Research On Stress Waves Of Phase Transition Material And Polymer Under Combined Stress

Various impact loadings are widely existed in engineering application and normal life,under such dynamic condition,it's significant to analyze the stress wave.After many years of development,the stress wave in traditional material has been widely studied,and develops a variety of complex wave theory,forms a lot of branches.How-ever,with the development of new materials of various novel property,the theory of stress wave should be upgraded,moreover,with the theory of stress wave,the deeper understanding of past experiments can be achieved.Base on this consideration,we stud-ied the stress wave of phase transition material and polymer under combined stress,and re-analysed some phenomena of past experiments from the perspective of propagation of stress wave.The tension/compression-torsion of thin walled tube and the compression-shear of plate are the most commonly combined stress condition when studying the propagation of stress wave.Firstly,we studied the stress wave with phase transition under tension-torsion combined stress of thin walled tube theoretically,based on the characteristic line theory,we obtained the theoretical result of coupling stress wave with phase transi-tion under tension-torsion combined loading,and investigated the influence of second phase strengthening effect on phase transition wave.The results show that,with the influence of second phase strengthening effect,there will be coupling shock wave with phase transition.Combining the constitutive equations and jump condition of shock wave,we obtained the equation group determining the generalized Hugoniot curves of coupling shock wave of phase transition,and discussed the solving strategy of different initial and final state of coupling shock wave with phase transition.By numerical sim-ulation independently with the theory of coupling shock wave,we calculated the form-ing process of coupling shock wave with phase transition under three different pursuing conditions,and observed the automatic adjustment of state before the wave front.The simulated results of stress state before wave front and the speed of the coupling shock wave are basically same with the theory results,proving the existence of the coupling shock wave and the validation of our theory.Since there is nearly no experimental study of tension-torsion stress wave with phase transition,we designed and built a system which can apply and hold twisting loading of a thin walled tube and convert the impact compression loading to impact tension loading.With a split Hopkinson pressure bar generating the impact compres-sion loading,we conducted the impact tension experiments of pre-torqued thin walled tube of 304 stainless steel and phase transition material NiTi alloy.In the experiment of 304 stainless steel,we observed the typical coupling plastic wave structure of rate-sensitive material,proving the feasibility of our system.In the experiment of NiTi alloy,we observed the coupling fast wave with phase transition directly,and compared the ex-perimental results with simulation.The simulated results fit well with the experiment,and by simulation,we analyzed the reason why the coupling slow wave is not obvious.Since the bearable loading amplitude of the system is limited,we have not observed the coupling shock wave with phase transition,but it's still make sense of observing the coupling wave with phase transition in experiment directly.As for the stress wave with phase transition under combined compression-shear loading,using constitutive equations considering both influence of hydrostatic pres-sure and deviatoric stress,we conducted preliminary study on the propagation of stress wave with phase transition under combined compression-shear loading,and analyzed the critical condition of phase transition with different asymmetry coefficient and Pois-son's ratio.We reanalyzed the experimental results of NiTi alloy under pressure-shear plate impact experiment,especially the special phenomenon that the measured trans-verse particle velocity is less than the velocity at the impact surface,and put forward a different understand of the wave structure in the experimental process.From our the-ory of stress wave with phase transition under combined compression-shear loading,we obtain a theory result fit better with the experimental result compared with other literature.Since the special phenomenon that the measured transverse particle velocity is less than the velocity at the impact surface in the pressure-shear plate impact experiment is also existed in polymer such as PMMA and nylon-66,moreover,the microscopic observations show that there is shear band near the impact surface and the mechanism is still not fully understood,it make sense to study this shear attenuation phenomenon.We conducted studies on the propagation of stress wave of polymer in stress space and particle velocity space,find that in the loading process:(1)In the stress space,with the increasing of shear stress,there is almost no changes of longitudinal stress,but the lateral stress keeps increasing.When the shear stress increase to the yield stress under pure shear,the normal stress at all directions of the material are equal to each other,which means that the material is under pure shear with hydrostatic pressure.(2)In the particle velocity space,the pure shear with hydrostatic pressure corresponds to a curve which stand for the maximal transverse velocity.The maximal transverse velocity is related to the longitudinal velocity,and can exceed the pure shear particle velocity limit.(3)When loading transverse velocity amplitude exceeds the above curve,there will be an interruption at the interface.If the bonding strength of interface is less than the strength of material,there will be slipping at the interface,otherwise,there will be shear band in the weaker material very near the interface.The above analysis shows that,the basic mechanism of the attenuation and shear failure is related to the shear strength of material.Besides,taking account the unloading process and the viscosity,the electrical signal measured in the experiment may also decrease,but there will be no failure near the interface.