Optimization of load application in sandwich structures

Sandwich structures offer very high lightweight design potential due to their very high specific stiffness and are therefore used in many aerospace structures. A key challenge in the design of sandwich components is the load transfer of local forces that occur at the joints to the sandwich. The sandwich core, which is usually very soft, and the thin cover skins can only absorb local loads to a very limited extent. Therefore, so-called inserts are used in the load introduction areas, usually simple load introduction structures such as sleeves or bushings that are inserted into the sandwich structure. In application, sandwich components have a large number of connection points, which is why there is a high potential for weight reduction here.

In the BLANCA project, new load introduction structures are being developed using additive manufacturing and optimized structural bonding, which are optimized in terms of the weight and effectiveness of the load introductions. Furthermore, through the further development of virtual and physical testing, the structural verification of corresponding aerospace structures will be made more efficient and robust and, as far as possible, virtualized using validated and verified simulation models. To this end, analytical and numerical methods for modeling and simulating material and structural behavior under structural load are being developed and validated via a sample and test program. In addition to reducing costs in the course of component development, this also succeeds in saving valuable resources, since a smaller number of physical tests are necessary as part of the validation.

Derivation of criteria for component development

• Definition of requirements and formulation of a specification sheet
• Design, process and material selection

Structural bonding

• Characterize surface topographies of additively manufactured metallic components and develop appropriate surface pretreatment processes to achieve high bond strengths under static and fatigue loads
• Coupon and element tests for the determination of universal characteristic values of adhesives
• Development and validation of a most reliable and robust calculation method for predicting the mechanical properties of complex bonded joints

Additively manufactured load introduction

• Material characterization of additively manufactured metallic components, taking into account directional dependence and different types of loading
• Modeling of material and failure behavior
• Structural optimization of load introduction elements considering aerospace relevant requirements (e.g. damage tolerance, multiple loadpath)

Sandwich component

• Experimental characterization of material and failure behavior of cover skin and honeycomb core by coupon level tests
• Modeling of the material and failure behavior and validation of the calculation models by tests on element level
• Structural optimization using FEM for weight reduction

Assembly

• Implementation of an optimized demonstrator component - here: Avionics plate of an air taxi
• Performance of structural tests to evaluate the newly developed methods

From a scientific point of view, the investigations in the BLANCA project contribute to a better material and structural understanding of load introduction in sandwich structures. By developing and applying new simulation and load introduction concepts, optimized load introduction elements could be designed. The structural behavior was optimized for both static and dynamic loads. In addition, a more reliable application of virtual calculation models was achieved through the extensive test series.

The methods and concepts developed can be applied in future studies for more complex assemblies as well as for other materials. This could help to further deepen the knowledge gained in the project. Further research would also be conceivable in the area of alternative core materials and associated load introduction design. Further research activities would also be interesting with regard to increased functional integration in sandwich components.

Our research project BLANCA is based on the funding initiative BayLu25. The funding program is mainly concerned with research, development and innovation projects basically in the field of action "Increasing productivity and material efficiency in the aviation industry".

Project duration: January 2021 to December 2023

Funding volume for the University of the German Armed Forces: 288,400€.

The project is funded by the Bavarian State Ministry of Economic Affairs, Regional Development and Energy.