Ultra Thick Laminates for Compact Load Introduction Fittings

• Main Author: Zimmermann, Kristian
• Format: Doctoral Thesis
• Language: English
• Published: KTH, Lättkonstruktioner 2011
• Online Access: http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-46730; http://nbn-resolving.de/urn:isbn:978-91-7501-156-1

Composites are increasingly often used for thick and compact structures with the clear aim to reduce the overall weight and cost of an aircraft. But classic applications of composites are thinner structures with limited out of plane loads. Analysis and test methods are therefore commonly developed and used for thinner structures and neglect the special challenges involved with thicker laminates. In addition composites are increasingly becoming interesting for fittings and joints since the surrounding structures are either built or being developed in composites a well. Using metallic fitting and joints can cause additional thermal stresses and /or corrosion due to the material mix of composites and metals. Due to the enlarged field of application for composites, there is an increasing demand for suitable analysis, test and manufacturing methods. Compact and highly loaded composite structures are prone to be subjected to high and multidirectional loads. This causes an atypical load situation for composites, which usually are subjected to plane loads to best exploit the strength of the fibers. Due to the orthotropic nature of the material a large amount of design variables are introduced. The design of any composite part is highly manufacturing driven, meaning that the final shape is determined by manufacturing capabilities. Thick composites provide a cost effective alternative and can generate a distinct weight benefit over standard metallic components and hence will a play a significant role in future aircraft developments. Analysis, testing and manufacturing methods have to be developed and adapted for that purpose. A reliable analysis is only possible if accurate 3D material properties are available. Analysis capabilities have to be assessed using empirical test data in order to judge the applicability. The presented work has its emphasis on the analysis and testing of structural components manufactured in thick composites. The generated data from a comprehensive manufacturing and test program is also used as basis for a cost and weight study under the assumption of a highly automized serial production. The results further underline the potential of thick composites.  In a first approach, standard 2D finite element methods are used for a topology investigation. In order to fully capture the behavior of the material 3D methods are quickly implemented. An extensive test program with full scale samples and coupons is used to improve and evolve the analysis. An open mold manufacturing cycle minimizes tooling costs and provides optimum flexibility for frequent design changes. A strong link between the analysis, the manufacturing and the design is maintained throughout the developments in order to generate a material suitable design solution. Although the ultimate goal is to manufacture a specific component, the topics are approached as generic as possible in order to provide a basis for future studies with similar boundary conditions. Despite the fact that the material creates countless design variables, an affordable approach for the analysis of thick composite structures is provided using standard 3D composite brick elements. The initial problem of missing reliable 3D material properties is counteracted with tests of full scale sub- components and modified short beam shear tests. A new cure cycle for thick laminates is presented and analyzed to assess process induced stresses and deformations. A large landing gear fitting component is designed and manufactured and can be regarded as an excellent demonstrator of ultra thick composites. With a maximum wall thickness of 90mm, the component provides a weight reduction of 18% and a cost benefit of approximately 20% compared to the metallic counterpart. The potential of composites applied to a compact and highly loaded fitting is demonstrated and suitable analysis methods are established. A need for future tests to provide reliable and generic 3D material properties is identified. To provide a weight and cost benefit it is crucial to find a design topology suitable for composites. === QC 20111114 === ALCAS