Adhesive bonding of fiber-reinforced polymer (FRP) patches is increasingly used to strengthen steel structures. While carbon FRP (CFRP) and epoxy adhesives are the primary materials in industrial applications, this study explores hybrid Carbon/Flax FRP as an alternative for reinforcing steel plates under flexural loading. Four composite layups were tested: F5 (flax), C5 (carbon), CFC, and FC (carbon/flax hybrids). These patches were bonded to steel plates using three adhesives: a flexible and ductile silane-modified polymer (SMP-FD), a medium flexibility-ductility acrylate (ACR-MFD), and a rigid and brittle epoxy (EP-RB), representing a wide range of adhesive properties. Three-point bending tests were conducted to evaluate mechanical performance compared to unreinforced steel plates. Results demonstrated that composite patch bonding significantly enhances load-bearing capacity. The EP-RB adhesive provided the highest reinforcement, followed by ACR-MFD and SMP-FD. Hybrid FC and CFC configurations achieved reinforcement comparable to or greater than pure carbon (C5), highlighting the potential of hybrid designs for structural applications.

Adhesive bonding of Fiber-Reinforced Polymer (FRP) patches is increasingly used to strengthen steel structures. Considering that carbon FRP (CFRP) and epoxy adhesives are the primary materials in industrial applications, this study explores the feasibility of hybridizing CFRPs with Flax FRPs (FFRPs) and the effects of using different adhesives on the mechanical performance of reinforced steel plates under flexural loading. Four configurations of composite layups were manufactured, namely: F5 (flax), C5 (carbon), CF3C and F3C2 (carbon/flax hybrids). These patches were bonded to steel plates using three adhesives: a rigid and brittle epoxy (adhesive A), a medium flexibility-ductility acrylate (adhesive B), and highly flexible ductile silane-modified polymer (adhesive C), representing a wide range of adhesive properties. Quasi-static three-point bending tests were conducted to evaluate mechanical performance of these novel hybrid composite-adhesive-steel structures compared to unreinforced steel plates, and Digital Image Correlation (DIC) was performed to monitor displacements and strains on the outer surface of the composite patch. Furthermore, finite element models were built and validated by experimental data, which were then used to predict development of stresses in each part of the assembly. Overall, it has been demonstrated that composite patch bonding can significantly enhance the load-bearing capacity of the reinforced steel substrate. Adhesive A provided the highest flexural strengthening but displayed brittle failure behavior, followed by adhesive B which showed second best flexural strengthening performance while keeping a ductile behavior at failure. Adhesive C showed poor flexural strengthening enhancement due to the poor stress transfer capabilities of the adhesive. Hybrid F3C2 and CF3C configurations achieved flexural performance comparable to pure carbon (C5), highlighting the potential of hybrid designs for structural applications.

This chapter discusses the mixed-mode loading of adhesive joints. The importance of mixed-mode loading is first introduced and then test methods commonly used to measure the mixed-mode fracture resistance of adhesive joints are presented and briefly discussed. The approaches to determine the fracture resistance are briefly reviewed and then the partitioning of mixed-mode fracture energies is discussed. The limitations of the local singular field and global approaches to mixed-mode partitioning are discussed and the use and application of a semianalytical cohesive zone analysis partitioning scheme is evaluated. The limitations of the global partitioning approach are further discussed in the context of developing a scheme to design and analyze adhesive joints with dissimilar adherends (a bi-material interface). A longitudinal strain criterion is proposed in addition to the matching of flexural rigidities and the approach is validated numerically. Finally, the practical issues of crack stability, failure path selection, and the use of mixed-mode failure envelopes is considered.