STRESS WAVE PROPAGATION AND DYNAMIC· FAILURE OF STRUCTURES UNDER IMPACT LOADS

Abstract

Structures are dynamic systems since any load applied to them is actually a dynamic load. A transient dynamic response results even when an apparently static load is applied. This transient response is washed out by structural damping leading to a steady state response. Under step loads the steady state response is the static structural response under constant load. The resultant dynamic stresses may be instantly higher than the steady state response, thus dynamic stresses may lead-to a dynamic failure of the structure which is not predicted by statical analysis . This thesis aims at studying dynamic response and failure of nonlinear, damped and un-damped truss structures under impact loads. Material nonlinearity with different' stress-strain paths for loading and unloading is considered. Dynamic structural response is calculated under constant, harmonic and random loads. A. stiffness-version finite element (FE) formulation for the truss problem in the frequency domain is developed. The method is applied to calculate stress and deformation wave propagation, and the response of elastic and elastic-plastic simple trusses. Calculations show that the dynamic response overshoot may be much higher than the static response. This may lead to dynamic failure not predicted by statical analysis. The frequency domain FE formulation has an advantage over time domain formulations in calculating the response to random loads. The reason is that both the random input load and output response are represented by power- spectral density (PSD) functions. Direct integration of the PSD response function provides the root mean square of the deformation and stress response.