چكيده به لاتين
In the present research, the strain rate dependent mechanical behavior of composite bolted joints was investigated by the theoretical model and the experiment. First, using micromechanical equations, a new theoretical model was presented to predict the bearing stiffness and damage initiation of composite bolted joints under different strain rates. For this purpose, the strain rate dependent elastic mechanical properties of unidirectional composite ply were characterized by using the strain rate dependent elastic properties and strength of the resin and fiber together with their Poisson’s ratio. Then, using these mechanical properties, layups, the geometrical parameters of the joint, and the modified spring-mass model, the bearing stiffness of the joint at any arbitrary strain rate was calculated. By defining the stress intensity factor and by using maximum on-axis stress failure criteria in composite materials, the damage initiation load of the single-lap joint at any arbitrary strain rates was calculated. The innovations of the present theoretical model are the consideration of the change in strain rate, fiber volume fraction, arbitraray layups, geometrical parameters, secondary bending, and nonlinear shear stress-strain behavior. Then, an extensive experimental program was conducted to investigate the effect of increasing the strain rate and adding the carbon nanofiber on the mechanical properties of single-lap composite bolted joint including damage initiation bearing stress, 2% offset bearing strength, ultimate bearing strength, bearing stiffness, ultimate bearing strain,.and energy absorption. For this purpose, laminated composite with the layups of [-45/0/45/90]s and [90/-452/45]s with and without carbon nanofiber was fabricated. The bolt with protruding head with near-fit clearance was considered. The specimens were selected so that the bearing failure mode based on ASTM D5961 took place. Also, by using the general factorial design, an influential factor on the mechanical properties of single-lap composite bolted joints with their percent of effect was calculated. The main result of the present experimental model is the significant effects of the increase in strain rate and adding the carbon nanofibers on the mechanical properties of a single-lap composite bolted joints. To validate the present theoretical model, the conducted experiments and finite element models were used. The results of the present model showed good agreement with the conducted experiments and the FEM model.
