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.