Observation of Impact Induced Failure in Slotted HMX Crystals Using X-Ray Phase Contrast Imaging

Hot spot theory details how energetic materials may ignite under the apparent threshold energy. Despite being the dominating theory for decades, the individual mechanisms for hot spot formation are still widely debated today and lack a comprehensive understanding, particularly in the experimental field due to the rapid and unpredictable reaction nature. This paper describes the use of X-ray Phase Contrast Imaging on HMX single crystals cast in Sylgard-184 & REG; polymer matrix during single-stage gas gun impact. To simulate a pre-existing open crack and to encourage crack formation within the X-ray observation window, the HMX crystals contain a milled slot at the impact face. Experiments were performed up to an impact velocity of 490 m/s. Observations primarily show fracture originating from the milled slot flaw, where stresses are concentrated, with propagation outward and characteristically brittle fractures in the bulk material. However, observations also include unique behavior where closed cracks formed throughout a sample containing a 100 & mu;m milled slot in a diagonal, crisscrossed pattern before final failure.