Novel Self-Strengthening Metamaterials via Additive Manufacturing

Safety-critical structural applications require materials with high toughness along with fracture strength. However, currently available tough materials often exhibit reduced mechanical strength under high strain applications. While recent developments in metamaterials can be attributed to the ability to produce complex structures via additive manufacturing, there is a lack of understanding in the design, mechanics, and fabrication of self-strengthening metamaterials. This original work introduces the novel concept of self-strengthening closed-cell metamaterials with unique nested strengthening elements that are engaged under extreme loading conditions. The internal strengthening elements engage with the outer structural elements under high strain thereby strengthening the structure in the direction of extreme strain and alleviating the stress in the outer structure. This engagement couples two critical mechanical properties which are otherwise mutually exclusive namely; quasi-static strength and toughness. Strength is the ability to resist permanent deformation (yield) while toughness is the ability to resist fracture under high strain. Under high strain, the engagement of internal self-strengthening elements in the metamaterial makes it stronger with higher ductility. This reduces the possibility of crack initiation in the outer structural elements which often lead to failure. In this paper, two- dimensional square and hexagonal self-strengthening unit cells are introduced. Unit cells were simulated using Abaqus to capture the geometric details of the strengthening elements. The simulations demonstrate the engagement of self-strengthening members under compression to study their effect on stress development. The unit cells were compared with unit cells without strengthening. The rate of stress development in the outer members was reduced by 69.45% in the square unit cell and 56.75% in the hexagonal unit cell. This damage-tolerant metamaterial design can be potentially used for safety-critical applications that require higher strength such as aerospace and biomedical applications.