Anomalous Plastic Deformation in Impacted RDX

To estimate the plastic deformation, assume that the deformation occurs mainly in the shear bands. Introduce into the plastic deformation rate. Equation (5) nominal characteristic values for RDX, ° so that the plastic deformation rate reduces to dy/dt = 2T(t,U)Pc(t)x10 s. Approximating dy/dt = Ay/At and letting At = 5 x lO s, which is the measured time of the impact, gives Ay = T(t,U)Pc(t)x10 s. It will be shown in the following paragraph that for mild impacts slightly less than that required for initiation of chemical reaction in RDX, T(t,U)Pc(t) = 10 in crystals at the outer perimeter of the impacted sample. The plastic strain is Ay 10 which is close to the measured values of Ay = 10 to 20, (1000% to 2(KX)%). 

In a recent series of impact tests the plastic deformation rates required to initiate chemical reaction were measured at the reaction site for 10 different explosive materials ranging from the sensitive explosives RDX and HMX to the insensitive explosives PBXN-128 and PBX 9502 [21]. The measured plastic deformation rate ranged from Ay/At =. 7 x lO s for RDX or HMX to Ay/At 3 x 10 s PBXN-128 to PBX-9502. Substituting these measured plastic deformation rates into the expression for the calculated plastic deformation rate in the previous paragraph implies that first reaction occurs in the impact tested materials when 10 T(x,U)pc 10 . As shown above, T(r, U)pc = 10 predicts the observed plastic strain in RDX crystals just prior to the initiation threshold. 

Consider next the plastic deformation rate that occurs in a crystal during a very high amplitude shock. For very large plastic deformation the amplitude of the 

---