores can be considered as flaws in the terms of fracture mechanics and the flaw size, calculated from rupture strength and toughness is related to the pore size distribution. Different kinds of gels structure (fractal or not fractal) have been synthesized by a control of the different steps of transformation such as sintering and plastic compaction. The relationships between structural and the elastic properties are discussed in terms of the percolation theory and fractal structure.     

Sub-critical crack growth in some phosphate glasses

The effect of alumina on the sub-critical crack growth is investigated on a set of calcium aluminophosphate glasses of molar composition 50% P₂O₅–(50 ? x)% CaO–x% Al₂O₃ (0% ? x ? 10%). The crack propagation is operated using the double-cleavage-drilled-compression (DCDC) method. In this test, the sample is loaded in compression with a mechanical testing machine in an environmentally controlled chamber (both temperature and water vapor pressure). The crack velocity is plotted as a function of stress intensity factor. Regions I and II are characterized by a dependence of crack velocity on the amount of alumina content in the glass. When the alumina content increases, glass shows, as expected, a trend less sensible to the stress corrosion correlated to the slope of the crack growth curves, but in the same time, these curves are shifted toward higher crack velocity values. For different compositions, an unexpected behavior is also observed with temperature and water vapor pressure. We assume the formation of a ‘gel-like product’ on fresh fracture surfaces which modifies the reactivity at the crack tip.     

Three-dimensional reconstruction of aerogels from TEM images

The knowledge of the internal structure of porous materials is of main importance to compute their physical properties. This article focuses on base-catalyzed and colloidal silica aerogels, which are fractal materials and we use an original method for the reconstruction of these aerogels from TEM images. The method used is iterative and leads to the same fractal dimension as the real material, computed from the internal two-point correlation function. Unlike the reconstruction of porous materials found in literature, our method is based on the distribution of matter and not of the porous network, and has the additional advantage of being only half statistical, i.e. only the coordinates in the z direction are statistically obtained while the coordinates x and y of the elements of matter are accurately obtained from TEM images of our samples     

Effect of Aging on Mechanical Properties of Resorcinol-Formaldehyde Gels

Resorcinol-Formaldehyde gels have been prepared in aqueous solutions. After a gelification stage at 80°C, an aging was performed in water or acetic acid solutions at ambient temperature or in the parent liquid at 80°C for different durations. Shear modulus of gels immersed in water is measured using the 3 points bending technique. The evolution of elastic constant with time depends on the pH of aging solution. The strengthening is more pronounced for aging under acidic conditions than in water. Nevertheless when the aging stage is performed at 80°C for a few days, quite identical values of shear modulus are obtained comparatively to acid treatment. The enhancement of mechanical properties of RF gels in acidic conditions is related to polycondensation reaction rates.