Role of bolt pretension in determining stress and load-bearing capacity of hybrid joints

This study investigated the effect of bolt pretension on the load-bearing capacity and stress distribution of hybrid joints combining adhesive bonding and mechanical fastening, using two brittle epoxies (SW7240 and DP460) and one ductile polyurethane (SF479). Experimental results showed that the brittle epoxies consistently maintained a load capacity above 300 kN, even at low pretension levels (10% of the maximum 65 kN), whereas the polyurethane exhibited marked sensitivity to pretension, with its load capacity increasing from 220 kN at 10% pretension to approximately 300 kN at full pretension. Despite these differences, all adhesives displayed brittle failure, characterized by cohesive failure near the adhesive layer's surface. Material characterization revealed linear-elastic behavior for the epoxies and bi-linear behavior for the polyurethane, which allowed for effective stress redistribution. Multiaxial stress analysis indicated that pretension-induced compressive stresses critically enhanced the polyurethane's performance by shifting the stress state to delay failure, while localized stress concentrations limited the benefits of pretension for the epoxies. Finite element analysis supported these observations, showing that pretension had minimal effect on the concentrated tensile stresses in epoxy- bonded joints but improved stress distribution in polyurethane-bonded joints, enabling them to sustain higher loads without exceeding failure thresholds. These findings emphasize the importance of adhesive selection and the interaction between pretension and stress management, with brittle epoxies offering robust performance irrespective of pretension, while ductile polyurethanes leverage pretension to achieve enhanced load-bearing capacity under combined loading conditions.