Identification of physical effects in flying spot photothermal non-destructive testing

Abstract

Among the existing non-destructive testing techniques, the flying spot photothermal technique [CITE] appears to be a good alternative to penetrant testing for open-cracks detection in metallic structures. The technique basically consists in the local heating of a structure under test and the measurement of the temperature elevation which depends on the inspected medium properties. During the scanning of the structure, thermal and optical effects contribute to the elaboration of the photothermal signals. The thermal effect is relative to the presence of a thermal barrier in the inspected medium (e.g. crack) and the optical effects are due to the variations of the absorptivity and diffusivity factors of the inspected surface (surface conditions). Both effects are competitive and can lead to hardly interpretable signals. In this paper, the authors present a method allowing to separately identify thermal and optical effects, in order to enhance the characterization of the structure under test. The method was applied on a mockup featuring open-cracks and rough surface conditions, and allows to construct separate optical and thermal images. The resulting so-called thermal image allows to significantly highlight the presence of the cracks. Besides, the optical image enriches the characterization of the structure, since it supplements the diagnosis of the cracks and also enables to characterize other defects such as surface topology and features.