Inspection of Laser Generated Lamb Waves using Shearographic Interferometry

The implementation of structural health monitoring systems in modern aircraft structures made of carbon fibre reinforced plastics requires a deeper understanding of the propagation of impact generated Lamb waves. In this paper a digital shearing interferometry method is presented that allows visualization of laser generated Lamb waves. The contactless full field measurement of these waves is performed by a Mach-Zehnder interferometer which combines spatial phase shifting technique and shearography. The latter is a laser-based coherent speckle-technique whereby the first order derivative of the displacement is recorded. The recording of the laser generated propagating wave field at predetermined times requires a stable experimental arrangement with high accuracy. We present here an appropriate interferometric setup that allows imaging out-of-plane surface displacements and in which the wave field is reconstructed from the measured first order derivatives. The shearographic results are validated by comparison with optical vibrometry.