Studies On The Mechanical Properties Of Energetic Crystals

Abstract:The mechanical properties(such as elastic modulus or fracture strength) and mechanical behaviors(for instance,dislocation slipping,plastic yielding,cracking failure and so on) of energetic crystals are crucial to the designing,preparing,detonation as well as security performances of energetic materials.In this paper,the mechanical properties of energetic crystals were investigated both from the perspectives of crystalline particle aggregates and single crystals.Owing to the high correlation of mechanical properties and crystalline qualities,RDX and HMX crystalline aggregates of different qualities were selected to be compressed so as to obtain compaction curves,which then were fit by the Kawakita and Heckel equations.The fitting constants were used to evaluate the mechanical properties and qualities of the crystals. Further studies were executed on segmentation of the compaction process and influences of the test parameters.What's more,the rules of the acoustic emission signals,which were produced in the compression,were investigated,too.The results show that the constants of the Kawakita and Heckel equations,which are of dimension of the reciprocal of modulus and bear the meanings of ensemble average,can be employed effectively to describe the mechanical properties of the crystalline aggregates.The constant is lower and the quality is better.While the recrystalline particles have lower constant values,which approves that their qualities are better than the commercial grade ones.The evaluation results are consistent with other means,which validates the effectiveness of the method by mechanical properties. Basing on the traits of the curvature of compaction curves and the process of compressing,the compression process is divided into three regimes,i.e.rearrangement,fragmentation and compaction stages.It is the best and true reflection to the mechanical properties and interactions of aggregates to fit with the mere data of fragmentation regime.The parameter settings of compression experiment were also discussed,and the conclusion is that:the size of the mould and the charge amount per unit area straightly contribute to the results;the initial height or density acts only when the charge manners vary greatly;while the particle size distribution and loading rate have little influence.The acoustic emission(AE) signals were detected when pressing the crystalline granules.The maximum amplitude,hit count(rate) and rise time present regularly in the procedure,which corresponds to the mechanical behaviors of the granules.Theβ-HMX single crystals with high quality and large dimension were loaded by instrumented nano-indenter in the(010) and(001) plane,respectively.The modulus and hardness were acquired for the first.The elastic moduli are 22.88GPa and 26.05GPa of(010) and(001) plane respectively,while the hardness values are 1.11GPa and 0.95GPa.Finite element simulations were performed for indentation experiment,in which the anisotropic elastic-plastic constitutive model and HMX constants published in papers were used.And the differences between experimental and simulation curves were discussed preliminarily.The results show that both the modulus derived by Zaug and Sewell are markedly lower to the indentation results.The simulation curves of Zaug are very different from the experimental curves,which may be caused by its very low shear modulus.While the unloading curves of Sewell are quite close to the indentation curves,indicating that the elastic modulus by Sewell approach to the real modulus in indentation experiment,yet they are different in the numerical values just because of their differences in the principles.In this paper,the pilot studies show that the elastic properties obtained by indentation,in respect that it is three-dimensional stress statue during the loading process,are much close to the properties the engineering practice.