Design of weight-reducing adhesive bonding interlayers and investigation of the mechanical properties of adhesively bonded joints fabricated with these interlayers

In this study, auxetic interlayers with re-emtrant, star-shaped, and triangular geometries were designed to reduce adhesive consumption in bonded joints. The interlayers were applied to the bonding region of single-lap joints, and their influence on the mechanical performance was investigated. All interlayers were modeled in the SolidWorks, after which the single-lap joint assemblies were created and subjected to mechanical analysis using the ANSYS. Mating female molds were generated to fully match the auxetic geometries, and these molds were defined as the adhesive material in the analyses. The adherend plates were defined as polylactic acid (PLA), single-lap bonded specimens were created, and axial shear analyses were performed. Axial shear experiments were conducted using an Instron 8874 servo-hydraulic test machine. During specimen preparation, surface cleaning, curing temperature, curing pressure, and external conditions were kept identical for all samples. Based on the analysis results and the mechanical performance of the PLA–PLA bonded joints, re-entrant horizontal and re-entrant vertical auxetic templates, which exhibited the highest strength, were selected and applied to composite and aluminum bonded joints. Specimens were produced using different adherend materials with indented auxetic interlayers, and their axial tensile strengths and elongation capacities were compared. It was observed that the reduction in failure strength of the interlayerede joints was lower than the reduction in adhesive volume. Additionally, while failure in fully bonded joints occurred abruptly, interlayered joints continued to carry load for a certain period after initial damage. This behavior suggests that auxetic-interlayered joints may be promising for bonded applications where high load-carrying capacity is not critical and damage evolution needs to be predicted.