Experimental Studies On Mechanical Properties Of A PBX Under Various Dynamic Loading Conditions

Polymer bonded explosives (PBX) are used in a wide variety of applications, ranging from rocket propellants to the main explosive charges in conventional munitions. Of great concern to explosive researchers is the possibility of accidental ignition during manufacture, transport or handling. It is thus essential to obtain a complete description and understanding of the mechanical response of PBX. This description and understanding are necessary for modeling those events that may cause unwanted ignition of the energetic materials and the resulting hazards.A method is proposed to combine the traditional Split Hopkinson Pressure Bars and Split Hopkinson Tensile Bars in a newly developed Hopkinson Pressure-Tensile Bars. A momentum-trap system is introduced to ensure single pulse loading during tests. The controlled multi-pulse loading is carried out by a stuffed striker, and the dwell time and amplitudes of pulses can be accurately controlled. The strain field of the specimen is estimated by the Digital Image Correlation (DIC) processing of the photos which are taken by a synchronous high speed camera. In addition, a self made laser gap gauge (LGG) is developed in the experiment.The modified Hopkinson bar system is developed to measure material responses at intermediate strain rates (1 10² s^-1). In this work, the insituation Hopkinson bar technique is proposed, where the LGG is adopted in the traditional Hopkinson bars system to measure the sample deformation and an X-cut quartz stress gauge to measure the dynamic load directly. For the flattened Brazilian disc (FBD) test, the LGG is used to monitor the transverse expansion of the disc perpendicular to the loading axis, from which the average tensile strain is deduced. The numerical simulation reveals a linear relationship between the tensile strain at the center of the specimen and the average tensile strain. This scaling factor is not sensitive to the material elastic parameters. This method is as accurate as the DIC process, but the latter is limited by the number of frames of the high speed camera. The results are then compared to those from other two methods, the direct tension with'dog bone'specimen and the semi-circular bending (SCB) tension test. The results from the first two methods are coincident with each other, while the result of the SCB test has to be properly adjusted by a'non-local'failure approach due to the stress gradient along the fracture path. The mechanical properties of the PBX at three density levels (1.5, 1.6, 1.7 g/cm³) are tested under strain-rates ranging from 10^-4 to 10³ s^-1. A constitutive relation has been established based on the experimental curves, including the features such as mass density, strain, strain-rate et al. The results show that the mechanical behaviors of the PBX bear obvious rate-dependence, and the corresponding compression/tension failure stresses increase with the original density or the loading strain rates.Two types of confinement, the passive confinement using jacket and the active confinement using pneumatic pressure vessel, are used in the triaxial test. The friction between the specimen and the confinement medium is analyzed for both methods. There is noticeable difference in the jacket confinement due to friction, and sliding fit with lubrication could minimize the friction error. However, the friction-induced error is ignorable in the pneumatic pressure vessel method as long as the confinement stress is fixed during tests. Both the axial strength and the corresponding confinement stress increases proportionally as the rising of original density or strain rates. There is a change from strain softening after the maximum at the lower confining pressures to work hardening at larger strains at the higher confining pressures.Fracture initiation toughness, fracture energy, fracture propagation toughness, and fracture velocity are key dynamic fracture parameters. A method is proposed to simultaneously measure these parameters for mode-I fractures in SHPB testing with a notched SCB specimen. The crack propagating process is monitored by a synchronous high speed camera, and thus the strain field history is observed by DIC process. Both the initiation toughness and the propagation toughness are found to increase linearly with increasing loading rates. The propagation fracture toughness also increases with the fracture velocity, and a limiting fracture velocity is obtained.A dynamic failure criterion is set up to reveal the nonlinear effect of PBX. Parameters are obtained based on the experimental results of compression, tension and confinement. The model could potentially be implemented in commercial software such as ABAQUS and LS-DYNA. The failure modes are discussed in terms of various theoretical models. The results suggest that the debonding strength is much smaller than the crystal fracture strength. The measured tensile strength is similar to the debonding strength while the compression strength is somewhere between them.