WrapToR composite truss structures: Improved process and structural efficiency

Significance 

During design of structural elements, the need to improve efficiency of the load bearing structure often arises; under such circumstances, the designer has two options at their disposal: to either change the material, or change the geometry. Trusses are a class of structural elements usually formed of multiple individual elements connected at joints that are able to surpass the efficiency limits of simple sections. Advances in material engineering has borne composite materials, such as: fiber reinforced concrete and concrete-filled fiber-reinforced polymers. As such, advanced composite materials are an increasingly common choice for improving structural efficiency. For trusses, the loading within its members is dominant along their length. Consequently, forming these members with fiber reinforced composites would seem intuitive. In fact, it would not only result in a combination of highly efficient geometry and material, but also maximize material properties in the primary stress direction by aligning fibers with the members length. Regardless, the use of composite trusses to date has been limited due to increased complexities in truss manufacturing as a result of the numerous members to be jointed.

To this end, studies have been undertaken aimed at developing composite trusses using novel manufacturing techniques that reduce or remove the need for bonding multiple members. One notable publication reported on the Wrapped Tow Reinforced (WrapToR) truss concept that basically uses an adaptation of the filament winding process to produce composite truss beams. Unfortunately, this novel technique suffers from several shortfalls that ought to be resolved. On this account, researchers from the Bristol Composites Institute in the Department of Aerospace Engineering at University of Bristol in England: Christopher Hunt (PhD candidate), Professor Michael Wisnom and Dr. Benjamin Woods developed a novel configuration to increase the buckling resistance of the shear web members, ultimately translating to a more efficient manufacturing process. Their work is currently published in the research journal, Composite Structures.

Foremost, the novel approach, previously untested, involved twisting of the fiber tow during winding to produce shear web members with circular cross-sections. Specifically, the researchers designed, detailed and build a computer numerical controlled (CNC) truss winding machine that was used to fabricate trusses. Afterwards, the fabricated trusses were tested purposefully to failure so as to investigate the effects of tow twisting on structural performance. Finally, a comparison to commercially available composite tubes was conducted.

The authors reported that tow twisting improved the consistency of the cross-section shape and increased minimum second moment of area. However, analysis carried out using the direct stiffness method was seen to overpredict truss stiffness suggesting invalidity in the assumptions used by the method. Nonetheless, experimental comparison of WrapToR trusses with conventional composite pultruded unidirectional tubes confirmed that at the length scales tested, the truss configuration provided large improvements in structural efficiency.

In summary, the University of Bristol study investigated the effectiveness of a simple design technique for increasing the structural efficiency of WrapToR composite structures. Generally, their new technique involved twisting of the carbon fiber tow during the winding process to alter the shape of the shear members for improved buckling resistance.

WrapToR composite truss structures: Improved process and structural efficiency - Advances in Engineering

About the author

Christopher Hunt

Chris is a PhD student, currently enrolled on the doctoral training program at the University of Bristol Composites Institute. In 2015 Chris was awarded an MEng in Aerospace Engineering from the University of Bristol. During the final year of his master’s, he published work investigating the effects of varying cure cycles on pre-impregnated composite materials. Following the master’s program, in 2016 he started his current role as a PhD student where he is now helping to develop a novel composite truss technology.

His research interests include composite manufacturing, recycling, and structural design, with current activities focused on reducing the environmental impact of polluting vehicles using light-weight composite structures.

About the author

Benjamin K.S. Woods

Dr. Benjamin K.S. Woods earned his PhD in Aerospace Structures from the Alfred Gessow Rotorcraft Center at the University of Maryland in 2012, having earlier received a B.S. with honors (2005) and M.S. (2007) in Aerospace Engineering from Maryland.  His PhD focused on the development of a novel pneumatic actuator solution for active helicopter rotor blades. He was also heavily involved in Maryland’s World Record breaking Human Powered Helicopter project.  He held a post-doctoral research post (Senior Research Officer) at Swansea University from 2012 to 2015, where he developed a number of morphing aircraft concepts.  He started his current post in the Department of Aerospace Engineering at Bristol in August of 2015 as a Lecturer in Aerospace Structures. He is currently an EPSRC Early Career Fellow and is leading the Horizon 2020 project SABRE (https://sabreproject.eu/). He holds 7 US patents, has authored 27 journal publications, two book chapters, and 53 conference papers.

About the author

Michael R. Wisnom

Michael Wisnom is Professor of Aerospace Structures at the University of Bristol and Director of Bristol Composites Institute.  He is a leading expert on the mechanics and failure of fibre reinforced composites, with over 400 published papers. He is a member of the steering board of the UK National Composites Centre, Editor in Chief and European Editor for Applied Science and Manufacturing of the international journal Composites Part A. He is a Fellow of the Royal Academy of Engineering, of the Institution of Mechanical Engineers and of the American Society for Composites.

Reference

Christopher J. Hunt, Michael R. Wisnom, Benjamin K.S. Woods. WrapToR composite truss structures: Improved process and structural efficiency. Composite Structures, volume 230 (2019) 111467.

Go To Composite Structures

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