| Fiber reinforced polymer(FRP)pultrusions have the advantages of light-weight,high strength,corrosion resistance,and superior uni-directional mechanical properties along the fiber direction.When applied to truss structures,the uni-directional mechanical properties of FRP pultrusions can be fully utilized,which can effectively solve the problem of traditional steel corrosion,improve the safety and lifetime of structures,and greatly reduce the maintenance cost of structures.However,currently,only a few FRP truss structures constructed and applied in the civil engineering.One of the main reasons is that the low joints' efficiency in FRP structures(at the level of 20% ? 30%)limits the utilization of the FRP,which can not highlight the lightweight and low cost-effective features of FRP.Currently,three types of joint technologies are commonly used,i.e.,bolted,bonded,and hybrid(bolted and bonded)joints.When bolted joints are applied in structures,the ultimate failure load of joints is very low with the brittle failure mode of shear-out due to the uni-directional behavior of FRP pultrusions.Brittle failure occurrs in the bonded joints since epoxy adhesive layer is commonly selected.For the hybrid joints,bloted and boned connections cannot carry the load together due to the large stiffness discrepancy between them.Thus,the aim of this thesis is to improve joints' efficiency to utilize the strength of the FRP pultrusions in FRP structures and failure modes to provide ductility and excellent energy dissipation for the whole structure.Based on the integrated design of members and bolted joints,different fiber architectures of basalt FRP(BFRP)laminates were selected to improve failure modes of bolted joints and optimize the fiber architecture for the FRP pultrusions.The static and long-term(fatigue and creep)behavior of bolted joints,bonded joints and hybrid joints are comprehensively and systematically investigated.The main contents and conclusions are as follows:(1)Integrated design of multi-directional fiber architectures for BFRP pultrusions.Six configurations of hand lay-up laminates were designed with different ply angles and proportions which were composed of 0°,45°,-45° and 90° layers to investigate the effect of the fiber architecture on the mechanical behavior of laminates and joints.The experimental results showed that the failure mode of bolted joints and ultimate failure load could be effectively improved with the multidirectional fiber architecture,but the reduction of laminates' stiffness could not be avoided.Therefore,based on the integrated design of laminates and joints,the optimal fiber architecture of pultrusions for truss structure was determined as follows: 70% ? 0°? 80%,and the multi-directional fiber architecture(45°,-45°and 90°)was designed with the same proportion.(2)Static and fatigue behavior of BFRP MD bolted joints.The effect of fiber architecture on the behavior of bolted joints were experimentally investigated and compared to the unidirectional(UD)pultrutions.The parameter investigation on bolted joints was experimentally conducted including bolt diameters,hole-clearances,pre-tightened torques,number of bolts,and pitch distances.The fatigue life and degradation of the MD bolted joints were investigated and compared to UD bolted joint configuration.The experimental results showed that MD bolted joints could not only change the failure mode from the brittle shear failure to the ductile bearing failure,but also improve the ultimate failure load compared to the UD bolted joints and the resistance of the former was twice that of the latter.The F-N model of MD bolted joints was established from the experimental results.Compared to the UD joints,the fatigue life and energy dissipation capacity of MD bolted joints were greatly improved.(3)Static and fatigue behavior of BFRP MD bonded joints.The behavior of bonded joints composed of acrylic adhesive(ADP)with low stiffness was investigated and compared to the epoxy adhesive.The parameter investigation on bonded joints was experimentally conducted including adhesive layer thickness,overlap length,and loading rate.The static and creep behavior of the bonded joints under elevated temperatures were conducted due to the low glass transition temperature of ADP adhesive(negative).The fatigue experiments of the bonded joints were conducted under different load levels.The experimental results showed that the bonded joints with ADP adhesive layer showed excellent ductile and bilinear behavior compared to the EP configuration.In addition,the shear stress in the ADP bonded joints along the overlap length was uniformly distributed.The F-N model of bonded joints was established from the fatigue experiments and the interaction of creep-temperaturefatigue of bonded joints was clarified.The ultimate failure displacement of bonded joints was determined by the strain criterion obtained from different configurations of the experiments.(4)Static and fatigue behavior of BFRP MD hybrid joints.Based on the behavior of bolted(UD and MD)and bonded joints(EP and ADP),different configurations of hybrid joints could be combined and investigated.The effects of loading rate and overlap length on the hybrid joints were investigated.The fatigue experiments of the hybrid joints were conducted.The experimental results showed that the ultimate failure load of the MD-ADP hybrid joints can be linearly summed,which showed that the ultimate failure load and ductility of the hybrid joints were improved.The F-N model of hybrid joints was established from fatigue experiments,however,the fatigue behavior of hybrid joints was determinded by the bonded joints.The ultimate failure displacement of hybrid joints was dependent on bonded joints and determined by strain failure criterior.(5)Evaluation and design of BFRP MD bolted,bonded and hybrid joints.The static behavior(ultimate failure load and ductility),creep behavior and fatigue behavior were evaluated among these three types of joints(bolted,bonded and hybrid joints).Bolted joints composed of MD pultrusions and bonded joints composed of ductile ADP adhesive layer showed excellent ductility and deformation capacity,thus the ultimate failure load of the hybrid joints was the linear summation of bonded and bolted connection parts.As a result,the joint efficiency was significantly improved.The ADP bonded joints were sensitive to the sunstained load and prone to creep.However,the existence of bolt in the hybrid joint limited the creep of the adhesive layer and further improved the fatigue life and energy dissipation capacity of hybrid joints.Based on the established F-N model from the fatigue experiments,bolted joints was less sensitive to the load levels compared to bonded and hybrid joints and the normalized fatigue load level was much higher than these two.For the given BFRP pultrusions,three types of joints were designed based on the guildlines to realize the high joints' efficiency and utilization of FRP pultrusions.The fiber-architecture of FRP pultrusions was determined by the integrated design of members and joints.The ultimate failure loads of bolted and bonded joints were improved and that of hybrid joints was almost the linear summation of bolted and bonded connection parts.The ductility of these three types of joints were improved.The F-N models of these were established and the degradation laws were clarified under fatigue loading.All these studies can provide references for the design of FRP truss structures. |
PDF Full Text Request