Investigation of brittle adhesive reinforcement for single lap shear joints with composite adherends

Damghani, Mahdi; Naidu, Calvin; Cole, Daniel

Abstract

Adhesive bonding of composite structures is a promising alternative to mechanical fastening due to improved load distribution and reduced stress concentrations. However, the brittleness of high-stiffness epoxy adhesives poses challenges, especially under high loads. This study investigates reinforcement strategies for brittle adhesives in single lap shear joints using micro-scale glass beads (GB) and milled carbon fibres (MCF), both individually and in combination. Four adhesive configurations were tested: pure epoxy (XA120), GB-reinforced (10 %, 15 %, 20 % wt), MCF-reinforced (2.5 %, 5 %, 7.5 % wt), and a hybrid formulation (10 % GB + 5 % MCF). Moderate filler contents (10 % GB, 5 % MCF) enhanced joint stiffness and peak force, while excessive filler loading led to agglomeration and reduced performance. The hybrid formulation demonstrated balanced stiffness and strength due to the combined effects of spherical and fibrous fillers, reducing stress concentrations. Failure analysis revealed that pure XA120 and MCF-7.5 % joints primarily exhibited cohesive failure, while high GB content (20 %) increased interfacial debonding. The hybrid configuration showed mixed failure modes, suggesting variable stress distribution from non-uniform filler dispersion. Bootstrap resampling (n = 1000) confirmed that moderate GB content (15 %) provided consistent performance with narrow confidence intervals despite limited sample sizes. These findings underscore the importance of optimising filler content to balance stiffness, strength, and durability. While the hybrid formulation offers promising strength–toughness balance, further work is needed to improve filler dispersion and consistency for critical applications.

• Journal Article Type: Article
• Acceptance Date: Jul 30, 2025
• Online Publication Date: Aug 2, 2025
• Publication Date: Oct 1, 2025
• Deposit Date: Aug 3, 2025
• Publicly Available Date: Aug 3, 2026
• Journal: Structures
• Electronic ISSN: 2352-0124
• Publisher: Elsevier
• Peer Reviewed: Peer Reviewed
• Volume: 80
• Article Number: 109844
• DOI: https://doi.org/10.1016/j.istruc.2025.109844
• Public URL: https://uwe-repository.worktribe.com/output/14793335