Digital Shearography

Digital Shearography is a contactless, interferometric, full-field method for the measurement of excitation induced surface deformation. The specimen can be excited for example by applying heat, pressure change or vibrations. The specimen surface is illuminated with coherent laser light. A shearing device is used image the specimen surface twice with a slight offset. Thus two distinct points of the surface are images by one pixel of the CCD camera. This creates a speckle interference pattern, which is then measured while the specimen is in an unexcited state. The excitation, for example thermal by using halogen spots or flash lamps, causes deformations on the specimen surface, which leads to alteration in the pattern (e.g. changes in the phasing of two interfering light beams). By measuring this altered pattern and subtracting the origin pattern from it, the detection of even the smallest deformations is possible.

Schematic setup for digital shearography measurements with thermal excitation. The temperature change leads to different thermal expansion of the CFRP specimen and the steel insert, which can be measured on the specimen surface

Applications

Detection of different bonding conditions:

In this work Digital Shearography was applied to detect and distinguish different bonding conditions between a CFRP skin and the aluminium honeycomb core. The bonding between the skin and the honeycomb core is crucial for the mechanical properties of a structure. Measurements were carried out on a CFRP sandwich part with an aluminium honeycomb core. Mill outs in the core simulate flaws, of which half were left unmodified and the other half filled with foaming adhesive. The air crack simulates a disbond between the skin and the core, whereas the ones with the adhesive are inserted to prove if they are distinguishable from the disbond flaws.

Left: CFRP Sandwich part with an aluminium honeycomb core and mill outs to simulate disbands. Right: Results from Digital Shearography measurements with thermal excitation.

Detection of inclusions of foreign material

In this work Digital Shearography was applied to detect inserts of foreign material e. g. pieces of foil. Measurements were carried out on a CFRP sandwich part with a honeycomb core made of Nomex. Foil pieces of different size and material were embedded either between the second and the third ply of the skin or between the skin and the core to simulate inclusions of foreign material.

LEFT: CFRP sandwich part with honeycomb core made of Nomex and embedded foil pieces to simulate inclusions of foreign material. MIDDLE: Location and size of the embedded foil pieces. RIGHT: Results of Digital Shearography measurements with thermal excitation.