Out-of-plane crashworthiness of hierarchical cellular topology with different wall thicknesses in hierarchies

Hierarchical honeycomb is a topology presenting better weight-efficiency than the conventional honeycomb. However, the existing research are mainly conducted based on an assumption of uniform in-plane wall thickness. Rare studies consider the effect of different wall thicknesses in hierarchies. This paper investigates the out-of-plane crashworthiness of vertex hexagonal-based hierarchical honeycombs with non-uniform wall thickness in distinct hierarchies experimentally and theoretically. The coupons with parent material of 316L steel are obtained using Selective Laser Melting fabricating technique. The experimental results indicate that the first order honeycombs with uniform wall thickness experience a failure mechanism transition from local elastic buckling to local plastic buckling of cell walls at the critical density of 0.0772. A progressive folding wave can be identified when relative density is lower than 0.0386. At any edge length ratio, the plateau crushing stress increases monotonously as the increase of the wall thickness ratio, but not for the half wavelength. Both the half wavelength and maximum plateau crushing stresses are linearly related to relative density. For the first order honeycombs, the effect of edge length ratio is more considerable on the plateau crushing stress than the wall thickness ratio. The second order honeycombs exhibit higher half wavelengths and maximum plateau crushing stresses than the first order honeycombs owing to the more considerable cell wall constraint among the hierarchies. Compared to the hierarchical honeycombs with uniform wall thicknesses at relative densities of 0.005∼0.0386, the non-uniform wall thickness enhances the maximum plateau crushing stress significantly, especially at a low relative density, and the maximum improvements for first order and second order honeycombs are 71.5 and 48.6%, respectively. However, the absolute improvements are similar, averaging approximately 1.18 MPa. This work provides a foundation for developing ultralight hierarchical material candidates applied in passive protection equipment, such as aircrafts and vehicles.