Strength Analysis Method And Experimental Study On Root Bolt Of Wind Blade In Pre-burying Bolt Sleeves

The root bolt of the wind blade is divided into two connection patterns， i.e.cylinder bolt sleeve connection and bolt sleeve pre-burying connection. The formerone has simple structure and mature analysis method. However， it can weaken thestrength of the blade root. Besides， it cannot meet the requirements of thelarge-size blade load. On the contrary， the latter one does not greatly weaken thestrength of the blade root， and it can meet that requirement. However， suchtechnology has complex structure and it started too late. At present， it is still in thephase of theoretical research. The key analysis method is mastered and kept secretonly by a minority of foreign enterprises， while there is no research on this aspectin China.This subject relies on the national863project “Design and ManufactureTechnology Development of Home-made Raw Material Wind Blade”， whoseproject number is2009AA034503. With the large-size wind blade project ofSinoma50.2-2MW， the method integrating engineering methods， finite elementsimulation with experiments is adopted. This subject puts great emphasis on thestrength analysis method and experiment research on the blade root bolt inpre-burying bolt sleeves. The main research content is as follows：On the engineering method， the design principle of standard VDI2230iscalculated on the basis of international bolts with high strength. It discusses andformulates the stress distribution diagram of blade root connection system.Accordingly， it analyzes the mechanics meaning of the load transmittingcoefficient and its influential factors. Besides， it makes research on the simplifiedcalculating method towards the flexibility of the components. Then， it deduces thecalculating method of maximum theoretical preload. Finally， the parameters，such as bolt shaft axial stress， bending stress， twist shearing stress， fatiguedamage value， etc.， are obtained by applying the above research achievements tothe engineering cases of Sinoma50.2-2MW.On the finite element simulation， the blade root1/184model is establishedby applying finite element software， and the stress transmission approaches anddeviation of the blade root model are analyzed. Through the transformation of bladeroot load， it imposes external axial load by grading. It attaches great importance tothe construction analysis and definition on the model boundary conditions andcontact surface. Moreover， it discusses the principle and method of transformingthe unordered fatigue load to ordered one. Finally， the parameters， such as bolt shaft axial stress， bending stress， fatigue damage value， etc.， are obtained byapplying the above research achievements to the engineering cases ofSinoma50.2-2MW.On full-scale slow test， it lays out1/4bridge strain gage on the blade bolt，and adopts the method of crane and sensor to conduct multi-point loading. Besides，it adopts dynamic and static strain collector and obtains combined stress of boltstatic load bending. On the full-scale fatigue test，according to the transforming ofdamage principle to the blade fatigue load， it shortens the period of fatigue test，adopts the method of variable frequency motor and eccentric wheel to conductsingle-point stimulation. Furthermore， it adopts dynamic and static collector torecord the blade amplification and cycle index.It summarizes and analyzes the analysisresults of the above three methods.The research shows that， the external load of the blade is distributed by the bolt，pre-buried bolt sleeve and variable bearing based on the flexibility， and only14%external load is transmitted to the bolt， and thus the preload is the main influentialfactor of the static strength of the bolt. As the blade root load exceeds the criticalvalue， it leads to the opening of the contact surface and stress redistribution.Besides， the relation between the bolt and the external blade root load presentsnon-linear increasing. The finite element simulation cannot stimulate the detailstructure of the screw thread. However， the empirical formula can be adopted forcalculation by the engineering method. Therefore， the analysis of engineeringmethod has relatively high integrity. The fatigue stress amplification of the bolt hasnothing to do with the twist shearing stress， and thus the fatigue damage analysisof the finite element simulation has relatively high accuracy.