Novel composites with asymmetrical elastic wave properties

The traditional structural composites are known as materials with a spatial order. The temporal order is introduced into structural composites in this paper. We theoretically describe and numerically demonstrate the architecture for a structural composite which exhibits tunable and nearly "full-banded" asymmetrical elastic wave property. Given its spatiotemporal order and unconventional wave properties, such composite can be named as spatiotemporal metamaterial (STMM). Firstly, the effective material parameters, which include some new material parameters, are obtained based on improved multi-scale homogenization techniques. Secondly, the unconventional wave properties are theoretically analyzed. It is found that the elastic wave group velocity surfaces do not satisfy the centrosymmetry in STMM, i.e. the time-reversal symmetry of linear elastic wave is violated. In certain instances, the waves even propagate unidirectionally. Finally, we present an example of the STMMs, which includes homogenized calculations and numerical simulations where the shear waves are forbidden in some directions. The unconventional wave properties of the STMMs proposed in this paper own attractive potential applications, such as acoustic absorbing, acoustic information processing, energy-saving, and energy-harvesting.