Acoustic backscattering enhancements resulting from the interaction of an obliquely incident plane wave with an infinite cylinder

Background and objectiveThe analysis of the acoustic backscattering enhancements from tilted cylinders is of particular importance in determining some of the (visco)elastic properties of the cylinder, and/or its surrounding fluid in ultrasonic non-destructive evaluation (NDE) and imaging (NDI) applications. Previous related investigations on an aluminum cylinder limited to incidence angles varying from 0° to 40°, revealed the existence of an anomalous “pseudo-Rayleigh” mode (above the critical Rayleigh angle) identified as the rigid-body translational dipole (n = 1) mode. The objective here is to provide a complete investigation on the backscattering enhancements for incidence angles larger than 40° for various elastic and viscoelastic cylinder materials.MethodUsing the partial-wave series solution for the linear scattering by an infinite circular cylinder, the acoustic backscattering from isotropic elastic and viscoelastic (polymer-type) cylinders excited by an obliquely incident plane acoustic wave is investigated. Total and resonance backscattering form functions are calculated for several elastic and viscoelastic cylinder materials immersed in water versus the angle of incidence 0° ⩽ α < 90°. The “pure” resonance peaks are isolated by subtracting a rigid background from the total form function, so the associated resonance modes are properly identified.Results and conclusionThe plots of the partial-wave series reveal acoustic backscattering enhancements (not shown in previous investigations) generally occurring at ka  ≲≲ 0.1 at a critical angle αc   bounded by the longitudinal and shear waves coupling angles θL=sin-1(c/cL)θL=sin-1(c/cL) and θS=sin-1(c/cS)θS=sin-1(c/cS) such that θL<αc<θSθL<αc<θS (where cLcL and cScS are the phase velocities of the longitudinal and shear waves inside the elastic cylinder, and c   is the speed of sound in the surrounding medium). It is shown here that the backscattering enhancements with a critical angle θL<αc<θSθL<αc<θS result from the excitation of the monopole (n = 0) resonance mode. Moreover, additional acoustic backscattering enhancements still occur in the range 1 ≲≲ka  ≲≲ 6 even though the angle of tilt is greater than the Rayleigh wave coupling angle θR=sin-1(c/cR)θR=sin-1(c/cR) (where cRcR is the Rayleigh wave velocity in an elastic half-space). The resonance scattering theory shows that such additional enhancements are associated with the excitation of a dipole (n = 1) resonance mode which may result from the interference of meridional and/or helical waves propagating along the cylinder’s surface. It is therefore essential to consider tilt angles ranging from normal to end-on incidence for a complete analysis of the backscattering by elastic and viscoelastic cylinders.