Research On Multiple Elastic-plastic Impacts Of Flexible Structures

The problem of multiple-elastic-plastic impact is important in theoretical analysis and engineering application for flexible structural systems. The phenomenon of multiple-elastic-plastic impact is complex in its mechanics behavior, due to the interactions of the dynamical effect of material, the propagation of elastic-plastic waves induced by impacts and the structural responses to impacts.This thesis studies five typical flexible impact systems, a rigid mass impacting against a simply support beam, a free-free beam impacting on a simply support beam, a free-free rod impinging on a simply support beam, a rigid mass impacting against a simply support beam with axial constraints, and a rigid mass impacting against a simply support beam with considering strain rate effect. A numerical method is presented to investigate the phenomenon of multiple-elastic-plastic impact of the five typical flexible impact systems.The main researches and conclusions are shown as followings:(1) Three local elastic-plastic contact deformation models for multiple-contact behavior have been presented by improving local elastic-plastic contact deformation models for single contact behavior. The comparison shows their agreements in describing the characteristics of the response of impact force, dynamical deformation and energy transformation, and their differences in the magnitude of impact force.(2) A so-called MCIS method is presented by combining the local elastic-plastic contact deformation models for multiple-contact behavior and the finite difference method to investigate the problem of multiple-elastic-plastic impact of flexible structural impact systems. It considers the propagation of elastic-plastic waves induced by impacts, the process of multiple impacts and the process of multiple separations.(3) The MCIS method has been applied to investigate the phenomenon of multiple-elastic-plastic impact in the five impact systems. Its numerical results have been compared with those of the finite element method, rigid-plastic theory and elastic-plastic analysis under stick assumption. The comparions illustrate that the MCIS method is valid, especially in the systematical investiagtion on the phenomenon of multiple-elastic-plastic impact of flexible impact systems.(4) The numerical investigation of the five typical flexible impact systems has shown that the impact process consists of complicated processes of multiple sub-elastic-plastic impacts. There are usually more than two impact zones which consist of several different types of sub-elastic-plastic impacts. The total impulse and total energy dissipation of the other impact zones are considerable to those of the first impact zone. Some subsequent impacts may have considerable magnitudes of impact force compared to that of the first impact. It concludes that the multiple impact zones and the multiple impacts have important influences on structural impact physical behavior.(5) The numerical investigation of the five typical flexible impact systems has shown that the multiple-elastic-plastic impact has obvious effect on the structural dynamical response. The effect of elastic-plastic deformation makes the concept of "fixing hinge" and "travelling hinge" can not be surely applied and their applicability is dependent on the structure type of impact. The multiple-elastic-plastic impact could cause some unusual phenomenon, such as "twice stable phase transformation" of dynamical response, from transient phase to the first stable phase, to transient phase, to the second stable phase.(6) A phenomenon of energy inverse feeding from impacted body to impacting body has taken place in the last impact processes of the five impact system, and is shown to be an elastic deformation behavior.(7) Appearance and disappearance of sub-elastic-plastic impacts is consistent with the structural transient deformation response, so the sub-elastic-plastic impact may be caused by the structural transient deformation.(8) The numerical investigation has shown that there are some inherent mechanisms unrevealed for the multiple-elastic-plastic impact phenomenon. The numerical results illustrate that the whole impact zone is relative to the first elastic vibration mode, the first impact zone is relative to the third elastic vibration mode and the first separation zone is relative to the second elastic vibration mode.