Experimental evidence of absolute bandgaps in phononic crystal pipes

The vibration filtering properties of a phononic crystal pipe whose unit cell consists of two segments of different materials and cross sections are studied numerically and experimentally. Such an architected bi-material pipe leads to the alignment of the dispersion branches in the same frequency ranges for all types of waves (flexural, longitudinal, and torsional), leading to an absolute bandgap. Each motion is studied by a 1D model in which the propagation of Floquet-Bloch waves in lossy media is considered. Numerical optimization is based on the simplex algorithm and aims to control both the central frequency and the bandwidth of the absolute bandgap on a selected target. Experimental characterization of a demonstrator confirms the filtering effects due to partial and absolute bandgaps even in the presence of quite high structural damping.