This paper presents a novel tri-directional negative stiffness (NS) mechanical metamaterial, consisting of disk structure elements arranged in three-dimensions. The NS behavior was achieved through the elastic instability of the disk structure. Through the combination of numerical simulations and experiments, the NS property, the deformation mechanisms, and the basic mechanical performance of the disk structure and the novel NS mechanical metamaterial were systematically investigated. The influence of the geometric parameters on the snap-through behaviors of the NS metamaterial was investigated through the experimentally verified numerical method, and further designs were conducted on the disk structure to tailor its mechanical responses. Also based on the instability of the disk structure, several novel multidirectional NS structures, which can achieve NS behavior along five and seven loading directions, were firstly proposed. This innovation improves the homogeneity of the NS metamaterials, and is expected to expand the application scope of such metamaterial, considering that the previous reported NS metamaterial can exhibit NS behavior along three-dimensions at most. Moreover, the mechanical performance of the proposed NS metamaterial is tunable, benefited from the special structure form of itself. To our knowledge, this work is first to design NS metamaterial using the disk structures, and report the multidirectional (more than tri-directions) NS metamaterials.

Metamaterials have thrived in recent years, with negative stiffness mechanical metamaterials (NSMMS) emerging as an essential branch. Extensive research in the past decade has yielded fruitful results in this field. This work encompasses an in-depth exploration of the origin of negative stiffness behavior, along with detailed demonstrations of the implementation mechanism and construction methods used in NSMMS. Furthermore, the paper highlights the diverse range of applications for these metamaterials, including energy absorption, advanced actuators, deployable and morphing structures, vibration control, and more. Lastly, a brief glimpse into the future development direction of this metamaterial is proposed.