Experimental investigation of bio-inspired Bouligand-type CFRP adherends in adhesive joints under multi-rate and multi-directional tensile loading

Abstract

This study explores the mechanical performance of adhesively bonded single-lap joints (SLJs) with composite adherends featuring various stacking sequences, including bio-inspired gradual Bouligand helicoidal (G), conventional helicoidal (H), unidirectional (UD), and quasi-isotropic (QI) configurations. Joints were tested under multi-directional (0°, 45°, 90°) and multi-rate tensile loading. A failure load envelope was developed, revealing that UD joints (known for its high stiffness) performed well in 0° loading but showed strong anisotropy at other angles. In contrast, G-type joints demonstrate superior multi-directional integrity, making them promising for applications with variable loading directions. These findings highlight the significance of stacking sequence in optimizing composite SLJ designs for structural applications. Since all joints demonstrated higher performance under tensile loading in the zero-direction, the study was extended to evaluate their behavior under multi-rate tensile testing. Correspondingly, while G- and H-type SLJs exhibited strength comparable to UD joints due to cohesive failure, they showed greater failure displacement and, consequently, higher energy absorption.