Adhesive bonding of polymer composites reinforced with carbon unidirectional non-crimp fabrics

Lightweight materials have always played a significant role in product design, but global trends toward emissions reduction and resource efficiency have further increased the importance of this topic. In particular, the introduction of CO2 emissions targets and correlated penalties is increasing the interest in lightweight materials, such as fiber reinforced polymers (FRP), which are set for significant market growth in the automotive industries. Automotive industries would like to use FRP more broadly; however, the use is still limited because of the high manufacturing costs and long cycle times. But the recent developments in the area of manufacturing technologies are now paving the way to more widespread adoption. Newly developed manufacturing technology, such as high-pressure resin-transfer moulding (HP-RTM) can reduce the manufacturing cost and time. The technology provides a strong potential to incorporate the CFRP composite materials in high-volume production vehicles. However, the current adoption speed in high-end products and critical load-bearing applications is still limited by the ability to join CFRP efficiently. Structural adhesive bonding represents a feasible joining method for FRPs because it entails several advantages, such as the reduction of stress concentration, and assembly cost and time. FRP surfaces are primarily composed of a polymer matrix and can be affected by the presence of contaminants such as silicones and fluorocarbon from release compounds that represent leftovers of the fabrication process. It is important to perform a surface preparation step before bonding to remove such contamination and possibly increase the strength of adhesive joints. In the case of FRPs, the outcome of the surface preparation step is strictly depends on the actual structure and composition of the composite material, especially in the near-surface region, which is the one that mostly dictates the ability of an adhesive to establish strong adhesion and a durable bond. Concerning surface preparation and bonding of carbon fiber reinforces polymers (CFRP) manufactured using the HP-RTM technology there is still a relative paucity of contributions, especially on the existing interplay between surface modification, near-surface composite structure and the mechanics of deformation and fracture. The goal of this work was to complement the existing studies in the field and ascertain the mechanical behaviour of adhesive joints comprising composite adherents that feature a non-crimp fabric (NCF) textile whereby carbon fiber tows (or yarns) are stitched together using polyester yarns. The research work includes surface pre-treatment and characterization, mechanical testing and fractographic analysis. Surface preparation is crucial for the mechanical performance of adhesive joints. In this work, both manual sanding and UV laser irradiation (355 nm) is investigated. The morphology and topography of the target surfaces before and after surface preparation is ascertained using optical and confocal microscopy studies. Besides, the wettability is assessed using contact angle measurements. Two load cases are considered for mechanical testing of the composite joints, the Double Cantilever and the End Notch Flexure test configurations. The aim is to determine the mode I and mode II fracture toughness of the joints. In-situ CCD imaging during testing, post-failure visual inspection, and optical observations are combined to shed light on the mechanisms of deformation and fracture of the joints. The obtained results highlight the peculiar interaction between surface preparation and the resulting complex surface structure of the composite. Although the treated surfaces did not show an improved wettability after sanding, the mechanism of failure of the joint was cohesive and the mode I fracture toughness was in good agreement with that reported in previous related studies and above the values commonly reported for CFRP/epoxy joints in the existing literature. The mode II fracture tests also displayed cohesive fracture but highlighted a mechanism of deformation whereby the ductility of the adhesive played a much more relevant role and led to the development of a fracture process zone that spanned several millimetres in length. Remarkably, the results of mechanical tests executed on adhesive joints with sanded interfaces were better than those obtained after UV-laser treatment. Although the investigation of the laser process was still preliminary and of exploratory nature, some interesting indications already emerged from this study, including the need for a fine-tuning of the processing variable that can prevent or mitigate surface degradation through photothermal ablation