Efficiency of single-lap hybrid screwed / bonded joints under cyclic tension-compression

The paper presents investigation results of the mechanical behavior of single-lap hybrid composite/steel joints strengthened by self-tapping screws under low-cyclic tension-compres-sion (from 1•103 up to 2•104 cycles). Joints made of glass-fiber reinforced plastic and metal adherents with transversal reinforcement by self-tapping screws with small diameter. Also the developed method of joints durability improving was examined. The method uses a filling of gaps between screw and hole-wall by room-hardened epoxy resin. Cyclic tests performed on Instron Electropuls 3000 test machine with symmetric loading for two types of specimens: original and modified using developed method. The main damage growth mechanisms in the specimens was identified and explained for the tension-compression cyclic load with the amplitude in the range 150 to 200 N. The damage progression resulted in the distinctive features of the “force-displacement” cyclic diagrams and the histories of equivalent compliance (the ratio between total amplitudes of hysteresis loop and load of specimen test diagram). The rapid increase of the equivalent compliance at the limit state was explained as a crack growth in a self-tapping screw. The low-cycle fatigue test results of modified specimens can be acceptably approximate by power functions. These functions allowed to devise two empirical equations, which can reasonably extrapolate test data up to million cycles. Modified specimens demonstrate 4,2 times higher cycle numbers before reaching the limit state at the same level of compliance and 1,5 times smaller scatter of the test results. It confirms the effectiveness of developed method of joints durability improving.