The dynamic mechanical response of an energetic material simulant: acetaminophen

Acetaminophen is an energetic material simulant with highly complex behavior under a variety of loading conditions. It is a molecular crystal with two known stable phases, monoclinic and orthorhombic, and is most relevant to the energetic material β-HMX. Its widespread use by the pharmaceutical industry has generated a wealth of data on its chemical, thermodynamic, and physical properties under quasi-static conditions. However, to date there exists a comparative lack of understanding on its dynamic behavior under high strain rates. Here we couple shock loading techniques and X-ray phase contrast imaging to characterize the shock response of acetaminophen. At the Advanced Photon Source (APS) synchrotron facility, Front Surface Impact (FSI) experiments were employed to extract the shock Hugoniot of acetaminophen and are followed up by experiments on Richtmyer-Meshkov instabilities in the material. This second set of experiments highlights the surprising ability of acetaminophen, a relatively brittle material, to undergo jetting, and moreover hints at the material crossing over a phase boundary as the jets flow at significantly higher, temperature, pressure, and velocity than the surrounding material.

To better understand this highly complex dynamic response, direct numerical simulations were performed at the continuum mesoscale. A complete equation of state derived from the Helmholtz free-energy for acetaminophen has been implemented, and the derived theoretical shock Hugoniot is compared to experiments.



This work provides a platform for future experiments investigating molecular crystals under high strain-rate, finite deformation scenarios and resultant phenomena such as viscoplastic flow, phase changes, fracture, and hydrodynamic jetting. Such efforts may play a key role in better predictively modeling initiation and detonation in reactive energetic materials.