Affordable Thermoplastic Matrix CFC/Metallic Framework Structures Manufacture

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Affordable Thermoplastic Matrix CFC/Metallic Framework Structures Manufacture

Host Institution: Cranfield University

Lead Investigator: Andrew Mills

Co-Investigator: Aurèle Bras, Lawrence Cook

Executive Summary

Many concept for thermoplastic matrix and metallic hybrid structures have been identified. Taking a sports car fully body structure geometry and carrying out a loading study, novel structures suitable for high rate manufacturing have been proposed. These utilise combinations of pultruded tubular sections and folded thermoplastic ‘organosheets’. The tubular parts are proposed to be joined using low cost metallic connectors. The concepts do not therefore require dedicated mould tooling for shaping the parts.

The following concepts are considered to offer the most potential for structural application and further investigation:

a. Metallic joint wrapping

b. Composite tube swaging

c. Metal joint interlocking with composite sections

a. The wrapping of section ends around other parts offers the potential to form framework structures able to resist the tearing forces during crash loading without the need for bolting or manufacturing large complex shape structures, which are so difficult to lay up by automation. This would greatly reduce both assembly time and cost. The wrapped joint investigation has shown that load transfer is achieved through straighter fibres and over a longer shear area than for conventional bolted or bonded overlap joining. Hence, less localised thickening or adding fasteners maybe eliminated for joints subjected to high tensile loading; such as for A, B or C pillars during vehicle crash.

a. and b. offer the potential benefits for manufacturing complexity and cost of being able to use standard sections or either curved plate or tube and to locally form connections to joint parts using the geometry of the joint part as form and jig tooling. Taking cut lengths of section and forming attachment during assembly will massively reduce both tooling cost and parts handling and tracking.

b. Tube swaging and interlocking metallic sleeves did not provide joints of significant tensile strength during this study. It is believed that this resulted from the non-availability of thermoplastic CFC tubing with axial tows and sections with triaxial fibres would show greatly increased joint strength.

The design of the metallic inner and outer joint components will also need to be investigated to establish whether this joint concept is sufficiently lightweight compared to bonded or mechanically fastened joints. A simpler tube end crimping and mechanical fastening may provide a lighter and lower cost joint. The thermoplastic CFC tube end crimping was demonstrated to be a simple and fast process.

c. The dimple interlocking concept offers a means of provide strong shear connections between sections and metallic joints since load transfer is provided by interlocking rather than adhesion. The provision of strong joints, which can be inspected by camera, is a major benefit to high rate manufacturing.

Recommended next research stages

This feasibility project has investigated many aspects of TP CFC and metallic framework design and manufacture. The techniques below all show the potential for more detailed investigation:

a. Metallic joint wrapping

b. Composite tube swaging

c. Metal joint interlocking with composite sections

Each of these techniques will require equipment for local heat and pressure application, which is capable of being adaptable eventually fast moving robotic assembly equipment. This will require both equipment and skills from a collaborative research group, such as a manufacturing catapult; for example the AMRC, MTC or NCC.

Alongside development of the joining techniques, continuous forming techniques for either tubular section or shaped plate as a feedstock for the framework sections. Continuing collaboration with the Technical University of Dresden is advised to investigate continuous forming and tailorable fibre angle sections.

For each of the three processes, further and much more detailed investigation of realistic joints using various permutations of inner and outer thermoplastic composite sections and metal joining pieces which are best suited to the structural loading requirements. This will entail the manufacture of a wide range of experimental joints.

Once effective forming techniques have been developed, the durability of the joints will need to be investigated. Whether interlocking, or local bonding / welding or complete adhesion/ welding in the joining areas is required. Adhesive application or welding techniques may need to be utilised.

The strength and weight of the joints produced using all three techniques will need to be compared with local fastener application. This is an additional step, but may provide the required load transfer for either reduced manufacturing cost or weight.

In addition to further framework design and manufacturing investigation, the effect of the forming processes on both the laminate compaction and fibre disruption will need to be studied. The effect of local heating may cause an unacceptable level of disruption to the thermoplastic composite pre-manufactured laminates.

Finally for the selected and developed techniques, simulation tools for the forming processes and the process parameters required to provide reliable forming will need be developed.

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