DLR Institute of Structures and Design increases maturity of thermoplastic composite fuselage structures

 

As part of the “LuFoV-3 TB-Rumpf” project, the autoclave-free consolidation of thermoplastic prepreg laminates and resistance welding is being further developed and validated as technology bricks for future aircraft fuselages. This work is being carried out by the German Aerospace Center (DLR), Institute of Structures and Design (Institute BT) in Stuttgart, collaborating with partners from the aerospace industry, including Airbus and other research institutes.

DLR Institute BT has demonstrated a fully integrated fuselage intersection, consisting of skin, stringers, frame and cleats. The demonstrator is based on an out-of-autoclave (OOA) consolidated curved skin with co-consolidated stringers and integration of frames and cleats via resistance welding. The welds have been characterized by mechanical tests and the weldline was further examined using an optical microscope. 

 

TB-Rumpf maturing OOA consolidation and welding

State of the art for consolidation of large-area components made of continuous fiber-reinforced high-temperature thermoplastics (e.g., polyaryletherketone or PAEK) is autoclave consolidation. However, through an optimized process setup and tailored process control, it is possible to achieve complete consolidation using only temperature and vacuum pressure. The use of self-heated molds or ovens can eliminate the need for an autoclave and thus generate cost advantages. The TB-Rumpf project aims to mature the process called VCT (Vacuum Consolidation Technique) and to determine the process limits, including maximum possible laminate thickness.

Another key technology for future applications of advanced thermoplastic composites is welding for assembly. DLR and Airbus have identified resistance welding as a leading technology due to its ability to achieve high-strength structures. Within the TB-Rumpf project, the resistance welding process and necessary welding elements are being optimized. The resulting demonstrators are structurally tested to validate the mechanical performance of the joint and the welded composite laminates.

 

Investigation of vacuum consolidation technique (VCT)

Matthias Horn, project leader at DLR Institute BT, explains that the TB-Rumpf demonstrator uses unidirectional carbon fiber-reinforced low-melt polyaryletherketone (LM-PAEK). This thermoplastic composite tape is processed using automated fiber placement (AFP) achieved using a KUKA (Augsburg, Germany) robot with an AFPT (Doerth, Germany) end-effector and 6-kilowatt laser for heating the tape. 

OOA consolidation using VCT:  “The goal was to validate the feasibility of this process for thick laminates,” says Georg Doll, Institute BT researcher and TB-Rumpf lead for consolidation. “We have shown good results for laminates up to 11 millimeters in thickness, without pores or other imperfections, validated by micrograph cross-sections and ultrasonic inspection. The keys to this success are an optimized VCT setup, a tailored heating cycle and homogenous and consistent prepreg quality.”  

OOA co-consolidation of stiffener elements:  In addition to using VCT for production of fuselage skin panels without an autoclave, the TB-Rumpf project has also succeeded in developing co-consolidation for direct integration of the stringer stiffener elements with the skin during a single VCT process cycle.

AFP layup of curved and double curved shapes: Laser-based AFP as a complementary process to VCT was investigated, including process and material limits (e.g., maximum tape steering), by manufacturing double-curved laminates measuring roughly 800 x 1200 millimeters. The results from these manufacturing trials can be used to advance OOA consolidation as well as in-situ consolidation, which is achieved during AFP without a secondary operation.

 

Resistance welding  

“To integrate structural fuselage elements, and thus make production of future thermoplastic fuselage concepts feasible, we investigated resistance welding with respect to increased process robustness and optimized strength values,” says Simon Bauer, Institute BT researcher and TB-Rumpf lead for welding. “The focus was on an optimized welding element setup, based on carbon fiber as the heating element, and improved electrical conduction, in combination with the best suitable welding parameters, such as the heating cycle, temperature, pressure, current and voltage.” Resistance welding was used to attach the cleats to both the frame and stringers; it was also used to attach the frame to the skin.  

Technology bricks for upcoming fuselage designs

 “Due to the demonstrated technology maturity, both out-of-autoclave consolidation as well as resistance welding can play an important role for future fuselage designs,” underlines Dr. Paul Jörn, head of the corresponding Airbus project.

To bring the technologies even closer to industrial application, further challenging aspects will be investigated in close future. For vacuum consolidation the size scaling and transfer to double curved contours will be focused. For resistance welding tolerance aspects as well as automation aspects will be addressed.     

The results presented here were achieved within the TB-Rumpf research project (FKZ: 20W1721D) in the framework of the Federal Aviation Research Program V-3, funded by Germany’s Federal Ministry for Economic Affairs and Energy.

For more information, please visit dlr.de/bt.