Experimental study on carbon fiber reinforced polymer laminate-steel lap joints connected with film adhesive at room temperature

In order to alleviate the difficulty in thickness control and forming for paste adhesive layer in fabrication of carbon fiber reinforced polymer(CFRP) laminate-steel lap joints, a type of film adhesive was used to replace traditional paste adhesives. A total of 15 CFRP laminate-steel double lap joints connected with a film adhesive, with five kinds of bond lengths, were fabricated, and experimental studies were conducted focused on the failure modes, effective bond length, force transfer law, bond-slip constitutive and bearing capacity at room temperature. The results show that the bond strength of the film adhesive used is slightly higher than the interlaminar strength of the CFRP laminates (i.e. the bond strength between carbon fibers and resin matrix). The effective bond length of the film adhesive-connected CFRP laminate-steel joints at the room temperature is approximately 80 mm. At the beginning of loading, the maximum shear stress is located at the end of the joint steel plate. Its position moves towards the end of the joint CFRP laminate with the increase of load. At the end of loading, its position is located at 20 mm (when the bond length is less than 80 mm) or 50 mm (when the bond length is greater than 120 mm) from the end of the joint steel plate. The bond-slip relationship of film adhesive-connected CFRP laminate-steel lap joints can be simplified as a trapezoid model which is significantly different from the triangle model of paste adhesive-connected joints, suggesting that the ductility of the film adhesive-connected joints has been greatly improved. The representative values (quasi mean values) of peak shear stress, fracture energy and interfacial stiffness of the film adhesive-connected joints are 1.2-3.0 times, 1.6-5.7 times and 5.4-7.5 times, respectively, of those of joints with four typical paste adhesives. When the bond length is not less than the effective bond length, the representative value of tensile capacity of the film adhesive-connected joints are 1.25-2.39 times of the joints connected with paste adhesives. The variation coefficients of tensile strength and maximum displacement of the film adhesive-connected joins almost have no difference from the joints connected with paste adhesives.