For decades, carbon-fibre composites have delivered the lightweight performance demanded by the aerospace and automotive sectors. Their exceptional strength-to-weight ratio has enabled significant reductions in vehicle mass, improved fuel efficiency and increased structural performance. Yet despite these advantages, composites have faced a persistent sustainability challenge: what happens when a component reaches the end of its useful life?
Traditional thermoset composites are notoriously difficult to repair, separate and recycle. Once cured, their polymer matrices cannot be remelted, making disassembly and material recovery both technically challenging and economically unattractive. As a result, many high-value composite components ultimately follow linear rather than circular material pathways.
A recent study investigating resistance-welded carbon-fibre-reinforced low-melt poly(aryl ether ketone) (CF/LM-PAEK) composites suggests a different future may be possible. By optimising both the welding and separation processes, researchers from Technical University of Braunschweig and Leibniz University Hannover have demonstrated that high-performance thermoplastic composite joints can be assembled, disassembled and potentially re-used, offering a compelling route towards circular composite structures.
MOVING BEYOND PERMANENT JOINTS
The emergence of thermoplastic composites has already begun reshaping thinking around lightweight structures. Unlike thermoset materials, thermoplastic matrices can be reheated and remelted, enabling welding processes that eliminate the need for mechanical fasteners or adhesives.
This characteristic offers several advantages. Resistance welding enables rapid assembly, reduced part counts and lower manufacturing complexity while preserving the excellent mechanical properties associated with continuous carbon-fibre reinforcement. Thermoplastic composites also offer strong impact resistance, high fracture toughness and excellent chemical durability.
However, the most important implication may be their potential contribution to circular manufacturing. Rather than creating permanently bonded structures, resistance welding creates joints that can potentially be separated in a controlled manner. This capability allows engineers to consider repair, refurbishment, component replacement and material recovery much earlier in the product lifecycle. For industries increasingly focused on lifecycle performance rather than simply manufacturing efficiency, this represents a significant shift.
OPTIMISING STRENGTH AND DISASSEMBLY
The study focused on understanding how welding parameters influence the performance of resistance-welded CF/LM-PAEK lap joints. Using a Taguchi experimental design and analysis of variance, the researchers evaluated the influence of welding time, power and pressure on joint strength.
The optimised process produced lap-shear strengths of approximately 50MPa, demonstrating that resistance welding can generate structurally robust joints suitable for demanding engineering applications. More significantly, a post-weld annealing process further increased performance, raising lap-shear strength to more than 63MPa.
These findings are important because circularity cannot come at the expense of structural integrity. Automotive and aerospace designers require joining methods capable of delivering production-ready performance while simultaneously supporting repair and recovery strategies.
ENABLING DESIGN FOR DISASSEMBLY
One of the central principles of the circular economy is designing products so that valuable materials can remain in use for as long as possible. For composite structures, this often means enabling inspection, repair, remanufacture and eventual material recovery. Historically, this has been difficult to achieve. Adhesively bonded or co-cured composite assemblies are frequently challenging to separate without damaging the underlying materials.
Resistance-welded thermoplastic composites offer a fundamentally different approach. Because the joining mechanism relies on localised melting of the thermoplastic matrix, joints can be intentionally separated under controlled conditions. The study specifically investigated both joining and controlled disassembly, highlighting the potential for reversible assembly strategies in future composite structures.
For automotive manufacturers, this could support component replacement and refurbishment programmes that extend vehicle life while reducing waste. For aerospace applications, it could enable repairable structural assemblies that minimise replacement of high-value composite components.
A STRONG FIT FOR AEROSPACE APPLICATIONS
The aerospace sector may be particularly well positioned to benefit from these developments.
Carbon-fibre-reinforced thermoplastics have attracted growing interest because they combine lightweight performance with weldability and rapid manufacturing. Researchers note that thermoplastic composites offer significant advantages compared with conventional materials, including high strength, corrosion resistance and improved processability. They also support lightweight structures capable of reducing aircraft mass and improving operational efficiency.
Importantly, welding technologies can also simplify maintenance operations. Resistance welding enables damaged sections to be repaired or replaced more easily than traditional bonded structures, helping to extend service life and reduce downtime.
As aerospace manufacturers face increasing pressure to improve sustainability across the full lifecycle of aircraft structures, recoverable thermoplastic composite assemblies could become an important enabling technology.
IMPLICATIONS FOR AUTOMOTIVE LIGHTWEIGHTING
The automotive sector faces a similar challenge. Lightweight composites offer significant opportunities to reduce vehicle mass, extend electric vehicle range and lower energy consumption. However, regulators and manufacturers are increasingly demanding evidence that lightweight materials can also support circular-economy objectives.
Resistance-welded CF/LM-PAEK structures align closely with these requirements. By supporting both high-performance lightweighting and controlled disassembly, they create opportunities for repair, remanufacture and material recovery that are difficult to achieve with conventional composite architectures.
TOWARDS CIRCULAR COMPOSITE STRUCTURES
The significance of this research extends beyond welding process optimisation. It highlights a broader transition occurring within advanced composites engineering—from designing purely for performance towards designing for performance throughout multiple life cycles.
By demonstrating that thermoplastic composite joints can be both structurally robust and intentionally separable, the study provides a practical foundation for future composite structures that are lighter, repairable and ultimately more circular. For aerospace and automotive engineers pursuing ambitious sustainability targets, that combination could prove increasingly valuable in the years ahead.
The study ‘Sustainable joints in thermoplastic composites: An experimental study of disassembly and re-welding’ is published in Materials Design
