Study On PBX’s Mechanical Behavior And Damage Feature

As modern high-powered weapon systems develop so fast, the safety problem ofthe explosive gradually becomes more and more spiculate. New-style normal weapongives higher requirement of explosives’ detonation initiation behavior. Polymer-bondedexplosive (PBX) is a kind of high energetic composite, having higher energy density,better mechanical behavior and higher safety property. Therefore, they are widely usedin many areas like national defense and civil economy. However, unexpectedexplosions occur sometimes even if the insensitive explosives were used. To satisfy therequirement of high property weapon equipments, it is crucial to investigate eachproperty of the insensitive explosives. Because PBX has complicate structure and itusually undergoes very complex mechanical state before explosion, to study PBX’smechanical behavior, especially the dynamic mechanical behavior is an important taskfor researching the safety behavior of PBX. This thesis aims at the research on thedynamic response and damage evolution. Using Split Hopkinson Pressure Bar (SHPB),we studied the dynamic compressive and tensile behavior of PBX, obtained the damagefeature at meso-scale of recovered specimen, and finally constructed the constitutiverelation involving the meso-damage evolution.There are three parts in the thesis. The basic mechanical researches of PBX wereperformed firstly. Damage developments and its influences on stress-strain curves wereexperimentally observed. From the experimental results, it can be inferred that PBX istypically brittle. The according damage modes are mainly micro-cracks’ initiation andspreading, and development of PBX’s inner micro-cracks needs to be experimentallyobtained for the investigation of damage evolution of PBX. Hence, in the second part,the experimental and numerical studies of PBX’s meso-damage evolution were carriedout by way of Brazilian test. After the obtainment of the damage feature of PBX, themathematical description of the evolution law is necessary to be founded. Therefore, atthe last part, the micro-crack development region constitutive model was adopted todescribe the influence of inner micro-cracks’ evolution on the macro stress-strain curve.Follows are the details.The first part mainly works on the macro dynamic response of PBX. Themeasurement of the SHPB tests was improved to obtain more accurate data, because thefailure strain of PBX was quite small. Using quartz crystal as stress gauge andsemiconductor strain gauge together, high Signal-to-Noise signals were obtained. Usingdifferent wave length and pulse shaper, it was found that the damage evolution wassensitive to impact loading, and the range of the loading strain rate was small due to thelow strain rate limit. According to the feature of the stress-strain curve, constitutive equations were developed and they could describe the stress-strain relation including theunloading stage after failure. Laser micro-displacement velocity interferometer wasdeveloped to accurately calibrate the coefficients of semiconductor and quartz crystalslices, by measuring the particle velocity of the bars directly. Results show thecoefficients obtained by laser interferometer are smaller by10%than that obtained bytraditional methods. Because the laser interferometer system can realize multi-pointmeasurement, the velocity history of specimen’s two ends can be obtained directly.Moreover, it is still can be developed to measure the signals of long pulse at middlestrain rate loading cases.Mechanical behaviors of three PBXs are compared by taking high temperatureSHPB tests to investigate the influence of the components on the whole mechanicalresponses. Several mechanical parameters’ variations with temperature were obtainedand the contributions of PBXs’ each component to mechanical parameters wereanalyzed also. Results show the dynamic modulus and the failure strength decreases asthe temperature rises, but the failure strain changes little. Among the three PBXs, PBX3has unique failure mode and most stable mechanical behavior. The addition of TATBcan insentisize HMX obviously and improve the macro mechanical properties of PBXgreatly.The second part concerns about experimental and simulative investigation of themeso-damage evolution. At first, the quasi-static and dynamic Brazilian tests wereperformed on polished PBX and the meso-damage features of recovered specimen wereobserved using optical microscope. It is found that cracks are the main damage mode inPBX, like many brittle materials. In case of quasi-static loading, most microcracks areinterface debonding cracks, and macrocracks are mainly transgranular fracture cracks.The interface cracks may initiate at early stage of loading and they have little influenceon specimen’s sustainability. Transgranular cracks may be the primary cause ofspecimen’s macro-failure. In case of dynamic loading, the observed macro-cracks andmicro-cracks both exhibit transgranular fracture.Discrete element method was adopted to simulate the cracking process of PBXduring Brazilian tests. To prove the validity of this method, homogenous material modelwas built using HMX’s parameters. The strain distributions of the disk specimen duringBrazilian test were obtained and they were similar to the theoretic analysis of Braziliantests. Meso-structure of PBX was presented. Specimens with different particle size weremodeled under the loading of Brazilian test. Simulation results show that interfacecracks occur before transgranular cracks, and specimen with interface debonding crackscan sustain the applied loading, also. The specimen fails at macro-scale after the crystalfractures. Specimen with smaller particle size can sustain for a long time withmicrocracks before transgranular cracks occur.The third part focuses on the constitutive relation concerning the meso-damage evolution. The damage evolution contains the cracks’ splaying, the friction and slidingof closed cracks and the bending spread of wing cracks. Computing results in case ofuniaxial compression loading show that this constitutive model can describe themechanical behavior. At last, the model was developed for the use in dynamic loadingcase theoretically. In further work, the model will be developed for the use inLS-DYNA.