Guided waves propagation in a rod surrounded by an infinite solid medium is an interesting research topic. One important application of this topic is ultrasonic non-destructive testing for the rockbolts. In engineering constructions, numerous rockbolts, which are long steel rods, are inserted into the groundwork to prevent the slippage of the rock and to ensure the stability of the groundwork. There are thousands upon thousands rockbolts at the important building sites.

An essential problem is how to use the elastic waves propagated in the steel rockbolt to estimate the bonding state between the rockbolt and the surrounding rock. Many researchers do lots of studies on this issue. However, these studies are all focused on the dispersion characteristics of the guided waves without considering the excitation characteristics of the guided waves. So CUI Hanyin, ZHANG Bixing and JI Shunxin of Institute of Acoustics, Chinese Academy of Sciences carried out a series of studies and investigated the dispersion and excitation characteristics of the guided waves in a rod surrounded by an infinite solid medium (cladding).

In the study, the researchers employ the bisection technique to find all the roots of the dispersion function on the basis of theoretical analysis and to obtain the complex phase and group velocity dispersion curves of the guided modes. Then they divide the guided modes into two categories: normal modes and Stoneley modes according to the different dispersion characteristics. Through investigation, they get the conclusion that the normal modes merely exist in the "hard cladding" model in which the cladding's shear velocity is larger than the rod's; while the Stoneley modes in cylindrical interface are highly dispersive and merely exist in the model whose acoustical parameters satisfied the existence condition of the Stoneley waves. Finally, the researchers simulate the excitation mechanisms of the guided waves, excited by three source models: symmetric point source, axial and radial force sources, respectively. And Attention is paid on the dominant mode which has better excitation sensitivity and the suitable excitation frequency range. Moreover, the researchers analyze and compare the propagation characteristics of the Stoneley modes with those of the normal modes.

This research result was published on the recently issued journal of Acoustical Physics (2010, Vol. 56, No. 4, pp. 412–421).