Miura-ori Based Mechanical Metamaterials With Graded Stiffness

Mechanical metamaterial is a kind of man-made materials whose mechanical properties are mainly defined by their structures.Origami inspired metamaterials have attracted great attention from physicists to engineers,due to their exotic behaviors such as negative Poisson's ratio and programmable stiffness.Currently,most origami metamaterials make up of identical unit cells and thus show uniform mechanical behavior,which makes it difficult for them to adapt to non-uniform environments.Inspired by the functionally graded structures in nature,here we propose three new Miura-ori based mechanical metamaterials,referred to as in-plane graded Miura-ori,out-of-plane graded Miura-ori and in-plane & out-of-plane graded Miura-ori which have varying geometry over their volume and graded stiffness.First of all,the geometric conditions of the graded Miura-ori are set up through kinematic analysis.The folding process of the graded Miura-ori is also studied.It is found that one or two self-locking points exist determined by the geometry totally,which divides the folding process into a rigid folding stage and a non-rigid one.Secondly,physical samples of Miura-ori and graded Mirua-ori are fabricated by brass(CuZn40).Moreover,quasi-static compression tests in a certain direction are conducted to investigate the mechanical properties of the two new designs,and good match results from corresponding simulations confirm the experiments.Finally,a parametric study is carried out to study the effect of geometric parameters on the mechanical behaviours of the graded Miura-ori metamaterial,through a finite element modelling approach.The results show that the sector angle plays a major role in determining the graded stiffness and energy absorption capability.In conclusion,we demonstrate both experimentally and numerically that graded Miura-ori can generate up to graded stiffness,which is tunable through geometric parameters.Moreover,the graded Miura-ori has superior energy absorption capability to the uniform Miura-ori.Our work provides a new avenue in the design of origami structures and mechanical metamaterials that are tunable and adaptable to external loads.