An Investigation On Some Issues Of SHPB Technique

SHPB is a widely used apparatus for testing dynamic materials properties. In this dissertation, based on review of literatures, it is suggested that some investigations into SHPB technique should be conducted in order to improve the precision and comparability of SHPB testing results. For this purpose, some studies on the SHPB technique for metals are carried out in this dissertation: the comprehensive analysis of inertial effect of cylinder specimen and interfacial friction effect between bars and specimen, studies on the impedance matching between bars and specimen, analysis of machining requirement for tolerance of parallelism of the bar ends, analysis of stress and strain uniformities in specimen and bars, investigations into constant rate experimental technique and other methods.① The comprehensive analysis of inertial effect of cylinder specimen and interfacial friction effect. By comprehensive analyzing the inertia effect and friction effect on the experimental results, a new theoretical model is presented to modify Gorham's formula (1989) for analysis of inertia effect. On the base of these analysis results, some requirements for SHPB testing are summarized for improving SHPB experimental precision. The specimen size ratio is studied in detail, and the design requirement for specimen size ratio is clearly pointed out.② Studies on the impedance matching between bars and specimen. A theoretical model is established by taking the specimen as a spring with varying stiffness in SHPB experiments. The corresponding analysis gives a foundation for choice of material and size for bars and specimen according to the impedance matching principle. Two dimensionless parameters α and β are defined, which describe the impedance match between bars and specimen. The range of α and β is gained.③ Analysis of parallelism requirement for bar ends in SHPB system. The effects of the ends parallelism on experimental results are analyzed by numerical simulation, and the quantified requirement of parallelism of the ends is obtained. Otherwise, the simulation results are also helpful for choice of measurements and analysis of abnormal waves in experiments.④ Analysis of stress and strain uniformities in bars and specimen. The stress and strain uniformities in bars and specimen are analyzed by numerical simulation under condition that specimen is elastic or elastic-plastic. In the analysis the contact state between the bars and specimen is ideal, and the elastic-plastic behavior of specimen is emphasized in the analysis. In SHPB experiments, cross-sectional areas and the materials of the bars andspecimen are usually different, the effect of these differences on the stress and strain uniformities in bars and specimen is analyzed by changing the parameters of specimen such as.- diameter, elastic modulus, yield strength and hardening modulus. The diameter ratio between specimen and bars is obtained, which satisfies the stress and strain uniformities of bars and specimen. The relation between results obtained by method of experimental data processes and results output by numerical analysis is analyzed.(5) Investigation on constant strain rate experimental technique. Two methods to realize constant strain rate of specimen in SHPB are proposed. They are proved and compared by experiment and numerical simulation. The effect of strain rate-time history on the stress-strain curves is studied. The results indicate that the effect should be considered in the investigations of dynamic material properties and constitutive relations. When the strain rate of specimen keeps constant in SHPB experiment, the comparability of SHPB testing results obtained by different investigators can be improved.? Investigation on other experimental methods of SHPB. The SHPB with bars of small diameter (the diameter is small or equal 5mm) and Direct Impact Hopkinson Pressure Bar system (DIHPB) are investigated. The dynamic compression stress-strain curves of some materials with the strain rate up to 10V are obtained. Some questions should be further researched are pointed out. Otherwise, the dynamic unloading experimental method is explored by prolonging the unloading time of incident pulse by the help of the cylinder-cone striker instead of the cylinder striker of SHPB.