Stiffening of single-lap hybrid joints reinforced with self-tapped screws

The paper discusses the method of stiffening of single-lap hybrid joints of glass-fiber reinforced plastic and metal adherents with transversal reinforcement by self-tapped screws. The method uses a filling of gaps between screw and hole-wall by room-hardened epoxy resin. The healing results to stiffening of joint due to the growing of element-adherent interaction area and the reduction of residual damages of GFRP, which occurred after a screw mounting process. Glass-fiber reinforced plastic adherent was made of polyester resin and glass mat with surface density of 300 g/m2; metal adherent was made of hot-rolled sheet of mild steel. Adhesive with thickness of 3 mm is low-modulus polyurethane composition. Steel self-tapped screws with diameter of 2.5 mm were used as reinforce elements. Cyclic longitudinal tension-compression tests were performed for 15 original and 15 mo-dified specimens (by 5 specimens for each one of 3 loading levels: ±500, ±600 and ±700 N). The growth of mechanical compliance for more than 30 % in comparing with initial condition was accepted as limit condition for original specimens; for modified specimens this condition formulated as the growth of mechanical compliance up to the level of original specimens. Modified specimens had equivalent stiffness (the ratio between total amplitudes of load and hysteresis loop of specimen test diagram) higher than non-modified by an average 35 %. Also the both types of specimens demonstrated the slow monotonic stiffness reduction. The estimation of stiffness reduction slope shows that a stabilization of the deformation process will appear after a few hundred cycles.