Strain and damage sensing at the mesoscale in energetic materials in response to localized thermal loads

Abstract

Plastic bonded explosives (PBXs), consisting of high energy density energetic crystals in a polymer binder, are a class of energetic materials which have been widely studied in regards to their shock and ignition response. Of increasing interest is the response of such energetic materials to non-shock mechanical insults, e.g. accidental drop and dynamic/vibration loads in transport, which have been observed to produce localized damage and thermal loading due to the formation of hot spots. In some cases, the formation of these hot spots can lead to sufficient levels of localized heating capable of sustaining chemical reactions and transitioning to detonation. While there are several proposed mechanisms which could drive the formation of hot spots, the primary driver(s) for sustaining the chemical reaction and triggering detonation are not well understood. This is in part due to the difficulty in experimentally characterizing the distribution and interaction of hot spots at the mesoscale, given the small length and time scales over which they exist. Recently, Seidel and co-workers have explored the application of the distribution of carbon nanotubes within the binder phase of energetic materials as a means of introducing a significant piezoresistive response within the energetic material. Doing this can provide a means for strain and damage sensing at the mesoscale. While initial fabrication and testing of ammonium perchlorate and sugar-mock PDMS- and epoxy-binder energetic materials have provided initial proof-of-concept demonstrations of strain and damage sensing, successful application towards locating and characterizing damage and hot spots requires greater understanding of the piezoresistive network at the mesoscale, and how it responds to localized heating. In this work, a mesoscale model corresponding to a representative volume element of an energetic material having a piezoresistive carbon nanotube nanocomposite binder is developed and subjected to localized heating. An electro-thermo-mechanical peridynamics formulation is developed which includes the generation of heat energy due to fracture and friction, and is applied to assess the differences between strain and damage sensing. Efforts are also made to assess the response of the mesoscale sensing network to localized heating and damage due to the presence of and interactions between increasing amounts of prescribed hot spots. Initial modeling results from these simulations reveal that the distribution of localized heating (leading to interactions between heat sources) and heating rate are strong indicators of whether or not such thermally induced damage will propagate beyond its local origin.