Small Scale Testing to Assess Mechanical Behavior of Anisotropic Molecular Crystals

Due to the inherent dangers associated with handling high explosive materials, it is often useful to have access to inert simulant materials that mimic certain physical or mechanical properties, called "mock" materials. Mock materials can take the place of explosives in experiments, allowing experimental results to be obtained with less difficulty and risk. Recently there has been an interest in identifying new mechanical mock materials for the explosives HMX and PETN. These energetic materials and their prospective mocks are often used and tested in the form of small submillimeter crystals, with which typical size and geometry make many mechanical tests difficult or impossible. Additionally, these materials are typically prone to brittle fracture, which can further limit the usage conditions of the material as well as the range of conditions in which mechanical testing results are valid. Nanoindentation is a useful technique to measure mechanical properties in particulate form without the need to grow large single crystals or do additional processing on existing crystals.
Here, nanoindentation tests were performed on PETN, HMX, and several inert molecular crystals selected as potential mocks based on density, crystal structure, and previous thermal testing results. Comparisons were made on the basis of hardness, elastic modulus, yield point behavior, indentation fracture response, and sensitivity to non-uniform indenter orientation. Based on the results of these experiments, the inert material idoxuridine was selected for further consideration as an HMX mock, and the inert materials meso-erythritol and 2,4,6 trifluorobenzoic acid were selected for further consideration as PETN mocks.
As a result of this study, potential mechanical mocks were selected for two energetic materials, nanomechanical properties were reported for the first time ever for 6 inert molecular crystals, and nanoindentation was shown to be a versatile tool for rapid initial screening of materials as well as detailed investigations of materials of interest.