On the modeling of the effective thermal conductivity of particulate composites and the influence of building porosity on the shielding of blast waves

Abstract

The main and overarching objective of this research endeavor is to devise data-driven soft modeling tools that can provide fast, fairly accurate and computationally efficient predictions for: (1) the thermal conductivity of particulate composites (PC), and (2) the level of shielding provided by a porous structure from a high-yield explosion. The computational tools developed in this work constitute cheaper and faster alternatives to the relatively large and cumbersome numerical simulations or experimental testing campaigns that would otherwise be needed to make predictions of comparable accuracy.

Accurate and computationally efficient predictive models for the effective thermal conductivity of composites are needed to accelerate the design of new materials with improved properties and behavior. The predictive capabilities of previously developed models for thermal conductivity of PCs apply to limited ranges of component properties and proportions. Furthermore, existing material models that account for particle contiguity and filler-matrix thermal contact resistance fail to distinguish between those two effects. In the first part of this work, two novel and complementary predictive models for the effective thermal conductivity of two-phase isotropic PCs are derived: (i) a simple yet efficient analytical model for non-contiguous filler particles, and (ii) a generalized semi-analytical model accounting for both filler particle contiguity and thermal resistance at the filler-matrix interface. The latter model is powered by a thermal conduction grid solver that allows the incorporation of an unlimited number of elements and components to match increasingly complex particulate composite material configurations and behaviors. The models proposed match previously published experimental data fairly well. The grid model is further leveraged to relate the effective thermal conductivity to filler particle size and size distribution. It is found that the formation of filler conduction chains is favored by well-graded particle size distributions.

Numerous and complex interactions occur between a blast wave originating from a high-yield explosion and the typical elements of a dense urban topography. Fairly accurate predictions of the blast loads experienced by structures in such context can be only achieved through relatively large, potentially cumbersome and computationally expensive numerical simulations. Although many existing works address certain effects resulting from blast-structure interactions, their scopes are mostly limited to small-scale partially-confined explosions affecting a few buildings or streets. Furthermore, only few studies touch on the effect of building porosity on the propagation of blast waves, while none focuses on the specific influence of a building's porosity on its shielding effect. The second part of this work develops and implements a set of high-cost high-fidelity numerical simulations to explore the influence of building porosity on the shielding effect. A rich manifold of numerical solutions is thus obtained, including the overpressure and the specific impulse over virtual facades shielded by buildings of different porosity, located at various standoff distances from a large explosive charge. A suitable scaling and modeling approaches including regression and machine learning techniques are then applied to the numerical dataset to devise simplified, yet more general data-driven surrogate tools that can provide fast and fairly accurate estimates of the blast wave shielding capacity of porous buildings. Those tools can contribute in guiding design engineers through the process of evaluating blast loads behind porous structures. This study also contributes to furthering the current state of knowledge regarding the propagation of blast waves in and around porous buildings. In general, the shielding effect wanes as the standoff distance to the charge, or the distance behind the shielding structure, increases. Also generally, the lower the building porosity, the longer the path taken by the wave and the more the rise to peak overpressure is delayed. It is also observed for instance that the intensity of a blast wave behind a highly porous structure can in fact increase. This counter-intuitive outcome is attributed to a local channeling of the wave inside the building floors between the parallel slabs and side-walls.