Lamb wave sensors for detecting wall defects in pipes

A number of sensors have been designed and fabricated for pipeline inspection. These sensors require commercially available ultrasonic transducers but use a new coupling mechanism. Some of these sensors require the presence of a coupling fluid between the ultrasonic transducer and the pipe. These sensors can inspect horizontal pipes. Other sensors use solid coupling medium and can be used equally well for inspecting horizontal, vertical or inclined pipes. Our objective is to generate different modes of the cylindrical guided waves in pipes. The new coupling mechanisms have been designed to generate different axisymmetric and non-axisymmetric Lamb modes in the pipe by changing the excitation frequency, inclination angle and number of transmitters. These Lamb wave sensors have been used to inspect a number of defect-free and defective pipes. The preliminary results show that these sensors can detect the pipe defects even when the defects are located at a distance. A comparison between the degree of damage and the Lamb wave generated received signal shows that a correlation exists between the two. Hence, this technique can be used for detecting the pipe damage as well as for predicting its degree of damage. Experimental results are presented to justify this claim.