Mode selection of guided waves for ultrasonic inspection of gas pipelines with thick coating

Mode selection of guided waves in an elastic steel plate with thick coating layer has been studied here. Effect of both thick elastic coating and damp ing of the coating layer on the dispersion and mode shapes has been investi gated and modes that are less affected by elastic coating and damping have been identified. For modeling the viscoelastic behavior of the coating laye r, the standard linear solid is adopted and two damping factors are introdu ced into the Lame constants. Viscoelastic property of the coating layer cau ses all wave numbers to be complex. In order to find the complex wave numbe rs for a given frequency, a Rayleigh-Ritz type method is used to get approx imate Values of the complex wave numbers and then the IMSL subroutine zanly t is employed to refine roots. It is found that a thick elastic coating int roduces new dispersive modes and also, in general, changes considerably the modes of the bare steel plate, and that damping of the coating layer has s ignificant effect of the dispersion and mode shapes. However, we found that there are certain modes of the bare steel plate that are less affected by the thick elastic coating and damping. These modes could be selected for th e ultrasonic inspection of stress-corrosion cracks in thin-walled gas pipel ines with protective coal-tar coating. (C) 1999 Elsevier Science Ltd. All r ights reserved.