Concept Article: A Novel Compact Millipede Bar Waveguide for Propagation of Longitudinal Stress Waves

A novel concept called 'millipede bar' is proposed for efficient propagation of long-duration longitudinal stress waves in a slender rod of smaller footprint compared to a straight bar of equivalent acoustic length. This millipede bar concept consists of several rod segments of rectangular cross section joined at alternate ends with 180(degrees) bend junctions in a serpentine fashion. Analytical and numerical analyses of wave propagation through this bend junction reveal that when the input pulse duration is significantly longer than the time required for the wave to transmit the bend-dimension, the longitudinal wave emerges undistorted on the other side of the bend junction but with opposite sign, similar to the wave reflection at the free end of a long straight rod. Hence multiple 180(degrees) bend junctions can be used to design a millipede bar in a smaller footprint and propagate a long duration stress pulse undistorted over a long distance. However, the assumption of axial motion of the rigid junction in the analytical and numerical models is impractical to implement experimentally. Therefore, an innovative design is proposed that orients alternate bend junctions in two orthogonal planes, thus providing room for imposing roller supports at each bend junction for its axial motion. Finally, the implications of the millipede bar for experimental mechanics, especially for the design of compact split Hopkinson pressure bars, are highlighted.