Reprogramming rate-dependent stress-strain curves of mechanical metamaterials by “stair-building” strategy

Metamaterials programmed with target rate-dependent mechanical properties are efficient platforms for realizing advanced functionalities. Yet, the loading rate-dependent mechanical property programming has received limited attention. Here, the “stair-building” strategy is employed in the rate domain by combining the bistability with viscoelasticity. An arbitrary target curve in the programmable space can be approximated by a “stair” built by two kinds of “bricks”. The “bricks” can be realized by a dual-bistable unit, constructed by two bistable structures in series. The dual-bistable unit can switch between two efficient stable phases without inducing changes in the global morphology. Such a unit exhibits N-shaped stress-strain curves at both efficient stable phases with different peak values, resulting in different heights of “bricks”. Moreover, the N-shaped curves have rate-dependent peak values, indicating that the heights of “bricks” change with loading rate. The “stair-building” strategy is realized by array-structured mechanical metamaterials based on dual-bistable units. Different stress-strain curves under various loading rates can be reprogrammed in the same piece of metamaterial by intentionally selecting the efficient stable phases of units. Besides, the rate effect of the metamaterial can also be tuned by reprogramming stress-strain curves under both low and high loading rates, respectively. This reprogrammable metamaterial is promising in smart vibration isolators and adaptive energy absorbers.