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. So far the development of such material systems is driven by the demand to produce objects which are hard and wear resistant at the surface but have a ductile and tough body - like cutting tools - to give all example. Ceramically coated turbine blades might be mentioned as another application where FGMs are used in order to reduce thermally induced mechanical stress concentrations at the ceramic / body interface. A rarely treated topic is the elastodynamic wave propagation ill 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 oil two dimensional numerical simulations and oil 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 a frequency range of 0.5 THz.