Shearographic inspections by an interferoscope

Abstract

This paper explains the procedure for analysing surface strains and the detection of near-surface defects by means of a novel measurement device. The measurement instrument combines the shearographic measurement principle with an endoscope. In the field of non-destructive testing, the optical measurement technique provides a possibility for the detection of near-surface defects. As a full-field, noncontact, coherent-optical and non-destructive testing and measuring method on surfaces, electronic shearography meets those requirements. Irregularities on body surfaces can be recorded up to a dimension that is within the range of the used laser light wave-length. With shearography, objects as well as whole structures can be checked independently of their material. Shearography measures the strains as well as the gradients of the deformation in the given direction of image-shearing. From the quantitative strain measurement, stresses in the specimen surface can be observed. By a qualitative evaluation of the shearogram, strain concentration and thus imperfections are located. Through the endoscope, the measurement method of shearography is expanded equipment-wise. In this way, shearography enables examinations of cavities in technical units through the smallest holes. The shearographic-imaging endoscope is the Interferoscope. The Interferoscope is used, where conventional shearography fails as the examined location is difficult to access. With the Interferoscope, examinations of objects are just as well possible under application of dynamic excitation. This brings up the advantage, that also deep-seated imperfections are noticeable on the surface and are thus detectable. Through the dynamic measurement excitation the detection of object specific eigenfrequencies becomes as well possible.