Author(s)

V. Sukackas

Abstract

A Lamb wave excited on one point of a hollow cylindrical point reaches every other point in different ways, such as: by the shortest way and turning around 1, 2, 3… times. This factor can be used to control the wave field in the space, as it is impossible in the other ways when the source is a point. The waves can be focused if the emitter is excited by a series of short pulses calculated so that at the same time the pulse that has spun the cylinder n times is received, the second pulse with n-1 rotation around the cylinder and so on, and finally the last pulse that came by the shortest way. The focus point can be changed, i.e. scanned when the pulse’s position in the series is changed. The only one transducer becomes equivalent to the array that has the period equal to the perimeter of the cylinder. In the same way the signal in the receiving point can be processed by delaying and summing it so that constructive superposition occurred only when the signal comes from the desirable point. In this way the dynamic focusing and scanning can be performed irradiating the object or receiving the waves propagated by some point. In the latter case this can be the point of the passive reflecting defect in pulse-echo NDT mode or the source of the acoustic emission. The scanner is very simple – it consists of one point-form transducer. In the same way the more complicated and typical only to cylinder modes can be excited. Keywords: Lamb waves, cylinder, scanner, dynamic focusing, virtual array. 1 Introduction Lamb waves with the propagating medium restricted by two surfaces perfectly suit for the testing of hollow cylinder type objects, such as pipes (Alleyne et al [1] and Alleyne and Cawley [2]) and their inner surface (Volkovas and Sukackas [3]). Three exciting modes can be distinguished there:

Keywords

Lamb waves, cylinder, scanner, dynamic focusing, virtual array.