الفهرس 
EXPERIMENTAL PROGRAMOnly 14 pages are availabe for public view
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Abstract
For the past few years, wind turbines have become greater in tower height and blades, intended to improve profitability. In order to reach high altitudes where wind speed is very height, the heights of towers supporting turbines are ranging between 60 and 120m. These supporting tower forms 20%-30% of the total cost of a turbine. It was observed that the major cause of failure in steel towers of wind turbine is fatigue due to turbulent wind loads and dynamic loads caused by the blades rotation.
In the current study, an innovative technique, involves the use of carbon fiber reinforced polymer (CFRP) to retrofit cruciform welded joints in the steel tower which subject to fatigue loading. Wind turbine steel tower consists of many welded joints, and These welded joints are weak points in fatigue resistance. The aim of our research is to enhance these joints against fatigue using CFRP.
Research outline is divided into four parts. First, the effect of using only CFRP sheets in enhancing the resistance of steel tower cruciform welded joints to fatigue failure is studied numerically. According to analysis results, CFRP sheets reduced stress intensity factor (SIF) with notable values for toe crack propagation while it is not sufficient for root crack propagation. Therefore, CFRP sheets with stiffeners were proposed. To determine the efficiency of using CFRP stiffeners with overlapped sheets, Experimental work was performed. Details of the experimental program carried out on cruciform welded joint retrofitted by CFRP were presented showing specimen fabrication and CFRP application way. Number of CFRP layers and different stress range values were considered as parameters during specimen preparation and test setup. In the experimental program, the two different spots of weld crack propagation were considered. Results of the experimental work showing fatigue life enhancement and comparing test results with AWS2002 code were demonstrated.
In the third part, the effect of using CFRP stiffeners with overlapped sheets was studied numerically. 3-D finite element model was constructed to simulate cruciform welded steel joint retrofitted with CFRP sheets and stiffener and to determine SIF using contour integral method. The effect of various parameters on SIF is also presented. Several parameters like Young’s modulus of adhesives, Young’s modulus of carbon fiber, layers of carbon fibers, initial crack length, and CFRP stiffener were considered during the parametric study.
It is necessary to consider Residual stresses resulting from the weld process during fracture failure. Therefore, in order to determine these residual stresses, numerical simulation was performed based on finite element method. The simulation consists of coupled thermal and structural analyses sequentially. SIF (Kres) due to residual stresses was determined.
Finally, crack progress analyses was conducted based on linear elastic fracture mechanics, and S-N curves were predicted after using CFRP layer considering the different parameters. Predicted S-N curves were compared with the experimental results. Suggestions for future research work are also provided.