In its final day, @AuManufacturing’s Fibres and composites transforming industry series looks at the issue of repair and sustainment. By Rodney Thomson and Michael Scott.
Today we live in an era where globalisation allows countries to focus on their strengths and deliver to a world market, yet sustainment is a critical national capability to ensure continuous operation of vital equipment and services.
Fortunately, Australia already has strong repair and sustainment capability in advanced fibre reinforced composite materials. Bonded patch repair technologies were pioneered in Australia within the Defence Science Technology Group. For the past few decades, Australian Air Force aircraft have been kept in service through the application of bonded composite repairs to address fatigue and stress corrosion cracking in metallic structures, extending the life of the various platforms. This program has saved millions of dollars and has been exported around the world.
Composite repairs involve applying a fibre reinforced composite laminate over the affected structure by either mechanical fastening or adhesively bonding. As shown in Figure 1, the repair diverts a portion of the loads in the structure, relieving stresses in the underlying structure. Composite repairs can be used to:
- rehabilitate or restore the strength of a structure deteriorated by corrosion, fatigue cracks or other damage;
- strengthen a structure to increase load capacity;
- stiffen a structure to reduce deflection or increase buckling capacity;
- increase the fatigue life of a structure; or
- compensate for design or construction defects.
Figure 1: Principles of a composite patch repair
With outstanding fatigue and corrosion resistance, composite materials are ideally suited to many repair scenarios. Composites offer very high strength-to-weight ratio and the orthotropic material properties can be tailored to a particular application. The repairs can be applied quickly and can follow complex or irregular surfaces. There is no hot-work involved, so no risk of fire, heat-affected zones or thermal residual stresses. Composite repairs can often be conducted without interruption to services. For example, traditional welded pipeline repairs require the line to be shut down for the duration of the repair, whereas a composite repair can be applied while the line is still operating. An iconic example of composite materials used in sustainment of steel and concrete structures can be seen currently on the 336 metre span Westgate bridge over the Yarra River in Melbourne. The bridge has undergone structural reinforcement using in total more than 20 tons of carbon fibre, making it one of the largest carbon reinforcement projects in the world .
To ensure long-term effectiveness, composite repairs require careful design. Glass fibre or carbon fibre reinforcement are most commonly used, though boron and aramid may be employed in some specialist applications. To maximise the efficiency of the repair, the fibre reinforcement directions should be aligned with the primary load paths. Different repair configurations can be used as shown in Figure 2, with more complex scarf repairs reducing the bondline shear stress compared with simpler patch repairs. Careful tapering of the repair patch is needed to minimise peel stresses at the edges that may limit the repair strength and durability. A repair design is typically represented as shown in Figure 3 with a required thickness, a required length from the defect and a taper length.
Figure 2: Repair configurations and bond shear stress distribution 
Figure 3: Representative repair dimensions
Composites may not always be a suitable material for repair, with several issues affecting the durability and behaviour including:
- Environmental factors, including exposure to chemicals, temperature extremes or thermal cycling;
- Galvanic corrosion due to incompatibility between the materials used in the repair and underlying structure;
- Fire requirements as organic matrix materials and adhesives may burn and produce noxious gasses; and
- Vandalism, as composites can be sensitive to impacts and cuts.
Composite repairs can be applied directly to the structure via wet layup or using a prepreg system, then cured in-situ. Alternatively, pre-cured laminates can be mechanically fastened or adhesively bonded onto the structure. Installation of adhesively bonded composite repairs first and foremost relies on high quality surface preparation. The surface must be clean and free of contamination, with the aim to create a high energy surface for the adhesive to form an effective bond. On composite structures, this step typically involves removing any damaged material and filling if necessary, cleaning with solvent, abrading the area where the patch will be bonded (often via grit blasting) and ensuring the resulting surface is free of particles or other contamination. For metals and other materials, a similar process is used, though a primer may be applied to ensure good adhesion. For bonded repairs, adhesive selection is critical. Epoxies are most commonly used and offer the highest performance but are sensitive to surface preparation. Acrylics (methacrylates) offer high strength bonding with minimal surface preparation but are temperature sensitive. Urethanes are flexible, durable and impact resistant, but offer lower shear strength.
Advanced Composite Structures Australia (ACS Australia) specialises in structural repair with composite materials including inspection, assessment, design and repair of damaged storage tanks, pipelines, wind turbine blades, aircraft, helicopters and marine structures. ACS Australia also offers training on composite repair and rehabilitation in fields of aerospace and infrastructure. Below are three case studies on applications where advanced composites prolonged asset service life:
1. ACS Australia developed a composite repair clamp for the oil and gas industry. The clamp comprises two half shell sections which are bolted in position around a leaking pipe and a specially designed sealing system is used for leak and pressure containment. The clamp is 85% lighter than conventional steel clamps and has improved corrosion resistance. The composite clamp was validated via extensive hydrostatic pressure testing including elevated temperature conditions, as well as short- and long-term water testing. This technology was an Australian export, having implemented the repair clamp technology in Malaysia in collaboration with Petronas.
2. A composite radar reflector dish was damaged in adverse weather, resulting in delamination and structural damage. ACS Australia assessed the damage on-site and the repair was carried out in our in-house facilities, reinstating the composite structure back to the undamaged state. Critical to the repair, the contour of the reflector surface remained unchanged. After rectifying damage to the underlying honeycomb core material, a carbon fibre epoxy scarf repair laminate was applied, restoring the radar reflector to its original state, as shown in Figure 4.
Figure 4: ACS Australia Senior Engineer Johannes Straub carrying out repair on composite radar reflector
3. ACS Australia conducted the full certification program to DNV requirements for a new pipeline composite overwrap repair system. The system was designed to repair corroded steel pipes and to be applied in seawater and cured using heat. ACS Australia developed the qualification plan, manufactured test coupons, conducted conditioning and performed testing to verify material properties and long-term durability of the repair laminate and bond under the harsh operating environment. Pressure testing on representative pipe repairs were conducted as shown in Figure 5 to demonstrate compliance with the relevant standards.
Figure 5: Pipe overwrap repair qualification testing at ACS Australia
The application of composite repairs is extensive and the benefits are many. Some industries have already developed standards for the design and application of composites repairs such as ISO 24817, ASME PCC-2 and DNV-RP-C301.
As existing assets made from metal, concrete, timber or composites are damaged in service or degrade over time, the demand for effective repair and sustainment technologies using composite materials will grow. As highlighted in this article, Australian-based capabilities in the design and application of repairs using composites materials will extend the service life of these valuable assets, and in doing so save many more millions of dollars.
 Baker, A. and Scott, M., 2016. Composite Materials for Aircraft Structures, Third Edition. Washington, DC: American Institute of Aeronautics and Astronautics.
Author: Rodney Thomson – Engineering Manager at Advanced Composite Structures Australia
Contributor: Michael Scott – Engineer at Advanced Composite Structures Australia
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