Study Of SHPB Method Optimization Andmechanical Response Mechanism Inhigh-Speed Penetration On Gelatin

Ballistic gelatin is widely used as a muscle analogue in analyses on the terminal effect of small caliber ammunition.When a gelatin block is penetrated by a high speed projectile,the interaction between the projectile and gelatin determines the penetration process.The understanding of the interaction is the base of penetration trajectory and gelatin cavity analyses.At present,interaction force models and penetration resistance models for gelatin penetration process were mostly fitted from the penetration experiments and have small correlation with the mechanical properties of gelatin,which makes the effects of gelatin properties on the penetration process unclear.The study in this paper is an analysis of the interaction during high speed penetration from the view of the gelatin mechanical response properties.The gelatin mechanical response mechanism was investigated base on the analyses of gelatin dynamic mechanical properties and strain rate sensitive mechanism.a)To solve the low reliability problem of the modified SHPB(Split Hopkinson Pressure Bar)methods for soft materials,the chosen HSHPB(Hollow Split Hopkinson Pressure Bar)method was investigated to improve its reliability by analyzing the transmitted signal gain mechanism in hollow transmission bars and the wave distortions during the signal gain process.The transmitted wave is too weak to capture when the traditional SHPB is used to test soft materials.Existing modified SHPB methods for soft materials are more or less unreliable which were reviewed in this paper.The HSHPB has some advantages in reliability which was chosen as a basic measure technique to analyze ballistic gelatin.The transmitted signal gain mechanism and transmitted wave distortions were theoretical and numeric investigated,and delayed distortion and high frequency interference in transmitted wave gain process were found,whose mechanism and law were also investigated.b)When using HSHPB to test ballistic gelatin,the signal gain ratio is too low to solve the weak transmitted signal problem,and the wave distortions were hard to restrain when the gain ratio was increased by increasing the hollow bar cross-section ratio.To solve these problems,theoretical analyses,numeric simulations and experiments were used to create parameters for HSHPB transmitted wave delay characterization,establish an evaluation method for HSHPB precision,decrease the delay distortion,retrain the end cap deformation interference,increase HSHPB signal gain ratio,and finally modify the HSHPB to fulfil the signal gain and precision requirements of gelatin dynamic test.Then,the modified HSHPB was used to test the gelatin dynamic properties.c)The mechanism of gelatin strain rate sensitivity effect in SHPB test is unclear,thus it is proposed that the radial inertia effect may be the main cause of gelatin strain rate sensitivity effect.Radial inertia stress equation was derived theoretically and extended to a version for the modified HSHPB method.The HSHPB test results and the D.Richler data were analyzed with two radial inertia stress equations.It is found that the radial inertia stress is basically the same with the total stress response which means the mechanical response of gelatin in these SHPB tests were mainly composed by radial inertia stress.d)Based on the conclusions above,the gelatin mechanical response mechanism during high speed penetration was analyzed by numeric simulations and experiments.Based on the conclusion that the mechanical response of the gelatin samples in SHPB and HSHPB tests is mainly composed by gelatin's radial inertia,a simplified linear elastic gelatin model with bulk modulus was proposed.The gelatin model simplifications of turning the EOS to a bulk modulus and turning the quasistatic elastic-plastic feature to pure quasistatic elastic feature did not effect the prediction accuracy of the numeric simulation,which means the stress deviator caused by the quasistatic feature is far less than the spherical stress tensor caused by gelatin inertia,and has little effect to the penetration process.The mechanical response caused by gelatin inertia is the main component of the mechanical response of gelatin during high speed penetration.And,the density is found to be the key factor.Finally,by comparing the flow state,hydrostatic pressure generation and cumulative process of gelatin in SHPB experiment and penetration process,the mechanical response process based on gelatin inertia effect is analyzed.Gelatin flows in the process of penetration,and gelatin inertia effect leads to hydrostatic pressure gradient in gelatin.The hydrostatic pressure accumulated by hydrostatic pressure gradient is applied to the contact surface between gelatin and projector to form a gelatin mechanical response.