The ability to connect and integrate components is a critical aspect of the manufacturing process. Mechanical fastening, achieved through mechanical joining, is a commonly employed technique. However, there are various drawbacks associated with the use of these fasteners, including the potential for damaging drilling components, increasing the overall weight of the structure, and introducing significant localized stresses [1]. Additionally, considering the involvement of manual labor, this method is time-consuming and lacks flexibility.
Conversely, adhesive joints offer several benefits such as even distribution of stress, improved resistance to fatigue, reduced structural weight, and the capability to join dissimilar materials [2, 3]. These advantages have rendered adhesive joints highly attractive in industries like aviation, marine, and automotive. It's important to acknowledge, however, that adhesive joints are often the weakest link in a structure due to their relatively limited fracture tolerance and susceptibility to harsh environmental conditions [4].
The performance of adhesive joints can be significantly affected by various factors, including the material properties of the adhesive, the thickness of the adhesive layer, the condition of the bonding surfaces, and the design of the joint [5–7]. Ensuring adequate bonding strength, resistance to fracture, and enduring durability within adhesive joints heavily relies on the attributes of the bonding surfaces. The primary objective of surface pretreatment is to remove any surface impurities like lubricants or oils and to address vulnerable boundary layers. This process aims to enhance the bonding area by optimizing wettability, establish mechanical interlocking, and induce alterations in the surface morphology through chemical means [8]. A range of surface treatment techniques, including mechanical roughening [9, 10], peel ply [11, 12], chemical treatments [13], and plasma treatments [14–15], have been utilized to attain desired surface characteristics for composite lamintes.
The surface texture and roughness play a direct role in influencing how adhesive flows and spreads across the substrate surface. Optimal surface roughness can facilitate adhesive penetration into the substrate, leading to an increase in the effective bonding area [16, 17]. Additionally, the concept of mechanical interlocking suggests that once the adhesive fills the gaps on the substrate surface, the cured adhesive establishes interlocking connections with the uneven surface topography of the substrate [17]. Furthermore, research indicates that the impact of mechanical interlocking is amplified with higher surface roughness, consequently resulting in enhanced bonding strength [18]. Gude et al. [19] conducted a study examining the relationship between surface attributes (specifically roughness and surface free energy) and the mechanical properties of adhesive joints. The experimental findings highlighted that mechanical interlocking primarily governed the adhesion mechanism for mode I testing. Interestingly, in the context of shear strength, the results indicated that an increase in surface roughness did not necessarily lead to an improvement, and instead, the polar component of the surface free energy exerted a more substantial influence on shear strength.
Numerous researches have investigated the impacts of diverse surface treatment approaches on adhesive-bonded joints. In the study by Kumar et al. [20], atmospheric pressure plasma treatment was employed to alter the bonding surfaces of adhesive single lap joints. The research demonstrated that this treatment method could be deemed efficient in enhancing the adhesive properties of graphite-epoxy laminate surfaces. In the work conducted by Sun et al. [21], an exploration was conducted into the impact of different patterns generated on the bonding surfaces through laser treatment on the fracture behavior of adhesively bonded joints. The outcomes demonstrated that introducing both longitudinal and transverse grooves on the bonding surfaces of the joints resulted in an elevation of the joint mode-I fracture energy value. In the investigation by Martinez-Landeros et al. [22], an examination was carried out to understand the impact of different surface pretreatment techniques, such as solvent cleaning, sanding, chemical etching, and peel ply, on the fracture characteristics of carbon fiber-reinforced composites. The findings suggested that the method of mechanical abrading yielded the most significant enhancements in terms of load-bearing capacity and fracture energy of the bonded joints.
The sanding technique, employed to prepare bonding surfaces, represents a pragmatic and straightforward approach that yields numerous advantages for enhancing the effectiveness of adhesive joints in composites. Through the regulation of surface roughness, sanding enlarges the bonding area and facilitates superior adhesive wetting. This, in turn, contributes to heightened maximum bonding strength, enhanced fracture resistance, and increased long-term durability. In a study by Yang et al. [9], the impact of sandpaper grit size and sanding direction on the tensile properties of carbon fiber-reinforced polymer (CFRP) single lap bonded joints was examined. The findings highlighted the significant influence of sandpaper grit size on surface characteristics. Moreover, it was observed that the highest strength was achieved when the surfaces were abraded in random directions.
The enhancement of bonding strength and fracture toughness in the polymer matrix of fiber-reinforced polymer composites (FRPCs) has also been explored through the incorporation of second-phase additives. These additives can vary in terms of size, material composition, and geometric characteristics. Out of the numerous options for second-phase additives, a notable focus has been on nanofillers, with a specific emphasis on carbon-based materials like carbon nanotubes (CNTs) and graphene nanoplatelets (GNPs). These nanofillers have garnered significant interest in recent times due to their ability to contribute to various toughening mechanisms during crack propagation, ultimately leading to an enhancement in the fracture resistance of polymers [23–26]. In a study by Gude et al. [27], they incorporated CNTs and carbon nanofibers (CNFs) into an epoxy adhesive. As a result, they observed significant improvements in the adhesive's fracture energy, with a 10% increase for CNTs and a 23.5% increase for CNFs. Khoramishad et al. [28] conducted a study to evaluate the improvement in the mode-I fracture characteristics of an epoxy adhesive through the introduction of MWCNTs. Their findings revealed a remarkable enhancement, showing a maximum 58.4% increase in the mode-I adhesive fracture energy upon adding 0.3 wt% MWCNTs.
While there are existing studies in the literature concerning the reinforcement of composite laminated adhesive joints [29, 27], it's worth noting that the synergistic impacts resulting from the combination of surface treatment techniques and the incorporation of nanomaterials at different weight percentages on the fracture behavior of glass fiber-reinforced polymer (GFRP) joints have not been explored in previous research. Hence, the present study aims to conduct an in-depth exploration of the combined influence of the sanding method using various grit sizes (60, 240, 800) and the incorporation of CNTs at different weight percentages (0.1%, 0.3%, 0.5% wt) on the fracture behavior of GFRP joints. This research seeks to shed light on the potential synergistic effects arising from these dual factors and their impact on the mechanical performance of the adhesive joints in GFRP structures. Various surface parameters of GFRP adhesive joints were meticulously examined, encompassing factors such as surface morphology, surface roughness, surface free energy, and contact angle. The study focused on determining the fracture energy of adhesive joints. Additionally, the research delved into analyzing how surface treatment and nanoparticle addition influences the fracture behavior of these adhesive joints.