“…When the high‐strength steel wire is with a free‐free boundary condition, and its ratio of diameter to length is less than 0.4, the Pochhammer–Chree dispersion equation (originally used to analyze dispersion characteristics of guided wave propagation in a free round rod) is also applicable to calculating the group velocity of the longitudinal guided wave propagated in the high‐strength steel wire. Therefore, when the infinite‐length solid steel wire is at the free boundary, the fluctuation of the longitudinal guided wave satisfies the Pochhammer–Chree dispersion equation 22,25–27 : In formula 1, r 1 is the diameter of the free round steel waveguide rod, k is the wavenumber of the wave, α is the component of the longitudinal wavenumber in the radial direction, β is the component of the transverse wavenumber in the radial direction, J 1 is the first‐order Bessel function of the first order, and J 0 is the first‐order Bessel function of the zero order.…”