A crack localization system based on the spatio-temporal gradient method

Abstract

For several decades, nondestructive tests, including x rays, gamma rays, radar, infrared thermography, and acoustic methods have been used not only for concrete structures but for other types of materials. Acoustic methods are the oldest and most widely used nondestructive testing. They are based on the propagation, and in some cases reflection, of elastic waves in solids or over their boundary surfaces. A well-known example is striking an object with a hammer and listening to variations in the ‘‘ringing’’ sound to detect the presence of internal voids, external cracks, or other defects. Usually, there are three techniques based on acoustic nondestructive tests sorted into the wave-velocity method, resonance methods, and echo methods. This paper proposes a fourth method with spatio-temporal analysis, which has two processes, as follows. In the observation, the resulting surface wave, its partial derivatives for x, y, and t, are calculated at several points. In the detection, the rank of the short-duration covariance matrix of time-series vector f [=(f fx fy ft)T] classifies the situation of the surface-wave field. Consequently, the proposed method can provide the wave front and the location of the cracks as the singularity in the sound field.