Numerical Simulation Study For The Impact Response Of Several Basic Phase Transition Structures

Phase transition(PT) can greatly affect the mechanical response of material and structure.It is a basic problem and major research task of solid mechanics and material science.The study of mechanical property is relatively mature,but it is very short of the study of the mechanical property of the phase transformation structures,especially the response under impact loading.In this paper,the impact response of phase transformation rod,bar and shell are systematically investigated by analytical and numerical methods, and some interesting phenomena and regularity are found.By characteristic method and finite difference numerical method,the rules for the propagation of macroscopic phase boundary under rectangular pulse load in a finite rod are investigated.It is found that the free surface always plays the role of stress unloading for the incident stress wave,while the fixed surface will affect the stress amplitude according the incident wave conditions.For the reversible PT materials rod,the impact amplitude,pulse width and the boundary conditions will all affect the propagation of macroscopic phase boundary and the unloading process,and the position where the tensile stress zone may appear will change correspondly.For the rod with free surface, under short pulse load,the tensile stress zone will first appear near the location where the phase boundary disappear,and when the pulse duration is longer,the tensile stress zone may first appear near the load surface.After unloading,both ends of the rod become free surface,which both play the role of stress unloading for the incident stress wave,and multiple spalls may occur.For the rod with fixed surface,the tensile stress zone will appear relatively late,and the spall may first occur near the fixed end.For the inversible PT materials rod,the effects of the boundary conditions to the phase transition are investigated.The possibility of producing symmetric distributed Functionally Graded Materials(FGMs) by controlling the shape,stress amplitude and duration of the loading pulse under different boundary conditions is proposed.By this theory,some new phenomenon,recently found in the phase transtion Taylor bar experiment can be explained successfullyFor the abnormal spall phenomena recently observed in impact experiments for symmetrical and the same thickness of pure iron plates and for the different thickness of FeMnNi alloy plates,in this article,the propagation of the elastic and plastic waves as well as the macroscopic phase boundary is studied quantitatively by characteristic line method,and the mechanism of the abnormal spall phenomena mentioned above is explained theoretically.The investigation shows that PT and reverse PT can strongly affect the profiles of the shock waves in the target,and the more complex interactions of the waves may cause abnormal spall.And for the impact of symmetrical and the same thickness of FeMnNi alloy plates,the rules of the position where the abnormal spall may occur with change of impact speed is found.Reserch shows that the abnormal phenomena will not occur for elastic-plastic materials,and it is unique for PT materials.A dynamic constitutive model for phase transformation which can describe the "stress-induced" phase transformation in isotropic material is succsessfully implemented in ABAQUS finite element software,thus numerical simulation study can be conduncted for Shape Memory Alloy(SMA) structures.The formation and propagation of phase transition flexurai wave(PTFW) in a pseudoelastic phase transition circular shell(PPTCS) which under radial impact at the top is numerically studied,and the effects of curvature on the PTFW are examined.Results show that PTFW is initiated by development and enlargement of elastic flexurai wave (EFW).PTFW always appers first at the impact point and remain there,forming a main PTFW peak.If the impact amplitude is large enough,several removing PTFW peaks may apper in the shell,which will move in the direction of the shell bottom at the same speed under different impact amplitude.Study shows that the flexural wave velocity is not sensitive to the shell curvature.For the PTFW peaks near the load point,the amplitude of bending moment is significantly affected by curvature,while for the elastic flexural wave of faster speed,shorter wavelength and small amplitude,the curvature effect can be ignored.For the shell with free bottom,the reflected flexural waves and the incident waves at the bottom of the shell are approximately symmetry,while for the shell with fixed bottom,additional PTFW will be caused by the bottom constraint,and the wave propagetion is more complex.The effect of the unloading flexural wave,which is dispersive and developing,on the loading wave is depended on the phasic difference when they meet.When the unloading wave and reflected wave from the bottom action at the same time,the spatio-temporal evolution map of the whole phase transformation zone is represented by some discrete zones.Numerical simulation study is conducted on the dynamic response of PPTCS under step or rectangular pulse load.Different pattern and the evolution procedure of phase transition hinge(PTH) is analysed.Under step load,different patterns such as hinge zone at the load point,single,double hinges in the shell may form.Under high but short pulse load,response of PPTCS may be divided into four stages:propagation of flexural waves and formation of PTH during loading,complex evolution of PTH after unloading, intensive whipping of the shell during the load point swinging toward the maximal displacement,and to-and-fro movement around the bottom of the shell;the first three phases occupy relatively short time but the majority of energy is dissipated.When the PPTCS,which is placed on a rigid boundary,is impacted by a rigid plate with an initial velocity,the dynamic response of PPTCS is investigated numerically. Results show that the dynamic response may be divided into three stages:formation and movement of PTHs,the evolution of PTHs,and the unloading of PTHs.When impacted by a rigid plate with large mass and slow velocity,the shell present a four PTHs formation and unloading process similar to quasi-static resoponse.When impacted by a plate with little mass but high velocity,there will be a siginificant PTHs removement process before the four PTHs form,during which there will been several separation and re-collision processes among the impact plate,shell and the rigid boundary.Besides,the study for response of PPTCS under three,four and six constraints condition show that when the number of constraints increases,though the dynamic response process is similar, more PTHs will apper in the shell,and the deformation and PT will occur more locally near the impact suface,as a result,buckling may occur more easily near the impact suface.With the increase in the number of constraints,shell stiffness will increase,while the energy absorption changs little.