The Effect of Waveguide Material and Shape on Ae Transmission Characteristics

Acoustic emission testing (AET) often utilises waveguides to separate delicate sensors from hot surfaces or to facilitate access to otherwise obstructed materials such as lagged vessels. In the authors’ work, restricted access to mechanical seals on a hot water pump necessitated the use of a short rod to transmit acoustic emission (AE) information from the ceramic seal face. Consequently, an appreciation was required of how this waveguide altered the seal’s AE signals. This paper presents the effects of varying the material and/or shape on the temporal and frequency characteristics of pulsed acoustic emission events travelling through small cylindrical rods. 36 waveguides, fabricated from either alumina ceramic, mild steel, stainless steel (SS316) or extruded Delrin were tested, of 3 different lengths, two diameters and four sensor face angles. The effect of a point at the source end of the waveguide was also verified. 1 RELEVANT ULTRASONIC THEORY The solutions to the general wave equation for acoustic waves travelling through isotropic cylinders are detailed in references [1]-[3]. However, there are several key points applicable to this research work that will now be summarised. Longitudinal (i.e. compressional) wave modes in a cylinder vary with radial and longitudinal distance along the cylinder but are symmetrical around the circumference. The relationship between phase velocity and frequency is known as the Pochammer-Chree equation and resembles the Lamb dispersion relationship in plates. From this equation, it can be seen that the fundamental mode approaches a constant velocity as k.a approaches zero, where k is the wavenumber and a is the cylinder radius. This limiting velocity corresponds to the longitudinal rod velocity: