Theoretical Investigation of Mechanically Coupled Chemical Kinetics and Phase Transitions in Energetic Materials

Major Goals: In this work we aim to understand how the mechanical response of energetic materials effects the chemical kinetics and to determine in what scenarios this effect is significant. Through the development of theoretical models that are validated by experimental observations, we aim to expose new phenomena. Accomplishments: - Theoretical model now published in Proceedings of the Royal Society, captures fully coupled dynamic cavity expansion, with the embedded explosive modeled as a reactive mixture, and can explain multi-stage explosions. Model explains the short timescale temperature spike reported in experiments and also confirms that for high activation energies the mechanical deformation can interfere with the chemical kinetics to essentially switch the explosion on and off at a frequency that is of the order on the natural elastic frequency. - Experimental work was extended to softer materials softer materials. Overall it shows that long time response, which exhibits multiple peaks of cavity size, cannot be explained by existing chemical kinetics models nor by mechanics. It appears to be governed by experimental artifacts such as blocking of the laser light by the cavity. This decelerates the heating as the cavity expands and accelerates it again after the cavity retracts. - From our experiments we can estimate the amount of energy released in the micro-explosion. This energy measure can then be used to assess the stability of an aggregate polymer bounded explosive, using theory of phase transitions, as detailed next. - Theoretical model reveals a threshold value for onset of instability of the reaction thus providing a clear (fully couple chemo-mechanical) approach to estimate the criteria for initiation from a hot spot. - Experimental work captures the effect of rate dependence on fracture patterns. - Work to understand shock waves is published in PRL and also featured in Nature Reviews Physics.