Nonlinear resonant ultrasound spectroscopy for nondestructive evaluation of thermally aged small pressed pellets

Abstract

As the high explosive Pentaerythritol Tetranitrate (PETN) ages, its microstructure changes in the form of microcracks, grain sintering, crystal coarsening, and via other mechanisms, which affects its explosive performance. However, current methods to interrogate the microstructure can be time consuming, hard to perform in situ, or do not necessarily produce meaningful information. Moreover, there is currently no proven benchscale method for characterizing the microstructure of pressed compacts, meaning this is a shortcoming for surveillance efforts. Nonlinear wave propagation in consolidated granular material, such as sandstones, concrete or in this case, pressed pellets, is a function of the microstructure and can be influenced by poor sintering of the grains, micro cracks and grain distribution. Nonlinear Resonant Ultrasound Spectroscopy (NRUS) is a tool that is able to measure the bulk hysteretic nonlinearity by resonating a sample at different amplitudes and observing the shift in resonant frequency. Typically, NRUS has been used on larger samples, but in this work, we use the technique to probe small pressed granular PETN pellets that have a diameter of 7.6 mm and a thickness of l.2 mm that are artificially aged by placing in an oven for a period of time. The samples are resonated by adhering a small piezo-transducer and measured with a laser vibrometer. We found a positive correlation between the degree of nonlinearity and the amount of time artificially aging under heat treatment, indicating that NRUS is a promising tool for diagnosing damage in PETN.