Numerische Quantifizierung des Einflusses räumlich verteilter Out-of-Plane Welligkeiten auf die Materialfestigkeit von Faserverbundstrukturen mithilfe probabilistischer Verfahren

Kurzfassung

Production-related defects have always posed immense challenges for the manufacture of aircraft structures made of carbon fiber-reinforced plastic (CFRP) in order to meet the high-quality requirements of aviation. Unwanted fiber deflections in the composite material, also known as waviness, represent a characteristic type of defect. In order to be able to adequately determine the changed material and load-bearing behavior under the influence of waviness, numerical calculation methods are now increasingly being used. Despite the availability of modern computing technology, however, simulation-based investigation of the topic is limited to selected geometries due to their complexity. The present work presents a numerical evaluation approach that maps the multitude of potential waviness geometries on the example of out-of-plane effects using a non-deterministic calculation method. For this purpose, a stochastic concept based on random fields for geometry parameterization is described and discussed. In addition, a FEM centered modelling approach is presented, with which the effects of the waviness on the load bearing reserves of the material can be investigated in terms of structural mechanics. This is done with the help of detailed models of the individual layers of the material composite, with which load-dependent reduction factors (KDF) are determined to quantify the material behavior. The KDFs are compared with each other in the thesis using various criteria to characterize the failure behavior in the CFRP composite. The presented probabilistic calculation approach is also demonstrated using representative laminate configurations using the example of the compressive strength Rx and the resulting statistical distributions of the KDF of the material parameter are determined. The comparison of the KDF shows major differences depending on the failure criterion considered, which motivate further investigations in this context. Finally, for a practical application of the method, various geometry-based metrics are compared with each other, on the basis of which an approximate determination of the KDF can be made possible. The investigations showed that the maximum waviness gradient is a suitable substitute variable for angles up to 19 degrees due to a low coefficient of variation. This offers the potential for using the method during production to evaluate realistic waviness defects.