A Failure Study of Composite T Joints Under Tensile Loading

Abstract

Composite T-joints subjected to out-of-plane tensile loading remain less studied than conventional lap joints despite their increasing use in hybrid composite–metal structures. This study experimentally investigates the mechanical behavior and failure mechanisms of glass fiber reinforced polymer (GFRP) T-joints mechanically fastened to steel components using preloaded bolts. Twenty-seven specimens fabricated from triaxial E-glass/epoxy laminates with a [0/±45]6 layup were tested under quasi-static tensile loading. The investigated parameters were the width-to-diameter ratio (W/D = 4, 5, and 6) and bolt tightening torque (15, 33, and 50 Nm). Load–displacement responses were analyzed to identify delamination initiation and ultimate failure loads. Delamination loads ranged from approximately 2.20 kN to 7.80 kN, while final failure loads ranged from 4.65 kN to 14.62 kN. Analysis of variance (ANOVA) revealed that the width-to-diameter ratio significantly influences both delamination initiation and ultimate failure loads, whereas bolt preload primarily affects the ultimate load capacity. Interaction effects between W/D and tightening torque were statistically insignificant. Failure typically initiated through matrix cracking and delamination in the fillet region before propagating toward the bolt location, where final failure occurred through net-tension, shear-out, or cleavage modes. Multinomial logistic regression indicated that net-tension and shear-out were the dominant failure modes, while cleavage occurred less frequently. The results provide experimentally derived insight into the structural response and failure behavior of hybrid GFRP–steel T-joints subjected to tensile loading and contribute data relevant for the design of mechanically fastened composite joints.