Sustainable design of composite–metal adhesive joints via re-entrant auxetic interlayers: balancing strength and adhesive reduction

Purpose This study aims to address the environmental and economic limitations associated with conventional epoxy-based adhesive joints by introducing a re-entrant auxetic interlayer concept that reduces adhesive usage while preserving structural performance in composite–metal joints. Design/methodology/approach Single-lap joints were fabricated using aluminum–aluminum, aluminum–composite, and composite–composite adherend combinations. A 3D-printed polylactic acid auxetic interlayer was incorporated between the adherends and bonded using Araldite 2015 epoxy. Mechanical performance was assessed through comparative testing between fully bonded joints and joints containing the auxetic interlayer, with specific attention to adhesive volume reduction and failure behavior. Findings The auxetic interlayer reduced adhesive consumption by 61.8% with only an approximately 26% reduction in joint strength relative to fully bonded counterparts. Additionally, interlayered joints demonstrated more gradual and predictable failure characteristics, indicating improved damage tolerance and more stable mechanical response. Originality/value This study presents a novel application of re-entrant auxetic geometry as an interlayer for adhesive joints, offering a sustainable design strategy that simultaneously decreases adhesive usage and enhances failure predictability. The concept provides a promising pathway for environmentally conscious and mechanically efficient adhesive joint design in next-generation composite–metal assemblies.