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Epoxy resin has a tight three-dimensional mesh structure after curing; due to this reason, the epoxy resin is brittle and not tough enough, which becomes the main reason for the destruction of the epoxy adhesive-steel/CFRP interface under fatigue loading of CFRP-reinforced steel structures. To prepare epoxy adhesives with good performance and suitable for CFRP-reinforced steel structures, the mechanical properties of epoxy adhesives are improved by adding polystyrene (PS) microspheres. In this work, five modified adhesives with PS weight fractions of 0 wt%, 1.25 wt%, 2.50 wt%, 3.75 wt% and 5 wt% are prepared by dispersion of PS particles through an ultrasonic cell crusher using a room-temperature curing process, and the tensile, flexural and impact properties of PS adhesives with different doping are investigated. Then, the microscopic morphology of the tensile section of the colloids is observed by scanning electron microscopy (SEM). The results show that the optimum dosing of PS is 2.5 wt%, and the tensile strength, tensile modulus of elasticity, flexural strength, flexural modulus and impact strength of the adhesive are increased by 77%, 147.7%, 71%, 35% and 22%, respectively, with this dosing. SEM analysis shows that PS particles produce large deformation to absorb energy when the matrix is fractured, and crack expansion needs to bypass or shear the PS particles, thus inhibiting crack expansion and achieving the purpose of toughening. Adhesion agglomeration of PS particles in the resin is the main reason for the decrease in the mechanical properties of adhesives.

Residual stresses change the stress ratio of fluctuating stresses, hence seriously affect the fatigue life of orthotropic steel decks (OSDs) under traffic loading. Residual stress distributions near the U rib-diaphragm joints are very complicated and need to be investigated further. In this paper, a systematic method has been proposed for calculating the residual stress field in the joint of U rib and diaphragm due to thermal cutting and welding. Firstly, a mathematical model of cutting heat sources was established to predict the temperature field. Then, a numerical elastoplastic thermomechanical model was built to predict the residual stress evolutions in a diaphragm-rib joint through the whole fabrication process involving flame cutting and welding. Moreover, the simulated temperature contours at the fusion zone and the residual stress distributions in the rib-diaphragm joint were compared and verified against the experimental ones. The numerical results showed a great agreement with the experimental ones, indicating that the heat source model can be used to accurately predict the temperature field during flame cutting. Finally, the validated numerical model was utilized to conduct parametrical analyses on the effects of thermal processing rates, e.g., the cutting and welding speeds and on the residual stress distribution in the rib-diaphragm joint. The results indicate that a faster cutting speed and a slower welding speed can decrease the residual stress magnitude at the rib-diaphragm joints and reduce the high-stress zone near the diaphragm cutouts.

To study the high-temperature mechanical properties of potassium magnesium phosphate cement mortar and the high-temperature resistance of its laminates. Potassium magnesium phosphate cement (MKPC) was prepared by using heavy-burning magnesium oxide and potassium dihydrogen phosphate as the main raw materials, borax as the retarder, and compounded with a certain amount of fly ash and silica fume. The effect of the mass ratio of magnesium to phosphorus (M:P), compounded fly ash and silica fume on the setting time and mechanical properties of MKPC was investigated. Furthermore, based on the better M:P, the compressive strength of MKPC mortar was studied after 3 h of constant temperature at 400 °C, 600 °C, and 800 °C, and the effect of fly ash and silica fume on the high-temperature resistance of MKPC was analyzed. The high-temperature resistance of MKPC was further evaluated by analyzing the temperature variation of potassium magnesium phosphate cement laminate during a constant temperature of 650 °C for 3 h. The results showed that the mechanical properties of potassium magnesium phosphate cement were influenced by different raw material ratios, and the mechanical properties of potassium magnesium phosphate cement were optimal when M:P was 2:1, fly ash was 5% and silica fume was 15%. The internal temperature of MKPC laminate increased slowly with time, and its high-temperature resistance was better.