Molecular dynamics study on fracture mechanism of functionally graded multilayer graphene origami under nanoindentation

• 3-dimensional (3D) multilayer graphene origami structure is achieved through pattern-based layer-wise hydrogenation. • Novel functionally graded multilayer graphene origami (FG-MGOri) structure is developed. • Fracture mechanism of 3D FG-MGOri under nanoindentation is studied by molecular dynamics (MD) simulations. • 3D FG-MGOri exhibits superior flexibility with pattern O offering the best fracture resistance. Owing to its outstanding mechanical properties, graphene has been widely used as an ideal reinforcement material in nanocomposites for various engineering structures. However, its practical application is significantly hindered by its inherent brittleness, i.e., it breaks at a low strain level under external loading. To address this formidable limitation, we leverage hydrogen functionalization and origami engineering to develop new graphene origami (GOri) and on top of that design innovative functionally graded multilayer graphene origami (FG-MGOri) structures with hydrogenation degree changing gradually in a layer-wise manner. Molecular dynamics simulation results on nanoindentation demonstrate that the transformation of two-dimensional flat graphene to three-dimensional GOri through hydrogen functionalization significantly enhances graphene’s flexibility, albeit with a trade-off in strength. However, this flexibility-strength trade-off can be effectively mitigated by using FG-MGOri. Our findings reveal that FG-MGOri with the O pattern (highest hydrogen atom in the middle and lowest hydrogen atom in the top and bottom) exhibits a remarkable 79.7% improvement in indentation depth compared to pristine multilayer graphene while maintains its high failure load. It is also found that the maximum indentation depth at the final failure of FG-MGOri increases as the total number of layers increases but the indentation load is rather insensitive. The present study reveals distinct nanoindentation features of FG-MGOri and provides valuable insights for the design of novel multilayer GOri structures with great potential applications in advanced materials and composites.