Experimental And Theoretic Studies On The Impact Buckling Of Columns And The Effect Of Stress Wave

Impact events occur in a wide variety of circumstance. Clearly, impact problems are often encountered in many technologies, such as landing of air craft, the collision of vehicle, the protection of nuclear power station, the interception of the missile, the design of structural safety and crashworthiness, etc. The impact loads are characterized by short duration and high intensity. The deformation and motion of a structure subjects to impact load is extremely rapid, hence the inertial effect has to be considered. The slender columns, thin plates and shells are the structural elements often used in practical engineering. The membrane rigidity of these elements is much bigger than lateral flexural rigidity. When they subject to impact load, the disaster failure easily caused due to the insufficiency of buckling strength. In recently years, the dynamic buckling of the structures under impact load is an active subject in solid mechanics because of the needs of the structural crashworthiness and manufacturing energy absorption facilities. In each developing period on the buckling investigation, the columns are always severed as first select object, and the mechanics model developed from columns can reveal profound connotation of physics. The dynamic buckling of the columns subjected to impact load has also special position in the structural dynamic buckling, and it is, in one respect, a more complex problem than buckling of shells under radial dynamic pressure. Axial impact load is, in fact, communicated to the columns by stress wave and the occurring and growing of buckling is a complex dynamic process related to the propagation of stress wave. In order to reveal the mechanism of the impact buckling of the columns and to determine the quantitative relation betweenthe propagation of stress wave and the dynamic buckling, a series of studies have made experimentally and theoretically in present paper. The important results are summarized in the following several aspects:1. The bar-bar impact experiments were carried out by means of air driver. The dynamic experimental studies on the buckling of the perfect columns with two kinds of bearing conditions and with three kinds of length under axial impact were completed. The time-history curves of strain on two symmetric surfaces of columns were recorded under different velocity. The relation curves between impact buckling load and buckling length under different bearing conditions are obtained from the amplitude and bifurcation time of strain time-history.2. The experimental results show that the constraint of wave front can be regarded as fixed constraint, and the constraint conditions at the impact end is very important when the dynamic buckling induced by stress wave is discussed. The lowest buckling mode are only recorded in our experiment, corresponding buckling load is distinctly greater than the static one under the same boundary condition and same length. The dynamic critical load is about 2.23 and 2.49 times of the static one in condition of movable clamped and hinged, and the ratio of dynamic load to static one has increasing trend as the impact velocity increases.3. The constraint conditions of wave front and the equation of lateral domination on the columns subjected to axial step pressure load are derived by using Hamilton principle. The qualitative analyzing buckling dominating equation shows that the scope of dynamic parameter sets the characteristics of the solutions, and the solutions stand for buckling motion when X < 0. The dynamic buckling load and buckling mode that the impact end is clamp and movable hinge are obtained by solving twin-parameter equation with direct-spread method. The dynamic buckling load is basically consistent with the one obtained by the experiment.4. The whole traveling process of elastic and elastic-plastic wave under impact processing was analyzed by characteristics method. The regularity of stress changes at both column ends and the first separating time of the rigid body and the columns were obtained.5. Using the energy principles and taking into account of the propagation and reflection of stress wave, the lateral disturbance equation on elastic and elastic-plastic columns was derived and power series solution was given. The critical buckling condition can be obtained from stability analysis of the solution. By numerical computation and analysis, the relationship among critical velocity and impact mass, hardening modulus, buckling time was given.6. Considering axial inertia, lateral inertia and the non-linearity of axial strain, the dynamic post-buckling dominating equations are derived. The post-buckling behavior of semi-infinite columns is analyzed by using finite difference method integrating non-linear equations with post-buckling initial conditions, which are characteristics parameter of linear bifurcation, and the load forms and the constrains conditions at impact end and so on influencing on post-buckling are discussed. The results show that the initial buckling mode grows into a series of high-order modes with the increase of the time, and the wave number of mode increases and the amplitude of mode continuously become greater in post-buckling period.