Frequency sensitive reflection, refraction, and transmission of bulk and shear waves in functionally graded materials

Functionally graded materials (FGMs) are defined as materials featuring engineered gradual spatial transitions in microstructure and/or composition thus having gradually varying mechanical properties. A rarely treated topic is the elastodynamic wave propagation in FGMs. It is of particular interest since the reflection, refraction, and transmission of mechanical waves is frequency dependent provided that the spatial area in which the material properties vary is in the order of the mechanical wave lengths to be distinguished. This opens a wide field of potential engineering applications like micromechanical frequency filters, spectrum analyzers, or acoustic isolation layers. Frequency sensitive elastodynamic wave propagation phenomena are demonstrated, based on 2D numerical simulations and on a series of short-pulse-laser-acoustic experiments. There, the frequency dependent reflection and transmission behavior caused by intermetallic interface layers of 10 to 20 nm thickness has been demonstrated, which corresponds to a micromechanical filter operating in the frequency range of 0.5 THz.