Experimental Study and Numerical Simulation on Mechanical Behavior of Polymer-bonded Explosives

Author:Liu Zuo Long

Supervisor:chen peng wan


Degree Year:2017





Polymer-bonded explosives(PBXs)are polymer matrix composites filled with high percent of explosive granules.As the damage inside PBXs may cause unexpected ignition of explosives,determining the mechanical properties of PBXs in various loading conditions is crucial for service life,security and reliability.In this paper,quasi-static mechanical properties of PBXs under different temperatures were studied and the macro and mesoscopic fracture modeling of PBX materials were carried out using cohesive zone model.The main contents of the paper are listed as follows.Standard Brazilian test was conducted under different temperatures from 20°C to 90°C to study the tensile properties of a PBX mock material(PBX-M),and the results show that the tensile capacity decreases with increasing temperatures;fracture toughness were measured by flattened Brazilian test.Based on the displacement and strain fields measured by digital image correlation(DIC)method,the deformation and fracture mechanism of PBX mock material affected by temperature were studied.The fracture morphology observed by SEM was used to study the damage modes and failure mechanism of samples.The results show that the mode of failure is predominately transgranular at room temperature and then debonding failure is dominate.The influences of length to diameter ratios on the quasi-static mechanical properties and failure of PBX under uniaxial compression were examined.In addition,three-dimensional DIC was applied to uniaxial compressive test to measure the out-plane deformation fields of PBX simulant under different temperatures with a length to diameter ration of 1.0.The results show that circumferential extension plays a main role in macroscopic failure mechanism under high temperatures.Debonding is common in PBXs and a new experimental method was designed to measure the interfacial debonding strength of the particles and binder of PBX.The tensile and compressive mechanical properties and fracture mechanism of PBX-1 based on TATB and PBX-2 based on HMX explosives under different temperature from-40°C to 200°C were studied using DIC method.The results show that PBXs are characterized as thermal softening and tension-compression asymmetry.The tensile strength and tensile modulus are much lower than compressive strength and modulus.The PBXs exhibit brittle behavior at room temperature and lower temperatures,and the plasticity increases when temperature is higher than 100°C.The tensile strength of PBX-1 is a bit higher than that of PBX-2,due to the mechanical properties of F2314 and the interfacial interaction between TATB and F2314.The compressive strength of PBX-2 is much higher than that of PBX-1;the reason could be that the compressive resistance of HMX in PBX-2 is better than TATB in PBX-1.Because of phase transition of β-HMX around 180°C,the compressive strength of PBX-2 at 185°C~200°C is distinctly different with that at 100°C~155°C.The constituents and volume fraction of binders in PBX-1 and PBX-2 lead to the differences of tensile failure mode and deformation.The embedded cohesive crack model(ECCM),based on the basis theory of cohesive zone model,was proposed and implemented in finite framework.After verifying the capabilities in single cell model,direct tension test and the three-point-bending test,ECCM model was applied to predict the critical load of PBX materials by semi-circular bending test with different notch lengths.The Theory of Critical Distances(TCD)was also applied to PBX materials to predict tensile stress concentration phenomena in order to study the fracture of notched specimens of PBXs.Both ECCM and TCD method show good agreement with the experimental results.From the results,we can conclude that reliable static assessment could be performed without the need to invoke complex non-linear constitutive laws of PBX.Considering the mesoscopic inhomogeneity,Weibull distribution was introduced to model the random heterogeneity of material properties and applied in the simulation of Brazilian test under different temperatures to predict the tensile strength of PBX-M.The study on the crack initiation and propagation of the PBX material at the mesoscopic scale is necessary to better understand macroscopic fracture behavior.The cohesive elements with zero in-plane thickness were pre-inserted along the initial mesh by self-compiled program to produce potential cracks.This method was applied to the finite element model based on the micro-morphology of PBX.The mechanical behavior of PBXs under tensile and compressive loading was simulated by this method.The results show tension-compression asymmetry.In addition,the influence of initial damage was also studied.The results show that defects in particles result in the transgranular fracture under tensile loading at room temperature and decrease the tensile and compressive strength of PBX.The effect of strain rate and the modulus of constituents on PBX compressive properties were analyzed.