Research On Problems Regarding Impact Dynamics Of Structural Systems Based On Mechanical Waves

The effect of a sharply applied,localized disturbance in a medium soon transmits or spreads to other parts of the medium.This simple fact forms a basis for the fascinating research subject known as wave propagation.In this paper,we only restrict our attention to the mechanical wave propagation in the elastic solid medium when disturbed,and its corresponding disturbance form is commonly from implusive load caused by impact or explosion.Especially when the impact time is in the same order of magnitude as the time required by a wave to pass through the structure,the effect of mechanical wave propagation is very obvious,in which case the dynamic response of the material to the external force can only be accurately described in the form of wave propagation.The theory of mechanical wave propagation has been widely used in structural damage detection and transient response analysis of structures under impulsive dynamic load.By the modeling method based on mechanical wave propagation,the dynamics problems caused by impact motion in some common structure systems in engineering are studied in this paper.The specific work is as follows:(1)The behavior of axial repeated impacts between non-uniform bars is related to the work efficiency of percussive drilling machine and the test of dynamic mechanical propertis on specimen in Split Hopkinson Pressure Bar set-up.The principles of incidence,transmission and reflection exist during the propagation of longitudinal wave in non-uniform bar,by establishing the discretized model of axial impact between non-uniform bars,using impulse response function,a procedure is developed to predict the parameters at impacted ends throughout the changing-status impact process where repeated impacts and separations occur.The calculation for an example impact model illustrates the self-recontact behaviour resulting from non-uniform dynamic axial extension caused by mechanical wave mtion,and reveals potential phenomenon of repeated contacts and separations in microsecond scale.(2)For the transverse impact of a projectile against a taut band-like membrane,because of low-velocity,sufficient pretension,it can be reduced to the linear model that a rigid-projectile has a normal impact against taut-string.This model can serve as the physical prototypes of cable-membrane structure resisting the impact of external objects and seat belts intercepting occupant torso.The responses under central and non-central impacts are solved analytically from the viewpoint of in-webbing longitudinally-propagating transverse waves,and the influences of parameters on the dynamics properites are analyzed combined with finite element(FE)simulation.For a central impact,after the first separation,second impact occurs in the system only when the ratio of two mass is close to 1.For a non-central impact,the projectile would be subjected to a combination of translation and rotation due to asymmetric wave propagations.The swing amplitude of a flat projectile or a projectile with small moment of inertia is susceptible to significant fluctuations.The band with a rather large off-centre ratio for the impacted zone and a rather short length of the shorter segment would facilitate a larger accumulation of swing amplitude in a single direction soon after the impact.(3)Seatbelt has been recognized as the most effective means to prevent occupant in car against being ejected out and sustaining injury caused by the impact with interior.Improving its limiting performance,such as reducing torso forward displacement and limiting force applied by seatbelt webbing,is still well worth studying.In this paper,the models of a rigid-projectile impacting against taut-string are employed to simulate the impact of torso against pre-tensioning seatbelt and the interaction between them.On the basis of conventional seatbelt model,a new model that its two outer ends can be controlled to slide forwards and rearwards is put forward.Studying from the standpoint of longitudinal propagations of in-webbing transverse waves,a displacement function,i.e.the superposition at origin by the displacements of an inwards-propagating and a outwards-propagating wave which both arrive at the origin at the same time is introduced;then analyse the properties required for the function to be competent;finally solve the inwards-propagating and the outwards-propagating waves inversely,and give the corresponding boundary displacement signal that needs to be input.Theoretically,the results show that this control technique can simultaneously reduce torso maximum displacement,maximum force,rebound velocity against backrest and the duration of interaction with seatbelt webbing,especially in the case with bigger ratio of torso mass to webbing mass and smaller product of initial angle of straightened webbing,wave speed and the reciprocal of impact velocity,which is apt to cause bigger maximum forward displacement,moderate maximum force and consequently high risk of casualty.The results also show that shortening lag time of the control input benefits the comprehensive reductions of the criteria.(4)In order to study the transient dynamics characteristics of the revolute joints of linkage mechanism and heavy-load artillery coordinator with high-speed under impact load,we establish the model of a Bernoulli-beam impacting against a rod-like stop and apply upper-and lower-displacement limit at the beam impacted point as boundary conditions to simulate the revolute joint.The alternating occurrence of impact and separation and the coupled effect of the flexural wave in beam and the longitudinal wave in rod lead to more complicated traveling ways of waves.In both impact phase and separation phase,the transient wave propagations are solved by the expansion of transient wave functions in a series of eigenfunctions.The numerical results reveal several transient phenomena involving the propagation of transient impact-induced waves,sub-impact phases,covert big impact impulse forces,sticking motion,and impact loss.Using ADAMS,the rigid and the rigid-flexible coupled models of coordinator are established.As simulation results,impact frequency is higher than that of the theoretical model since 3-D effect of the revolute joint and the periphery limitation of pin hole to pin shaft are considered.When clearance becomes larger,the amplitudes of impact forces during sub-impact phases become larger but sub-impacts occur less frequently and last shorter.Compared with the multi-rigid model,the elastic deformation of flexible coordinator arm can reduce the collision force of pin to pin hole,but it can increase the component's rebound speed and acceleration.