Additive Manufacturing of Structural Cores and Washout Tooling for Autoclave Curing of Hybrid Composite Structures

This paper presents a study combining additive manufactured (AM) elements with carbon fiber-reinforced polymers (CFRP) for the autoclave curing of complex-shaped, lightweight structures. Two approaches were developed: First, structural cores were produced with AM, over-laminated with CFRP, and co-cured in the autoclave. Second, a functional hull is produced with AM, filled with a temperature- and pressure-resistant material, and over-laminated with CFRP. After curing, the filler-material is removed to obtain a hollow lightweight structure. The approaches were applied to hat stiffeners, which were modeled, fabricated, and tested in three-point bending. Results show weight savings by up to 5% compared to a foam core reference. Moreover, the AM element contributes to the mechanical performance of the hat stiffener, which is highlighted by an increase in the specific bending stiffness and the first failure load by up to 18% and 310%. Results indicate that the approaches are appropriate for composite structures with complex geometries

Composites Part Production with Additive Manufacturing Technologies

Additive Manufacturing (AM) is of particular interest in the context of composite part production as AM promises the production of integrated, complex structures with low lead times. Currently, AM is used for tooling and sandwich cores with added functionalities. This paper presents four design principles that improve the production of composites parts during layup, handling, curing and post processing in the layup process. Design principles are applied to a hat-stiffener, a highly integrated aircraft instrument panel and a novel insert eliminating drilling operations. Results show that AM can reduce the part count, assembly steps and deformations during curing

Exploration and validation of integrated lightweight structures with additive manufacturing and fiber-reinforced polymers

This thesis presents and explores a set of novel approaches for the development of hybrid, integrated, lightweight, structures by combining additive manufacturing (AM) with fiber-reinforced polymers (FRP) in layup processes. The presented methods combine the strengths of both technologies: While AM takes a forming, structural and functional role, FRPs primarily contribute with their outstanding mechanical properties at low weight to the performance of the structure. From these characteristics three major design concepts are derived, including AM tooling for composite fabrication, AM structural sandwich cores with additional functionalities and AM load introduction elements. The understanding of materials and manufacturing approaches is a precondition for the embodiment of the design concepts. Therefore, various manufacturing routes are investigated including AM technologies such as fused deposition modeling, binder jetting and selective laser sintering. The thermo-mechanical stability of polymeric AM parts is crucial for autoclave curing of hybrid high-performance structures. To successfully design complex AM elements for the curing process of FRP, the thermo-mechanical creep properties of polymeric AM materials are characterized in three-point bending creep and tensile tests. Alternative concepts include sand and salt structures to produce sacrificial tooling. The embodiment of the three design concepts comprises two directions: designing for the performance of the part during service operation and designing to support the manufacturing of the structure. In this thesis both directions are addressed: Design for Performance: The specific mechanical performance of AM-CFRP structures is assessed in comparison to state-of-the Art structures by comparing the weight, the first failure load, the breaking load and the bending stiffness. Hat-stiffeners with structural cores made by AM were over-laminated with CFRP prepregs, cured in an autoclave and tested in three-point bending. AM core designs include honeycombs, trusses oriented along principal stresses, hollow structures filled with salt and a machined PMI foam core with a local load introduction element made by AM. AM-CFRP hat-stiffeners exhibit an increase in the specific fracture load ranging from 54% to 107% compared to the reference. Weights vary from an increase by up to 55% to a reduction of 5%, and the specific bending stiffness is increased by up to 41%. Results thereby confirm the mechanical competitiveness of AM-CFRP structures. Design for Processing: Additive manufacturing can support the production of composite parts along the process chain ranging from tooling, layup, handling, curing and post-processing. Four major design principles are presented and classified into integrated positioning and fixation elements, layup and handling aids, structural curing aids and post processing aids. Case studies show that the consideration of the processing during the design phase can reduce the deformation of the part during curing, the number of parts and the number of work steps and even eliminate drilling operations for the post-processing of inserts. The AM-CFRP approach is validated by incorporating the material data, the design principles and concepts into three components on system level. First, a novel aircraft instrument panel consisting of a multi-functional sandwich core shows that AM-CFRP can reduce the structural weight by 40%, the part count by 50% and the assembly steps by 50%. The second case study validates the mechanical performance of the AM-CFRP approach on component level by assessing the ultimate and the fatigue strength of a novel AM-CFRP prosthetic knee. The third case study consists of a robot leg structure and yields in weight reductions by 54% compared to state-of-the art references. The combination of AM with CFRP thus is a suitable approach for the manufacturing of individualized, lightweight, and geometrically-complex structures with integrated functionalities for low-volume applications, e.g. in the field of aerospace, flying vehicles, robotics and biomedical structures

Parametric Design of Conforming Joints for Thin-Shell Coilable Structures

This paper addresses the problem of designing and building structural connections (joints) for ultra-thin shells employed in large coilable structures for space applications. A conforming joint design concept for ladder-type coilable thin shells is proposed. A parametric design tool was developed to study the geometry of two joints that become overlapped in the coiled configuration as a function of the coiling radius and joint radii. Parametric design results are validated through finite element simulations. The proposed design tool provides a high level of design flexibility and is of interest in the spacecraft design process making use of ultra-thin shells

Combining Additive Manufacturing with Advanced Composites for Highly Integrated Robotic Structures

The combination of additive manufacturing with advanced composites offers potentials in the development of highly integrated lightweight structures. This paper investigates a novel manufacturing process route where binder jetting is used to produce a water soluble sand core for hand layup of autoclave prepreg composites. The novel approach is applied to a hollow high performance robotic part demonstrating the following two design potentials: First, binder jetting of water soluble sand material is a suitable technology for the production of very complex composite parts at low tooling costs. Second, tailored load introduction elements made of selective laser melting enable lightweight designs. Weight savings of 54.3% compared to a state-of-the-art aluminum robotic part indicate that the approach is competitive for complex low volume parts.ISSN:2212-827

Innovative Fahrzeugkonzepte für Shanghais letzte Meile

Die zunehmende Globalisierung führt in asiatischen Megastädten zu einem rapide ansteigenden Wohlstand, erhöhter Kaufkraft und dadurch zu dem Bedürfnis der Bevölkerung nach einer zeitgerechten, kostengünstigen und umweltschonenden Anlieferung der Güter in die urbanen Strukturen. Im Rahmen des internationalen Studentenprojekts globalDrive ermittelten deutsche und chinesische Studenten gemeinsam vor Ort die aktuelle Logistiksituation in chinesischen Megastädten am Beispiel Shanghais und leiteten Anforderungen für neue Fahrzeugkonzepte ab. Ziel des Projekts, das innerhalb der Kooperation des Lehrstuhls für Fahrzeugtechnik (FTM) der TU München, der Tongji University of Shanghai und mit Finanzierung durch die MAN Truck & Bus AG durchgeführt wurde, war es, einen Prototyp für ein urbanes Lieferfahrzeug aufzubauen. Die Diplomarbeit über die konstruktive Umsetzung des Fahrzeugkonzepts für die letzte Meile (vom Konzept bis zum Prototyp) wurde mit dem 2014er-Herrmann-Appel-Preis der IAV im Bereich zukünftige Mobilität ausgezeichnet

Design and manufacture of hybrid metal composite structures using functional tooling made by additive manufacturing

This paper presents a novel manufacturing technique for complex-shaped, hybrid metal composite structures leveraging the design freedom of additive manufacturing (AM). The key novelty of this research is an approach for an autoclave-suitable and removable tooling, which consists of a 3D-printed functional shell and a structural filler material. In this process, a layup shell is produced with AM and filled with a temperature-resistant curing support to form a removable inner tooling. The functional shell has integrated design features for the positioning and the fixation of metallic interface elements and is removed after curing through integrated breaking lines. The feasibility of this manufacturing technique is demonstrated by fabricating a novel lightweight structure for the hydraulic quadruped (HyQ) robot. Selective laser sintering (SLS) was used to produce the functional shell tooling. Titanium interface elements made via selective laser melting (SLM) were assembled to the shell and co-cured to carbon fiber using an autoclave prepreg process. The resulting multi-material structure was tested in ultimate strength and successfully operated on the HyQ robot. Weight savings of 55% compared to a reference design and the mechanical viability of the multi-material structure indicate that the proposed manufacturing technique is appropriate for individualized hybrid composite structures with complex geometries

Composites Part Production with Additive Manufacturing Technologies

Additive Manufacturing (AM) is of particular interest in the context of composite part production as AM promises the production of integrated, complex structures with low lead times. Currently, AM is used for tooling and sandwich cores with added functionalities. This paper presents four design principles that improve the production of composites parts during layup, handling, curing and post processing in the layup process. Design principles are applied to a hat-stiffener, a highly integrated aircraft instrument panel and a novel insert eliminating drilling operations. Results show that AM can reduce the part count, assembly steps and deformations during curing.ISSN:2212-827

Development of VariLeg, an exoskeleton with variable stiffness actuation: first results and user evaluation from the CYBATHLON 2016

Abstract Background Powered exoskeletons are a promising approach to restore the ability to walk after spinal cord injury (SCI). However, current exoskeletons remain limited in their walking speed and ability to support tasks of daily living, such as stair climbing or overcoming ramps. Moreover, training progress for such advanced mobility tasks is rarely reported in literature. The work presented here aims to demonstrate the basic functionality of the VariLeg exoskeleton and its ability to enable people with motor complete SCI to perform mobility tasks of daily life. Methods VariLeg is a novel powered lower limb exoskeleton that enables adjustments to the compliance in the leg, with the objective of improving the robustness of walking on uneven terrain. This is achieved by an actuation system with variable mechanical stiffness in the knee joint, which was validated through test bench experiments. The feasibility and usability of the exoskeleton was tested with two paraplegic users with motor complete thoracic lesions at Th4 and Th12. The users trained three times a week, in 60 min sessions over four months with the aim of participating in the CYBATHLON 2016 competition, which served as a field test for the usability of the exoskeleton. The progress on basic walking skills and on advanced mobility tasks such as incline walking and stair climbing is reported. Within this first study, the exoskeleton was used with a constant knee stiffness. Results Test bench evaluation of the variable stiffness actuation system demonstrate that the stiffness could be rendered with an error lower than 30 Nm/rad. During training with the exoskeleton, both users acquired proficient skills in basic balancing, walking and slalom walking. In advanced mobility tasks, such as climbing ramps and stairs, only basic (needing support) to intermediate (able to perform task independently in 25% of the attempts) skill levels were achieved. After 4 months of training, one user competed at the CYBATHLON 2016 and was able to perform 3 (stand-sit-stand, slalom and tilted path) out of 6 obstacles of the track. No adverse events occurred during the training or the competition. Conclusion Demonstration of the applicability to restore ambulation for people with motor complete SCI was achieved. The CYBATHLON highlighted the importance of training and gaining experience in piloting an exoskeleton, which were just as important as the technical realization of the robot