Ultrasonic Evaluation Using Higher Order Lamb Wave Modes

Long-ranged guided wave inspection systems are often used in a low frequency–thickness range below the cut-off frequency of the higher wave modes to simplify data interpretation. In this paper, the potentials of ultrasonic high order guided wave modes are studied in the inspection of plate structures. The wave propagation characteristics of high order wave modes and the corresponding sensitivity and detectability are investigated. Using the commercially available software ABAQUS, finite element simulations are carried out to model the ultrasonic wedge transducer and the inspection system for simulation of A1 and S1 wave mode distribution. Using a variable angle transducer, experimental pulse–echo technique is employed to evaluate the measurement relative errors obtained from two high order wave modes, A1 and S1, for a 2 mm steel plate at 4 MHz.mm of frequency–thickness. The experimental measurements show that for various transducer positions, the relative errors obtained by the S1 mode are less than 10% for short-range measurements.