The implementation of structural health monitoring systems in modern aircraft structures requires a deeper understanding
of impact and piezoelectric generated lamb wave propagation on carbon fibre reinforced plastics. In this paper a digital
shearing interferometry method is presented that visualizes lamb waves excited by impact events or piezoelectric
actuators. The contactless full field measurement of these waves is realized by a Mach-Zehnder interferometer which
combines spatial phase shifting and Shearography. The latter is a laser based technique whereby the first order derivates
of the displacement is indicated. Since a dynamical process is observed the spatial phase shifting technique is required.
The optical implementation of both techniques within the interferometically setup and experimental results with the
possibility to measure the out of plane displacement are presented. Therefore the underlying wavefield is reconstructed
from the measured first order derivatives. Subsequently these results are compared with a one point measuring method
and FEM simulation.
This paper presents a theoretical model for anisotropic wave attenuation in composites. The model has been implemented in a software called FIBREWAVE in order to predict dispersion and attenuation of S0, A0 and SH0 Lamb wave modes. The required input data are the complex stiffness matrix coefficients of the unidirectional plies of the laminate, which have been measured by a laser interferometry method. Complex stiffness data for an unidirectional CFRP laminates are moreover presented. Satisfactory agreement has been observed between predicted and experimental group velocities and wave attenuations.
In this work an analysis of impacts on carbon fibre structures using holographic interferometry is presented. Impacts are caused e.g. by stones or hail at high motion speeds of vehicles. An impact is defined as a load having a duration that is shorter than the travelling time of the impact waves through the structure. The measurements are therefore performed using a pulsed ruby laser, making it possible to record digital interferograms at different times after the impact [1]. The impact is excited by an air-driven projectile and the holograms are stored digitally using a CCD camera. The holograms are reconstructed numerically and the phase difference between the unloaded state before the impact and the loaded state at a predefined time after the impact is calculated. The out-of-plane displacement can then be extracted from these phase differences. The experiments cover the influence of different parameters. Investigated are the occurring wave forms and their group velocities. Results of experiments on the effect of different composite designs on the shape of the wavefront are presented.
Due to the anisotropic properties of carbon fibre composites not much is known about the damage tolerance and failure limits especially in the dynamical case. Therefore the goal of these experiments is to gain a deeper understanding of the dynamical behaviour of these materials and to provide the dynamical material parameters which can be used for numerical simulations on one hand and for design and construction on the other hand.
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