Effect of Mechanical Pretreatments on Damage Mechanisms and Fracture Toughness in CFRP/Epoxy Joints

Abstract

Adhesive bonding of carbon-fiber-reinforced polymers (CFRPs) is a key enabling technology for the assembly of lightweight structures. Surface pretreatment is necessary to remove contaminants related to material manufacturing and ensure bond reliability. The present experimental study focuses on the effect of mechanical abrasion on the damage mechanisms and fracture toughness of CFRP/epoxy joints. The analyzed CFRP plates were provided with a thin layer of surface epoxy matrix and featured enhanced sensitivity to surface preparation. Various degrees of morphological modification and fairly controllable carbon fiber exposure were obtained using sanding with emery paper and grit-blasting with glass particles. In the sanding process, different grit sizes of SiC paper were used, while the grit blasting treatment was carried by varying the sample-to-gun distance and the number of passes. Detailed surveys of surface topography and wettability were carried out using various methods, including scanning electron microscopy (SEM), contact profilometry, and wettability measurements. Mechanical tests were performed using double cantilever beam (DCB) adhesive joints. Two surface conditions were selected for the experiments: sanded interfaces mostly made of a polymer matrix and grit-blasted interfaces featuring a significant degree of exposed carbon fibers. Despite the different topographies, the selected surfaces displayed similar wettability. Besides, the adhesive joints with sanded interfaces had a smooth fracture response (steady-state crack growth). In contrast, the exposed fibers at grit-blasted interfaces enabled large-scale bridging and a significant R-curve behavior. While it is often predicated that quality composite joints require surfaces with a high percentage of the polymer matrix, our mechanical tests show that the exposure of carbon fibers can facilitate a remarkable toughening effect. These results open up for additional interesting prospects for future works concerning toughening of composite joints in automotive and aerospace applications.

Snap-through Crack Propagation in Architected Bonded Interfaces Analyzed Using a Mechanoluminescent SAO/E Coating

“…Motivated by the increasingly widespread usage of adhesives in automotive and aerospace, toughening of adhesive joints is currently the subject of an intense research effort across academia and industry. − Structural interfaces represent highly stressed regions where potential defects can nucleate and grow, thereby leading to catastrophic failures of adhesive bonds. Surface pre-treatments, such as grit-blasting, chemical etching, laser texturing, and plasma cleaning, are traditionally used to improve adhesion, joint strength, and fracture toughness. ,,, …”

“…Newly developed toughening strategies specifically target the interfacial adhesion landscape, the bondline architecture, or a combination of both, to control the path of crack propagation, reduce crack tip stress intensity, and improve the joint toughness. − For example, tailored interfacial defects (i.e., areas with reduced adhesion) were employed by Tao et al , to enable the formation of adhesive ligaments in adhesive-bonded composite joints. The energy absorbed by ligament deformation was re-distributed throughout the adjoined layers, thereby reducing the crack tip stress intensity and delaying fracture.…”

“…Surface pre-treatments, such as grit-blasting, chemical etching, laser texturing, and plasma cleaning, are traditionally used to improve adhesion, joint strength, and fracture toughness. 1,2,4,5 Newly developed toughening strategies specifically target the interfacial adhesion landscape, the bondline architecture, or a combination of both, to control the path of crack propagation, reduce crack tip stress intensity, and improve the joint toughness. 5−10 For example, tailored interfacial defects (i.e., areas with reduced adhesion) were employed by Tao et al 6,7 to enable the formation of adhesive ligaments in adhesive-bonded composite joints.…”

Comparative Study of Durability Behaviors of Thermoplastic Polypropylene and Thermosetting Epoxy Exposed to Elevated Temperature, Water Immersion and Sustained Bending Loading

“…Generally, thermosetting matrix (such as epoxy and polyurethane) can be used as the matrix of FRP [ 14 ], which has the advantages, such as simple processing technology and production efficiency, high strength and lower viscosity [ 15 ], which lead to a much wider use of epoxy resin composites. However, the three-dimensional network cross-linking structures of epoxy lead to poor fracture toughness, which is the key factor in brittle failure and fatigue damage for FRP [ 16 , 17 ]. In addition, the molecular structure of epoxy contains a large number of hydroxyl groups, which easily form hydrogen bonds with water molecules in a service environment, resulting in the hydrolysis and plasticization of the matrix and fiber/matrix interface debonding [ 18 , 19 ].…”