Effect of thickening configuration on tensile after impact of thickened CFRP bonded single lap joints

Adhesive bonded joints are highly susceptible to impact induced damage and subsequent strength degradation. In this study, effect of local thickening and thickening layup orientation on impact endurance and damage tolerance of bonded single lap joints (SLJ) is experimentally investigated. Standard and locally thickened laminates were fabricated with carbon fiber andepoxyresin system and bonded joints using adhesive. Four joint configurations were investigated. The baseline configuration consisted of un-thickened pure carbon (PC). The remaining three were thickened configurations, in which additional layers were applied at the overlap region of the baseline joint to mitigate impact vulnerability by providing a cushioning effect and protecting the bond line from critical damage. These thickened configurations employed different layup sequences for the added layers: [±45°] for angle-ply thickened carbon (APTC), [0/90°] for cross-ply thickened carbon (CPTC), and a combination of 0°, 90°, and ±45° plies for mixed-ply thickened carbon (MPTC). Low velocity transverse impact tests were conducted using drop weight impact system at impact energies of 3 J,5 J,7 J and 9 J, followed by tensile after impact (TAI) to evaluate residual strength. Mass normalized TAI concludes that CPTC configuration exhibited the highest residual strength, achieving an improvement of 358.1% compared to un-thickened PC joint, whereas the APTC configuration showed lowest residual strength of 22.8%. Scanning Electron Microscopy (SEM) revealed that elevated peel stresses in APTC promoted Mode-I opening of impact-induced adhesive cracks, leading to cohesive failure, while increased interfacial shear in CPTC and MPTC joint resulted in adhesive dominant failure. These findings demonstrate that optimal ply orientation in thickening region provides an effective design strategy for enhancing impact performance and failure endurance of composite adhesive bonded SLJs.