Study On Fatigue Strength Of Butt-welded Joints Reinforced With Carbon Fiber

The paper is to study the fatigue life extension of steel reinforced by carbon fiber reinforced polymer(CFRP)with high specific strength,fatigue resistance and corrosion resistance for the fatigue failure behaviors of the butt-welded joints on the web of excavator boom.The bonding layer(epoxy resin)in the steel/CFRP composite structure is the weak layer of the reinforced structure.Therefore,the bonding interface and the bonding layer have been strengthened basing on the mechanical mechanism of aramid fiber bridging.The influence of weld reinforcement,arrangement scheme of carbon fiber sheets and the bonding layer reinforced by aramid fibers are studied through theoretical analysis and finite element analysis of the butt-welded joints reinforced by carbon fiber sheets.The main research contents are as follows:(1)The mode-I fracture load was established to get the micromechanical resistance mechanism of aramid fibers and epoxy resin at the crack surface according to the toughening activities of aramid fibers on the bonding layer between carbon fiber sheets.The influence regulars of steel surface roughness,volume content of aramid fibers between resin layers,random distribution and scale effect of aramid fibers are carried out and the optimized parameters are obtained;(2)The surface roughness of steel was increased by surface notching technology.The specimens with the grooved steel surface reinforced by aramid fibers were prepared and the ended notch flexible(ENF)was carried out.The toughening effect of aramid fibers in the bonding layer under the Mode-II fracture load was studied basing on the mechanical mechanism of aramid fiber bridging and the short aramid fibers are mainly pulled out between the adhesive layers.The fracture energy under different shear friction stress is studied and the theoretical prediction results fit well with experimental data,which verify the toughening effect of aramid fibers in the bonding layer;(3)The finite element model of the butt-welded joints reinforced by CFRP was established by ABAQUS basing on the constitutive model of the bonding interface witharamid fibers.It can only be reinforced by single side carbon fiber sheet as it is the special butt joints with baking plate in the box structure.The stress distribution pattern of the welds was studied by the post weld grinding technology to reduce the stress concentration in the welding zone.The stress level at the welds was determined by the IIW standard and the damage degree of the butt joint was calculated by Miner linear damage accumulation rule.It was found that the improvement effect of the butt joints was remarkable by the post weld grinding technology;(4)The optimizations of the strengthening mode of CFRP on the butt-welded joints were studied by the lengths and numbers of layers of CFRP and the interface forms of the bonding layer to the improvement effect of its equivalent time.The results showed that the equivalent working time of the butt joints was the longest using three layers of carbon fiber sheets with211 mm in length and the reinforcing mode of Grooved/aramid fibers/multi-walled carbon nanotube.Meanwhile,based on the damage effect of the bonding layer under the fatigue load,the parameters of the bonding layer were modified and the damage empirical model of the Cohesive Zone Modeling(CZM)of the bonding interface was established by the damage factors of different fatigue cycles.The simulation results show that its equivalent working time decreases slightly when the butt-welded joint is subjected to fatigue load.