The sensitivity of porous energetic materials depends upon the meso-structural heterogeneities such as voids, defects, grain boundaries and cracks. This is because shock wave interaction with these heterogeneities can lead to the formation of localized heated regions termed as “hotspots”. Depending on the size and temperature of the hotspots chemical reaction can initiate in energetic crystals. There are various mechanisms responsible for the formation of hotspots, amongst which in the high shock strength loading regime, collapse of voids has been observed to be a vital mechanism that leads to hot-spot formation and reaction initiation in such materials. A plethora of literature exits covering this topic to detailed depths using 2D simulations. In such studies, 2D mesoscale simulations have shown that sensitivity of voids is strongly dependent on void shapes. In addition, circular voids have been shown to be less sensitive than elongated voids in both TATB-based and HMX-based energetic materials. Moreover, conical and elliptical voids have shown higher sensitivity compared to spherical voids. Thus, 2D analysis provides evidence regarding the dependence of void sensitivity on its shape. Unfortunately, the effect of void shape on sensitivity has not been studied in 3D except through 2D axisymmetric analysis. Worse off, some of the few published studies on void collapse in 3D have highlighted on the significance of discrepancies between the prediction of 3D and 2D analysis for circular voids, for other void morphologies, such as plate-like or rod-like voids there is no 3D study performed in the past.
Recently, University of Iowa scientists: Dr. Nirmal Kumar Rai and Professor H. S. Udaykumar from the Department of Mechanical and Industrial Engineering studied the behavior of voids of various shapes commonly present in the microstructures of energetic materials by performing three-dimensional reactive void collapse simulations in otherwise uniform, isotropic HMX. Specifically, they used four void shapes, namely: sphere, cylinder, ellipsoid and plate. Their work is currently published in the research journal, Physical Review Fluids.
In brief, the research method employed entailed executing a comparative study between the sensitivity of 3D voids and the corresponding 2D counterparts so as to comprehend the differences between the physics and void sensitivity measures that arise due to three-dimensionality. They employed a sharp Cartesian-based Eulerian framework to perform the void collapse simulations. They then modelled the HMX chemical decomposition using Tarver’s three-step reaction model. The dynamics of void collapse and hot-spot formation coupled with sensitivity quantification for all the four shapes were then analyzed. Lastly, a comparison of the sensitivity prediction of the 3D voids and their 2D counterparts was done.
The authors observed that for all the four shapes, collapse generated complex three-dimensional baroclinic vortical structures. Additionally, they noted that the differences in the vortical structures for the different void shapes considerably impacted the relative sensitivity of the voids. Furthermore, it was seen that voids of high surface area generated hot spots of greater intensity; intricate, highly contorted vortical structures that lead to hot spots of corresponding tortuosity and therefore enhanced growth rates of reaction fronts. All 3D voids were seen to be more sensitive than their two-dimensional counterparts.
In summary, Nirmal Kumar Rai-H. S. Udaykumar study presented an in-depth assessment of the effects of three-dimensionality on void collapse and reaction initiation in HMX. In general, the 3D void collapse simulation offered much insight concerning the collapse behavior of the voids and their sensitivity. The sensitivity of voids in 3D was observed to be significantly higher for a wide range of void shapes commonly present in the microstructures of porous energetic materials. Therefore, 2D assumption ought to be applied with caution when establishing criticality of the conditions for reaction initiation in energetic materials. Altogether, the results presented by University of Iowa scientists provide physical insights into hot-spot formation and growth, and point to the limitations of 2D analyses of hot-spot formation.
Nirmal Kumar Rai, H. S. Udaykumar. Three-dimensional simulations of void collapse in energetic materials. Physical Review Fluids, volume 3, 033201 (2018).Go To Physical Review Fluids