  In this paper, we present an analysis of the guided circumferential elastic
waves in soft EA tube actuators, which has potential applications in the
in-situ non-destructive evaluation or online structural health monitoring (SHM)
to detect structural defects or fatigue cracks in soft EA tube actuators and in
the self-sensing of soft EA tube actuators based on the concept of guided
circumferential elastic waves. Both circumferential SH and Lamb-type waves in
an incompressible soft EA cylindrical tube under inhomogeneous biasing fields
are considered. The biasing fields, induced by the application of an electric
voltage difference to the electrodes on the inner and outer cylindrical
surfaces of the EA tube in addition to an axial pre-stretch, are inhomogeneous
in the radial direction. Dorfmann and Ogden's theory of nonlinear
electroelasticity and the associated linear theory for small incremental motion
constitute the basis of our analysis. By means of the state-space formalism for
the incremental wave motion along with the approximate laminate technique,
dispersion relations are derived in a particularly efficient way. For a
neo-Hookean ideal dielectric model, the proposed approach is first validated
numerically. Numerical examples are then given to show that the guided
circumferential wave propagation characteristics are significantly affected by
the inhomogeneous biasing fields and the geometrical parameters. Some
particular phenomena such as the frequency veering and the nonlinear dependence
of the phase velocity on the radial electric voltage are discussed. Our
numerical findings demonstrate that it is feasible to use guided
circumferential elastic waves for the ultrasonic non-destructive online SHM to
detect interior structural defects or fatigue cracks and for the self-sensing
of the actual state of the soft EA tube actuator.
