High speed X-ray phase contrast imaging of energetic composites under dynamic compression

Abstract

Fracture of crystals and frictional heating are associated with the formation of “hot spots” (localized heating) in energetic composites such as polymer bonded explosives (PBXs). Traditional high speed optical imaging methods cannot be used to study the dynamic sub-surface deformation and the fracture behavior of such materials due to their opaque nature. In this study, high speed synchrotron X-ray experiments are conducted to visualize the in situ deformation and the fracture mechanisms in PBXs composed of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) crystals and hydroxyl-terminated polybutadiene binder doped with iron (III) oxide. A modified Kolsky bar apparatus was used to apply controlled dynamic compression on the PBX specimens, and a high speed synchrotron X-ray phase contrast imaging (PCI) setup was used to record the in situ deformation and failure in the specimens. The experiments show that synchrotron X-ray PCI provides a sufficient contrast between the HMX crystals and the doped binder, even at ultrafast recording rates. Under dynamic compression, most of the cracking in the crystals was observed to be due to the tensile stress generated by the diametral compression applied from the contacts between the crystals. Tensile stress driven cracking was also observed for some of the crystals due to the transverse deformation of the binder and superior bonding between the crystal and the binder. The obtained results are vital to develop improved understanding and to validate the macroscopic and mesoscopic numerical models for energetic composites so that eventually hot spot formation can be predicted.

Modeling impact‐induced damage and debonding using level sets in a sharp interface Eulerian framework

“…Experiments under quasi‐static tensile loading show that the main damage mode appears at the particle/matrix interfaces due to debonding of the binder from the crystals. At higher loading rates, the fracture of the crystals can also occur . The current simulations are restricted to interfacial debonding.…”

“…At higher loading rates, the fracture of the crystals can also occur. 70 The current simulations are restricted to interfacial debonding. The simulation of debonding between crystals and the matrix in a level-set-based sharp interface framework requires specific features to be implemented with respect to the interface representation.…”

X‐ray Phase Contrast Imaging of the Impact of Multiple HMX Particles in a Polymeric Matrix

“…Sylgard‐184® has a number average molecular weight of 6,190 g/mol and a weight average molecular weight of 27,700 g/mol giving a polydispersity index (ratio of weight average to number average) of 4.47 . The Fe 2 O 3 helps to provide contrast between the matrix and particle while using PCI and is added after the Sylgard‐184® is mixed, but before curing. Next, the dish was placed in an oven to cure the polymer to provide a base for the HMX particles to sit on (∼30 minutes at 100 °C).…”

X‐Ray Phase Contrast Imaging of the Impact of a Single HMX Particle in a Polymeric Matrix

“…This technique allowed for many samples to be prepared at once as the sample size was much smaller than the petri dish. All samples were prepared with iron oxide added to the Sylgard to increase the contrast between particle and matrix using x-ray PCI [25]. This initial molding method was used for the first experiments which had impact velocities of~200 and 450 m s À1 .…”