Pipes made of fiber-reinforced polymer (FRP) combine high mechanical performance with low weight and good corrosion resistance. That makes them particularly well suited to demanding pipeline systems. The advantages of the pipe material can only be fully realized, however, if the joints can also withstand the loads that occur.

Especially at high internal pressures, the joint becomes the critical interface: it must remain leak-tight over the long term, transmit the pressure-induced forces, and at the same time match the distinctive material structure of thin-walled, continuously fiber-reinforced pipes — including chemical resistance. Conventional joining methods quickly reach their limits here.

Why joining FRP pipes is demanding

Fiber-reinforced polymer pipes are used when high pressure resistance needs to be combined with low weight and good chemical resistance. These properties must not be lost at the joint: it, too, must reliably transmit the forces that occur and remain resistant to the medium being conveyed.

Classic joining methods are only of limited use for this.

Welding through the thermoplastic matrix is often not sufficient. The load-bearing continuous fibers are interrupted at the joint, so the connection can no longer transmit forces in the fiber direction. In addition, heat-based processes can locally alter the base material and the fiber–matrix structure. In most cases, the result is also a permanently non-detachable joint.

Bolted connections are likewise problematic with FRP. Creep and relaxation of the material cause preload to drop rapidly: by about 10–20% within the first 24 hours, and by up to 40% within a month. At the same time, low bolt preload and large hole clearance lead to tilting of the bolt and thus to uneven bearing pressure. The fiber structure around the hole is locally damaged. Bolted connections on thin-walled FRP pipes should therefore be avoided.

Adhesive bonding is possible in principle, but not trivial. It requires defined surfaces, controlled process conditions, and typically curing times. The joint can then usually not be released without destroying it.

Steel flanges and metallic fittings can take high forces and internal pressures, but depending on the medium they offer lower corrosion and chemical resistance. They also increase system weight and can create additional challenges due to different thermal expansion and possible galvanic corrosion.

A detachable, material-matched joint made of a fiber–thermoplastic composite compatible with the pipe is therefore the obvious approach. This reduces jumps in material and properties and largely preserves the mechanical and chemical advantages of the pipe system in the joint region as well.

Functional principle

The concept described here consists of the two pipes plus three essential joining elements: a connecting piece placed over the pipe ends, and two support rings on the outside.

Assembly takes a few steps:

1. A support ring is first slid onto each pipe segment.
2. The connecting piece is positioned over the abutting pipe ends. Its inner diameter is slightly smaller than the outer diameter of the pipe segments, so a light interference fit is already created.
3. The two support rings are then pressed axially onto the connecting piece.

Because the inner diameter of the support rings is smaller than the outer diameter of the connecting piece, a defined radial preload is generated during press-fitting. This preload produces the contact pressure required between support ring, connecting piece, and pipe segment.

The result is a mechanically joined, fluid-tight, detachable connection without heating, welding, or bonding the pipe ends.

Why this joining concept is attractive for high internal pressures

How fiber–thermoplastic composites and pipes made from them can be designed for high internal pressures was already shown in our blog post Pipelines for Renewable Energy. For the pipe system to deliver its advantages in operation as well, the joint must not become the weak point.

The components of the joint are therefore also made of fiber–thermoplastic composites. In this way, the mechanical and chemical advantages of the pipe material can be carried through into the joint. Using the same material system alone is not enough, however: especially at the part transitions, geometry and lay-up must be tailored to the loads that occur.

A pressure-resistant pipe joint must fulfill two tasks: it must safely transmit the forces arising from internal pressure between the pipe segments, and at the same time permanently maintain the contact pressure required for leak-tightness. What matters, therefore, is not only the wall thicknesses of the individual parts, but above all the load paths within the joint.

Hoop-reinforced support rings maintain the joining pressure

The support rings have continuous fibers oriented predominantly in the hoop direction. They can therefore take the hoop forces generated during press-fitting particularly efficiently. Their high hoop stiffness limits radial expansion and stabilizes the joint.

The radially acting joining pressure $p_F$ is converted into hoop forces within the support rings and thus introduced specifically into the most load-bearing fiber direction. The joint therefore uses the high tensile stiffness and strength of the continuous fibers, rather than carrying the load mainly through the weaker through-thickness direction or the polymer matrix.

This material-matched load transfer distinguishes the concept from joining solutions originally developed for unreinforced plastic pipes.

A radially compliant connecting piece distributes the joining pressure

The wound connecting piece is designed for high stiffness in the pipe’s axial direction, while being able to conform in a controlled way in the radial direction. This controlled compliance allows it to seat against the contact faces of the pipe segments without losing its stabilizing and load-transferring effect in the axial direction.

When the support rings are pressed on, the connecting piece is pushed uniformly against the pipe ends. The joining pressure is thereby distributed over a larger contact area. At the same time, small manufacturing deviations and surface irregularities can be compensated, and local stress peaks reduced.

This produces as uniform a contact pressure as possible — an essential prerequisite for the media tightness of the joint.

Fiber orientation matched to each role

Support rings, connecting piece, and pipe segments perform different functions within the joint. Their lay-up is therefore tailored to the loads that occur:

The joint is therefore not executed as a homogeneous, as-stiff-as-possible block. Instead, each part is given exactly the fiber orientation and stiffness it needs for its respective role. Only the coordinated interaction of all components enables a pressure-resistant, material-matched joint.

Simple assembly directly on site

A key advantage of the mechanical joint is the low assembly effort. The pipe ends need neither to be melted nor bonded nor extensively thermoformed. Special pretreatment of the joining zones is also not required with the principle described here.

For assembly, the parts are positioned and the support rings are applied axially with a suitable pressing tool.

This offers several practical advantages:

• no welding equipment and no local heat input — and thus low thermal loading of the pipe segments
• no curing or cooling times; the joint is load-bearing immediately after press-fitting
• few, clearly defined assembly steps
• little space required for the joining equipment
• significantly lower system weight than with metallic flanges and fittings

Because the joining elements and the pipe use the same material pairing, jumps in material and expansion at the joint are avoided. Thermal residual stresses therefore remain low.

This makes the concept particularly interesting for modular pipe systems, on-site installations, repairs, and test setups.

Reproducible joint quality

When the support rings are pressed on, the resulting joining pressure can be measured. From the measured pressures, conclusions can be drawn about joint quality: a defined, reproducible contact pressure is the prerequisite for leak-tightness and load transfer.

This opens up potential for documentable quality assurance in manufacturing and assembly — without destructive testing of the joint.

Detachable instead of permanently bonded

Welded and adhesively bonded joints are suitable for many applications, but can usually only be separated by destroying or cutting out the joint. That makes maintenance, replacement, and adaptation of a pipe system more difficult.

With the concept described here, no bonded joint is created. The pipe segments remain in their original state and are neither melted nor permanently bonded to one another. The connection therefore remains detachable in principle.

For operators and plant builders, this yields concrete advantages:

• individual pipe segments can be replaced more easily
• test or production plants can be modified in a modular way
• maintenance areas remain more accessible
• repairs do not necessarily require replacing a long pipe section