Effects of cell size vs. cell-wall thickness gradients on compressive behavior of additively manufactured foams

Graded foams show great potential in impact protection and blast resistance applications but a limited experimental study on their compressive behavior has been reported. Thus, this paper investigates the gradient effect on the compressive behavior of foams experimentally and numerically. The cell size graded foam (SGF) and the cell-wall thickness graded foam (TGF) are both built by the Voronoi method and then fabricated by the additive manufacturing technique. Meanwhile, uniform foams with different cell sizes (SUF) and cell-wall thicknesses (TUF) are also produced to be compared with graded foams. Quasi-static and dynamic compressive tests are conducted respectively by using a universal testing device and a direct-impact Hopkinson pressure bar. Experimental results reveal that SGFs deform continuously from lower to higher density regions and hence possess a gradually increasing plateau stage in the stress-strain curve. TGFs show three stepwise plateau stages because of their three layers with different cell-wall thicknesses. Moreover, SUFs and TUFs with constant relative density present similar mechanical properties despite their different cell morphologies. Several empirical formulae are proposed for uniform foams and fit well with experimental data. Further simulation is verified by experimental results and indicates that TGFs with adequate layers also possess a gradually increasing plateau stage just like SGFs. It means that the strength of each layer in a graded foam depends on its local density rather than cell morphology.