Optical NDT of delaminated plates under shear wave excitation

Abstract

Adhesive bonding has been accepted as an important process in the manufacturing and repair industries. However, the tendency of delamination at the bonded region due to improper bonding procedures, and unfavorable loading conditions and service environment has necessitated development of reliable inspection techniques for ensuring the structural integrity of these bonded structures. Optical inspection techniques have, over the last three decades since the invention of the laser, gained wide acceptance because they are non-destructive and non-contacting, and accurate measurements in the order of light-wavelengths may be obtained very rapidly. Optical flaw-detection generally requires the application of a load-increment on the structure, the response of the illuminated surface of the structure is then compared with that of a non-defective structure. The load-increments used are in various forms, such as vacuum stressing, heating, static mechanical loads, and steady-state mechanical excitations. This paper describes an optical NDT method for detecting delamination of a thin, small-sized adherend that is adhesive-bonded to a large metal sheet (substrate). The load-increment applied during testing is due to shear wave generated by piezo-electric sensors that are bonded on the substrate at various locations near the adherend. By changing the voltage applied to these sensors, different excitation frequencies and, hence, different shear waves, are generated. Measurements using a scanning laser vibrometer show a change in the velocity responses at the delaminated area when compared with a non-delaminated bonding. Results have also demonstrated that the location and size of the delamination may be determined using this technique.