Boosting Adhesive Joint Strength: Experimental Study on Chemical Etching of Carbon Fiber Reinforced Polymers

The 20th century noted the arrival of carbon fibre-reinforced polymer (CFRP) composites, however, since then they have found applications in many engineering domains. Their high strength-to-weight ratio made them ideal for aerospace applications. However, today, they are crucial in distinct industries, ranging from sports equipment to the automotive industry. Even though they are widely utilized, traditional methods of joining like bolting, riveting, and mechanical fastening pose grave challenges for CFRP. Knowing that the composite is anisotropic, these techniques are prone to delamination, generate stress concentrations, and damage the structure of the material. Whereas, the thermoset matrices of CFRP degrade at high temperatures, rendering welding inappropriate. On the other hand, the usage of adhesives is a superior joining method. Not only does it evenly distribute loads, but also minimizes stress concentrations, and maintains the integrity of the CFRP without adding weight or causing damage. However, surface roughness plays a major role in boosting the adhesive bond strength. The present work explores enhancing adhesive bond strength via incrementing surface roughness by chemical etching using sulphuric acid solution. The key controlling parameters of the process are: time, temperature, and acid concentration. Having been etched, the samples are adhesively bonded to either other etched CFRP samples or aluminium alloy plates in a lap joint. Post-curing, the joints undergo lap shear tests under tensile and compressive loads. The fracture surfaces are later analysed using Scanning Electron Microscopy (SEM) to understand the cause of failure. Experiments were performed using four kinds of CFRP substrates, two types of aluminium alloys, and four adhesives. Conclusion is that chemical etching doubles the adhesive bond strength in most cases, while others show at least a 25% gain in strength in comparison to their non-etched counterparts. The process also overpowers mechanical abrasion, however, the etching process must be customized in different CFRPs, owing to variations in composition and manufacturing processes.