# Utilizing Excess Resin in Prepregs to Achieve Good Performance in Joining Hybrid Materials

## TL;DR

This study examines how different adhesives and layer thicknesses affect the strength of joints between carbon fiber composites and steel, showing that a thick structural epoxy significantly improves performance.

## Contribution

The study introduces a novel comparison of thin prepreg resin and thick structural epoxy adhesives for hybrid material joints.

## Key findings

- Sika Power-533 adhesive increases critical energy release rate by up to 30 times compared to prepreg resin.
- Thick Sika adhesive performs consistently with both uncoated and Al–Si-coated steel, failing at the composite/Sika interface.
- Thin resin adhesive shows poor bonding with uncoated steel, detaching during preparation.

## Abstract

This study investigates the fracture toughness of adhesive joints between carbon fiber-reinforced polymer composites (CFRP) and boron-alloyed high-strength steel under Mode I and II loading, based on linear elastic fracture mechanics (LEFM). Two adhesive types were examined: the excess resin from the prepreg composite, forming a thin layer, and a toughened structural epoxy (Sika Power-533), designed for the automotive industry, forming a thick layer. Modified double cantilever beam (DCB) and end-notched flexure (ENF) specimens were used for testing. The results show that using Sika Power-533 increases the critical energy release rate by up to 30 times compared to the prepreg resin, highlighting the impact of adhesive layer thickness. Joints with the thick Sika adhesive performed similarly regardless of whether uncoated or Al–Si-coated steel was used, indicating the composite/Sika interface as the failure point. In contrast, the thin resin adhesive layer exhibited poor bonding with uncoated steel, which detached during sample preparation. This suggests that, for thin layers, the resin/steel interface is the weakest link. These findings underline the importance of adhesive selection and layer thickness for optimizing joint performance in composite–metal hybrid structures.