Prospects and limitations of eddy current shearography for non-destructive testing of adhesively bonded structural joints

ABSTRACT

Structural adhesive bonding has become increasingly important for the joining of dissimilar materials as well as for the reduction of weight of joined structures. To ensure the quality of these bonds, non-destructive testing (NDT) such as ultrasonic testing (UT) or radiographic testing (RT) is used. However, both testing methods have the disadvantage that the measurement takes too long to be used for mass productions.

Shearography is an optical method with suitable capability for automated NDT of adhesively bonded joints. It is a contact-free method with an extensive measuring field. For the measurement, an object is illuminated with coherent light. The reflection on the surface causes a speckle pattern on a detector. During the measurement, the object is stressed with the help of thermal or mechanical excitation (e.g. flashlight, underpressure). Changes in deformation of the object’s surface can be calculated by recording two stress scenarios. Flaws in the bonding area cause a local change of deformation and can be detected with shearography.

This paper presents the prospects and limitations of shearo-graphy for adhesively bonded structural joints. The stress in the specimen was achieved by thermal expansion caused by eddy currents. The evaluation included similar and dissimilar structural bonds of metal (e.g. deep drawing steel, aluminum) and CFRP substrates. The target of the examination is the characterization of the detectability of different bonding flaws such as missing adhesive, voids within the adhesive and kissing bonds.