Hybrid flax/carbon bonded composite patches for strengthening of steel plates: layup and adhesive effect

Abstract

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.