Local Defect Resonance-based Shearography to Increase the Selectivity of Defects

Abstract. The optically excited lock-in shearography (OLS) is an optical method for remote non-destructive testing by monitoring the displacement field of the object to be inspected. OLS is designed for quick and one-sided non-destructive testing of large components. The surface displacement is measured by means of an expanded laser beam. For non-destructive testing, usually not the displacement of the whole inspected object is of interest, but only the changes in the displacement field that are caused by defects. To determine the depth of defects we combined it, in the past, with the lockin technique where modulated excitation generates a thermal wave in the sample which is monitored by a shearography sensor. By using the optical excitation, in some cases, the defects signals are superimposed by the large displacement fields. Additionally, conventional application is often too long which makes the new approach with measuring times of about a few seconds a promising alternative. Thus, the elimination of these drawbacks will be helpful to broaden the range of applications of shearography. Non-destructive testing is an important part of the whole quality strategy of manufacturing and operation of CFRP components. However, routine application is often too long with regard to industrial inspection requirements. One possible solution to this problem is local defect resonance (LDR), a recently proposed method, which is combined in this paper with shearography. The LDR-based shearography (LDRS) measurements are performed within a few seconds. In practical use, the component is excited at frequencies in the range of 5- 50 kHz and the component surface observed as live video. We report on achievable detection sensitivities when using the LDR and present testing results of different types of CFRP-components