Detailed validation of a non-linear finite element code using dynamic axial crushing of a square tube

Abstract

A non-linear finite element (FE) code was validated using the results from a series of drop-hammer experiments where uniform square section mild steel tubular specimens were struck axially by a known mass travelling at a known pre-impact velocity. The specimens collapsed in a uniform progressive mode for which analytical formulae were given in the literature. Therefore FE results not generally available experimentally, such as average strain, could be compared with available estimates. An isotropic elastic, linear strain-hardening material model was used in the FE analysis and the strain-rate sensitive nature of mild steel was modelled using the Cowper-Symonds constitutive equation with modified coefficients. After comparing the FE predictions of drop-hammer velocity history, mean force and specimen crush to the experimental results, a more detailed investigation was made into the energy absorbed by a basic collapse element (BCE). Existing expressions for the dimensions of a BCE, its energy absorption capacity and the final average strain were compared to the FE predictions. An estimate of the average strain-rate using the FE results was also compared to a previously available estimate. The FE analysis results were in good agreement with the experimental data and the BCE energy absorption capacity and average strain predictions were in acceptable agreement with available estimates. However, the average strain-rate in the FE specimen was much lower than that predicted by the previously available estimate.