Mechanical behavior of energetic crystals like RDX, HMX and PETN and their polymer-bonded composites under high pressures and high strain-rates has been a subject of investigation from a long time. Often, energetic simulants like sucrose are used as inert explosives to study only mechanical behavior of explosive crystals without the added complexity of chemical reactions. However, the shear strength behavior of such materials has not received any attention. In the present work, Pressure-Shear Plate Impact experiments have been conducted on (a) HTPB (an elastomeric binder), (b) Sucrose (simulant crystal), (c) HTPB/sucrose composite at normal stresses of the 3-10 GPa and shear strain rates as high as 105-106 s-1. HTPB exhibits a very pressure-sensitive shearing resistance, increasing from 120 GPa to 470 MPa as the normal stress increases from 3 GPa to 9 GPa. Sucrose, on the other hand, has a nominally constant value of peak shear strength (~300-400 MPa). However, pronounced shear-strain softening is observed in sucrose at high shear-strains-even a dramatic drop in some cases. Such a drop is attributed to a shear band like thermo-viscoplastic instability in the material. PSPI experiments on the composite reveal the importance of the binder in shielding the explosive crystals from accidental impacts since the shearing resistance of the composite is shown to be dictated primarily by a very small amount (~10% by weight) of the polymeric binder. Based on the experimental data, constitutive models have been developed for HTPB and sucrose. HTPB is modeled using a quasi-linear viscoelastic model. A thermodynamically-consistent thermo-elastic thermo-viscoelastic model, equipped with a complete equation of state, is developed for modeling sucrose. Special emphasis is laid on developing a complete equation of state with a temperature-dependent specific heat as such an undertaking has important consequences in accurately predicting hot-spot temperatures in energetic crystals. Using these material models, one-dimensional canonical microstructures of the composite are then simulated.
Events like shear-band formation, pore-collapse, etc. which are deemed to be the underlying mechanisms for hot-spot formation in the highly heterogeneous polymer-bonded explosives are very localized in nature and occur at extremely small spatio-temporal scales. A high-speed microscopic imaging system with the capability of resolving sub-micron features at a temporal resolution of 250 ns is developed and the capability demonstrated on capturing shear-band initiation in polycarbonate.
About the Speaker
Pinkesh Malhotra is a last-year PhD student in Solid Mechanics at Brown University and is working under the guidance of Prof. Pradeep Guduru and Prof. Rodney Clifton. His primary research experience lies in experimental mechanics, especially towards high-pressure and high strain-rate testing of materials. He has designed and built Kolsky bar apparatus, a gas-gun and a new high-speed microscopic imaging system at Brown. He also has experience with constitutive modeling of different classes of materials, ranging from polymers to molecular solids, and implementing material models in ABAQUS through user sub-routines.
Host: Professor Randy Ewoldt