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Aspects of Joining Pultrusions

Gareth C McGrath, Phd Technology Manager,
Adhesives TWI Abington,
Cambridge, UK
Tel: + 44 223 891162 Fax: + 44 223 892588

Index

1. Introduction
2. Design
3. Fabrication
4. Joining
5. Design for Manufacture
6. Applications
7. Concluding Remarks
8. References

About the Author

Gareth joined TWI in 1989, he has been involved with composites and adhesives for more than ten years after an initial career in metallurgy. Since joining TWI, Gareth has developed techniques for helping to explain composite failure. These have included impact damage analysis, and time dependent studies. He is now Technology Manager for the expanding adhesives activity at TWI.

Summary

When designing with pultrusions, it is necessary to consider the joining system at the conceptual design phase so that prototype testing can be included in the design process. Two types of joining systems are available: mechanical fasteners and adhesive bonding. This paper will summarize the advantages and disadvantages of each system. By highlighting these aspects of the design, the specific fabrication requirements associated with each joining system will be explained. Adhesive bonding techniques will be qualified by reference to adhesive type/surface preparation and quality assurance. Mechanical fastening techniques will be explained and the choice of fastener related to the substrate combinations. The paper will concentrate on the most critical aspects of the conceptual design: design for manufacture and joint design. These will be exemplified by the use of novel applications for pultrusions in different industrial sectors including building, civil engineering, sports, chemical and nuclear.

Introduction

Pultrusions are predominantly axially fibre reinforced plastics and have transverse and shear properties that are poor in comparison with their axial properties. Consequently load diffusion into these materials requires careful design and adhesively bonded joints should be considered, to minimise stress concentrations. This reduces the tendency towards shear failures that can occur when using mechanically fastened joints for joining composites. In spite of this, many examples of bolted pultrusions are found and the presentation will highlight some of these. Alternative joining methods will be considered with reference to specific examples.

Design

When fabricating with pultrusions it is necessary to consider the structural requirements of the product at the conceptual design stage. Unlike conventional metals and alloys, which are characterised by their ductility, homogeneity and isotropy; fibre reinforced composites are characterised by heterogeneity and anisotropy and may be brittle. Reinforced composites can be thought of as being analogous to timber in many ways, possessing excellent properties along the grain but being relatively weak transversely. In order to achieve the potential offered by composites, a change of design 'approach is generally necessary in which consideration is given to the distinctive characteristics these materials possess. An excellent example of an appropriate approach is the Advanced Composites Construction System (1).

Whatever design approach is selected, manufacture and assembly systems must be considered at the conceptual stage if the most cost effective and efficient structure is to be achieved.

Fabrication

Almost all fabricating methods used currently in the processing of wood, aluminium, steel or other competitive materials are available for the fabrication of fibre reinforced composites or in this case, pultrusions. The most important characteristics of pultrusions to keep in mind is the anisotropy. Although working with a pultruded section is like working with wood, it is uniquely a fibreglass pultrusion and has its own character traits. Before fabricating pultrusions several characteristics of the product should be recognised:

1. Fibreglass is extremely abrasive and tools should be adequately maintained, or better still diamond coated edges used.
2. Fibreglass is dusty during fabrication and adequate health and safety precautions must be taken.
3. Although pultrusions are strong, they are also elastic and support is needed for all fabrication procedures.
4. Pultrusions may soften if exposed to friction heat, poor edges may result and water cooling should be used in conjunction with optimum cutting speeds.
5. Exposed fibres absorb moisture and contaminants and it is recommended that fabricated areas should be coated with resin, to prevent environmental degradation.
6. Pultrusions are composites that are engineered into laminate sections and therefore appropriate allowance must be made for this.
7. Joining procedures MUST be carefully appraised and prototype testing should be included in the decision process.

Joining

Because pultrusions are fibreglass composites many variables must be taken account of, such as ultraviolet exposure, chemical and temperature factors as well as mechanical requirements. It is vital that the designer does not simply replace an aluminium component with a pultrusion if the maximum benefit is to be achieved. Too often this approach is adopted and weight penalties incurred as a result of it.

In simple terms, the product application should be considered in terms of design for manufacture and as such the choice of joining system is of course critical to the successful outcome.

Adhesively Bonded Joints

T'here are many possible adhesive systems but the basic systems are:

1. epoxies
2. acrylics
3. polyesters

Table 1 summarises these adhesives.

Adhesives offer significant advantages:

• uniform stress distribution
• Reduced weight and cost
• joining materials of different thickness
• improved aesthetic appearance
• Exposed surfaces are not defaced
• Dissimilar materials can be joined
• They can accommodate differences in thermal expansion
• Sealing

However, as with other advantages in life, a number of offsetting disadvantages occur:

• The vast number of products makes material selection difficult
• A lack of design data
• Quality assurance
• Training

Joint Design

It is beneficial to design a component in such a way that the adhesively bonded joint will be fortified by the geometry of the final component rather than being destroyed or weakened by it. This is achieved by designing the joint so that the forces experienced tend only to compress the adhesive or induce, if possible, both compression and shear loads. If this cannot be done the design should aim to keep the unwanted cleavage forces away from the vulnerable edge of the joint, see Fig.l.

Fig 1-Design principles for adhesively bonded joints.

The design of bonded joints has been investigated in great detail by Hart-Smith (2), who shows clearly the importance of adherend thickness on the relative strength of the different joint configurations, as illustrated in Fig.2. The weakest joints are those where failure is limited by interlaminar failure of the adherend or peel of the adhesive. The next strongest joints are those in which the load is limited by the shear strength of the adhesive. The strongest joints will fail outside the joint area at a load equivalent to the strength of the adherend. Such failures can be related to the stress distributions described earlier.

Fig 2-Relative uses of different bonded joint types (hart smith)

An unsupported single-lap joint is the weakest configuration and, for practical lay-ups, will never be as strong as the adherends being joined. However, acceptable efficiencies can be achieved provided the overlap-to-thickness ratio is sufficiently large. Thicker adherends would need to be tapered at the ends of the overlap.

For thicknesses of about 1.5-1.75mm for quasi-isotropic lay-ups (less for unidirectional laminates) a double-lap configuration is needed to transfer the strength of the adherends. The optimum overlap-to-thickness ratio is about 30:1. If the adherends are uniform, thickness is limited to about 4.5mm, whilst tapering can increase the limit to 6.35mm.

For thicknesses greater than 6.35mm stepped or scarf joints should be considered. Theoretically, merely by making the angle small enough, it should be possible to make a scarf joint stronger than the adherend being joined. In practice, particularly for wide joints, it will be impossible to make the required knife-edge.

The highest performance may be gained by bonding panel sections into either an aluminium extrusion or into a pultrusion. Joints such as these, based on the concepts of Fig.3 are very robust, given an appropriate insert length.

Fig 3-Alternative versions of a contained joint based on pultrusions.

The designer's choice of material, its preparation for production bonding and that latter process itself are limited by the reaction chemistry of the main family types and the durability of the end product of that reaction. Durability is the key and its achievement a controlling factor.

Surface preparation

As stated previously, the bond strength is no greater than the lap shear strength of the laminate itself. A special problem is encountered when bonding pultrusions: The pultrusion has been made with a mould release and adhesives will not work with maximum efficiency until this is removed. Mould release agents migrate to the surface during the manufacturing process and removal of the outer layer will remove the majority of the mould release agent:

1. Solvent wipe with alcohol
2. Wipe dry
3. Abrade surface with abrasive paper or light grit blasting
4. Air blast surface to remove dust - avoid recontamination by rehandling!
5. Apply adhesive
6. Jig joint
7. Allow adhesive to cure in accordance with manufacturers recommendations.

Mechanical Fastening

It should be remembered that drilling the pultrusion will reduce the load bearing capability of the profile as a result of the destruction of the fibres which give the pultrusion its strength.

A wide variety of mechanical fastenings is available, as shown in Fig.4.

Comparison of Joining Techniques

To help in selecting the most appropriate joining system, Table 2 summarises and compares the aspects of adhesive and mechanical systems.

Design for Manufacture

Effective production requires the creation of a defined product within specification at minimum cost. Given that such cost influencing issues as design, compatibility and long term durability have been resolved, then for a large structure the final considerations are:

• Ease of use
• The cost incurred if mixing is needed
• The cost of the fastener and application system
• The cost of surface preparation or fabrication
• The practicality of surface preparation
• Joint design.

Applications

The following applications highlight the use of pultrusions for building, civil engineering, sports, chemical and nuclear.

Building

When the billion-dollar Taj Mahal hotel and casino opened recently in Atlantic City, New Jersey, pultrusions' were behind the scenes.

More than 1200 custom-designed mullions adored the walls in the parking garage behind the main structure. In addition, standard pultrusion profiles were part of the support members for the fascia and spandrel panels.

Design for manufacture included: stepped-up tooling time, prototyping and testing, colour matching and cosmetic quality control. In addition to custom design and production.

Civil Engineering

Maunsell Structures Plastics was responsible for the design of a footbridge over the River Tay - the Aberfeldy footbridge.

Students at the University of Dundee had been asked by the Golf Club to help build the footbridge - but this ruled out the use of heavy civil engineering work. A lecturer at the University was aware of work Maunsell had been developing for the design of lightweight durable advanced composite structures which were perfect for bridge construction.

Working closely with final-year engineering students (who provided the bridge erection team) Maunsell guided the project through planning, design and construction.

The bridge is a cable-stayed structure with a main span of 63m and two back spans. It is stayed from two 18m high 'A' shaped GRP pylons using Parafil cables. The 120m long lightweight bridge deck was assembled on site in only four weeks. A unique method of erection of towers, cables and deck was used, which due to the lightweight components, needed no site cranage.

Assembly of the bridge deck was carried out on-site in a tented region to keep the working area clean and dry. The epoxy adhesive used to bond the deck components started to cure about 45 minutes after application, so it was necessary to complete assembly within that time, and then to leave the components to cure for another 24 hours before carrying out further work nearby.

The adhesive was a two-pack system, supplied in cartridges with an automatic mixer nozzle which provided excellent control. Application was mostly by pneumatic guns, and the rate of application was controlled by monitoring the size of the 'bead' and by measuring the length of bonded joint achieved for each cartridge of adhesive.

A long term monitoring programme is being put in place by the University of Dundee (funded by the Department of Transport), to measure creep and thermal effects, and to observe the structure's dynamics.

The Aberfeldy footbridge has been heralded as 'a milestone in bridge construction' and it provides a strong, stable, no maintenance solution to the original brief. The project has helped illustrate that advanced composite materials are likely to take their place alongside steel, concrete and aluminium in the construction industry in the 21st century. And for now, golfers can wheel their trollies across a little piece of engineering history on their way to the tenth!

Sports

Platform tennis has enjoyed substantial popularity and growth in the past twenty years. In that time, there were two types of courts available: aluminium and wood. Wood offers significantly lower cost but has one major drawback - its life expectancy is less than 15 years. Aluminium was an early acceptable alternative but, with inflation, has now become increasingly expensive. A replacement aluminium deck alone was prices in 1988 in excess of $25,000 for the materials only. Now there is a third alternative.

Fibreglass reinforced plastic (FRP) is, when properly designed and fabricated, a structural material consisting mainly of fibrous glass reinforcements and polyester resins. There are many fabrication techniques to make FRP parts; however, for this application, the pultrusion process is preferred.

There are many advantages of pultruded FRP decks in platform tennis. These include: Durability - studies have shown only minimal strength losses of FRP parts after 20 years continuous exposure to several hostile environments. Wearability - FRP surfaces are compatible with a variety of high performance coatings such a urethane and epoxy paints. Sound - there is no noticeable ping or echo when playing on FRP compared to aluminium. Cost - just to keep the club treasurer happy, the cost of an FRP replacement deck is substantially less than its aluminium counterpart.

Chemical

The introduction of stairways and access to plant presents a threefold problem: To provide a safe access, freedom from maintenance and a pleasing appearance.

Stairways and platforms were designed, engineered and custom fabricated in accordance with OSHA standards. The structural framing and handrail were fabricated from pultrusions that have a Class 1 fire retardance, with an E-84 Class 1 flame rating. The grey structural framing, highlighted with the safety yellow handrail and grating, not only created a safer access to the cooling tower but it also gave the tower a more pleasurable appearance.

New FRP stairways and platforms were purchased to replace the existing structures made form pressure treated lumber. Weather and constant cooling tower mist had deteriorated the wood and produced prohibitive maintenance costs. The customer justified switching to FRP by eliminating his maintenance costs and by greatly reducing the installation cost of the replacement. The new structures were fabricated in three sub-assemblies and delivered to the job-site. The contractor was able to unload directly from the truck and set them into place (without the use of a crane due to the light weight of FRP) and complete installation in one day.

Nuclear

Constructed entirely of pultrusions, a Nuclear Test Tower was delivered to the ground zero nuclear test facility located hundreds of feet below the earth's surface in Mercury, Nevada. The Tower was built for the Department of Defense to support diagnostic equipment used in conjunction with a magnetic fusion experiment. It stands almost 30' high, 56' wide and forms an octagonal shape sweeping 240'.

All principle FRP components used in the construction of the nuclear Test Tower were furnished by Creative Pultrusions Inc. The structure utilised Pultex® series 1525 polyester pultruded fibreglass beams, angles and Superstud threaded rod and nuts.

Superstud® and Nuts® to assemble, specialised construction methods and components were utilised in this totally FRP structure. Special FRP angles of 15, 30, 75 and 105 degrees were utilised in construction and assembly of this structure.

Pultex® was selected for this application by the client, not due to its superior corrosion resistance, but rather due to its high strength to weight ratio and electromagnetic transparency. The total FRP structure offered considerable weight savings over the use of conventional materials which aided in field erection. Given the design parameters involved, Pultex® and Superstud® were the choice materials for this application.

Concluding Remarks

Pultrusion structures, particularly those based on thermoset resins can be readily and durably joined, both to themselves and associated metal structures. Particular attention should be paid to:

• joining technique selection,
• joint design,
• surface preparation and
• production aspects.

References

1 Head P R: 'Further developments in the use of a modular pultruded construction system in building and civil engineering structures'. Bigger and Better Pultrusions, EP'FA World Conference, Venice, Italy, June 2 and 3, 1994.

2 Hart Smith: 'Design of adhesively bonded joints'. In F L Matthews (ed) Joining Fibre-Reinforced Plastics Chpt 7, pp 271-311, Elsevier Applied Science.