AbstractThe low thermal conductivities of silica aerogels have made them of interest to the aerospace community for applications such as cryotank insulation. Recent advances in the application of conformal polymer coatings to these gels have made them significantly stronger, and potentially useful as lightweight materials for impact absorption as well. In this work, we perform multiscale computer simulations to investigate the tensile strength and failure behavior of silica and polymer-coated silica aerogels. The gels' nanostructure is simulated via a Diffusion Limited Cluster Aggregation (DLCA) procedure. The procedure produces fractal aggregates that exhibit fractal dimensions similar to those observed in real aerogels. The largest distinct feature of the clusters is the so-called secondary particle, typically tens of nm in diameter, which is composed of primary particles of amorphous silica an order of magnitude smaller. The secondary particles are connected by amorphous silica bridges that are typically smaller in diameter than the particles they connect. We investigate tensile failure via the application of a uniaxial tensile strain to the DLCA clusters. In computing the energetics of tensile strain, the detailed structure of the secondary particles is ignored, and the interaction among secondary particles is described by Morse pair potentials, representing the strain energetics of the silica gel and the polymer coating, parameterized such that the potential ranges are much smaller than the secondary particle size. The Morse parameters are obtained by separate atomistic simulation of models of the interparticle bridges and polymer coatings, with the tensile behavior of these bridges modeled via molecular statics. We consider the energetics of tensile strain and tensile failure, and compare qualitative features of low-and high-density gel failure.
Failure Characteristics of Joint Bolts in Shield Tunnels Subjected to Impact Loads from a Derailed Train
