Crash performance of cellular foams with reduced relative density part 2: rib deletion

Abstract

The finite element method has been used to investigate the crashworthiness of regular conventional and re-entrant cellular foams in pristine and defect forms. Defects were introduced by randomly deleting vertical ribs (cell walls), diagonal ribs and vertical-plus-diagonal ribs from the honeycomb models, which with suitable boundary conditions approximated to three-dimensional foams. Generally, the calculations show that deleting ribs leads to a reduction in magnitude of the Young's moduli and Poisson's ratios, although there are some notable exceptions. In particular, the Young's modulus and Poisson's ratio along the x-axis (perpendicular to the vertical ribs) actually increase in magnitude slightly when vertical ribs are deleted. In the case of the re-entrant honeycomb, a transition from auxetic (negative Poisson's ratio) to non-auxetic (positive Poisson's ratio) behaviour occurs at a critical threshold for diagonal rib deletion. For the conventional honeycomb, the Poisson's ratio for loading in the y-direction remains approximately constant when diagonal ribs are deleted.

Key words:

• Honeycomb
• defects
• FEM

ACKNOWLEDGEMENTS

The authors gratefully acknowledge financial support for part of this work from the ESF (JPMW – MSc studentship & Software Design Skills) and EPSRC (JPMW – PhD studentship, award number 99317310).