Impact measurements using double pulse digital holography

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.