1. Introduction

Thin-walled structures are widely used as kinetic energy absorbers for their low cost, light weight and high energy absorbing capacity. The UltraSTEEL dimpling process, developed by Hadley Industries plc, is an industrial manufacturing process that cold-rolled forms a plain steel strip into a dimpled strip. Quasi-static tensile test results showed that the UltraSTEEL process can increase the yield and ultimate strengths of the plain steel material up to 51% and 34%, respectively. It has been reported that energy absorption capability increased continuously with yield strength.

To understand the energy absorption characteristics of UltraSTEEL structures when subjected to dynamic axial crushing loads, explicit finite element simulations were conducted. Failure modes and specific energy absorption (SEA) of plain and UltraSTEEL columns were compared. Effect of the dimpled geometry and solidness ratio were also analysed through parametric studies.

2. Numerical Modelling

Explicit dynamics method was selected to complete the numerical simulations. The closed square hollow cross section was adopted in this study. The main features of finite element models used were:

• Shell element with a uniform element size of 0.55mm was selected, based on the geometry and a mesh density convergence study.
• Cowper-Symonds material model was employed, since the column crushing is a non-linear dynamic process and the material is strain rate sensitive.
• The finite element model was simplified by applying symmetric boundary conditions, and the rationality was justified.
• Triggers were introduced to initiate the progressive axial crushing, and the effect of the introduced triggers was evaluated.

3. Results

UltraSTEEL and plain columns have a similar failure mode when subject to axial crushing loads. The specific energy absorption of 1mm gauge UltraSTEEL column was found to be 12.4% higher than the same gauge plain column. The influence of the increased yield strength and dimpled geometry are studied separately.

Parametric study has shown that the variation of SEA with respect to thickness of UltraSTEEL columns is different from conventional plain columns, because the distance between two layers of lobes is highly dependent on the dimpled geometry. Advantage in SEA of UltraSTEEL columns becomes more significant as the thickness decreasing.