Structural Failure Of The Primary Load-carrying Component Of Large Composite Wind Turbine Blades

With the large-scale development of wind turbines,the increases of size,weight,and load of composite blades raise higher requirements to the reliability of blade structure.The safety concerns of the up-sizing blades are increasingly considerable.The blade damage will cause serious economic losses and could show complicated failure modes.Hence,it is very necessary to understand the failure behavior of blade structure,afterward providing effective methods on performance evaluation and failure prediction.Due to the technological risk and high cost,the practical challenges are still great for the full-scale collapse testing,but more attention is received for subcomponent testing.Despite the structural complexity and material variety of blades,to a large portion of blades in the spanwich direction,the overall stiffness and ultimate strength of blade are determined by the primary load-carrying member which is consisted of spar caps and shear webs.Therefore,the experimental and numerical studies featuring the primary load-carrying component and blade airfoil segment are carried out to explore the structural failures of wind turbine blades at the subcomponent scale.This dissertation focuses on the failure mechanism,parameter evaluation,high-fidelity failure prediction and three-dimensional modeling strategy of the blade and load-carrying component.The load-carrying beams with box-shaped section are designed and manufactured with reference to the structural features and molding process of the primary load-carrying component of large wind turbine blades,which are taken as representative for experimental and numerical researches.The monolithic composite laminates of spar caps and sandwich lamiantes of shear webs are adhesively bonded together to form a box beam.Besides,the different thickness and pre-delamination defect are considered in the design of spar cap.The collapse testing of box beams is conducted along flapwise direction and edgewise direction respectively,which is applied by ultimate static load under three-point bending.Multiple failures throughout the loading history are examined to understand what the entire failure sequence of box beams loaded to the final failure,and the benchmark data is presented for the subsequent development of numerical models.Global displacements,local strains and video images are recorded following the change of loading levels to capture failure initiation,propagation and the strain state contributing to post-collapse characteristics.The local deformation and damage are related to the macroscopic mechanical properties of the box beams to clarify the time sequence and spatial distribution of different failure modes such as buckling,Brazier effect and various material damages.The failure mechanisms of the box beams involving geometric,material and contact nonlinearities are discussed in detail.The study shows that the composite box beams may show different failure modes due to competing failure mechanisms:compressive crushing failure driven by local buckling of shear webs determines the ultimate strength of the box beams under flapwise loading,and adhesive joint debonding,initiated by local adhesive cracking and spar cap buckling,is the critical failure mode of the box beams under edgewise loading.The Brazier effect and shear nonlinearity contribute to the initial failure depending on the loading directions.Debonding rather than delamination characterizes post-collapse behavior of all box beams.A comprehensive and general finite element modeling method is developed to simulate interactive failure process of composite box beams used in wind turbine blades.A continuum-damage mechanics based progressive failure analysis approach is developed in three-dimensional stress/strain domain to simulate failure behavior of the box beams.Structural nonlinearities associated with geometry,materials and contact are included in the models.The multiple material failures considered in this study are composite failure with interlaminar and interlaminate failure modes,foam core crushing and adhesive failure.The in-plane nonlinear shear stress versus strain relation of unidirectional composites is included in the material damage model.The material damage models are verified at the element scale and laminate scale respectively to check their reliabilities.Comprehensive comparisons are made between numerical simulations and experimental observations with respect to strain response,ultimate loads,failure modes and failure progress.The modeling approach is found to be capable of predicting both strength and failure of composite box beams with reasonable accuracy and it exhibits great potential to predict failure response and facilitate damage tolerance design of the entire blades.The structural failure of box beams is usually driven by the local buckling of spar caps and/or shear webs.Due to this correlation between the buckling and potential failure,a linear buckling analysis method is proposed for evaluation of various factors affecting the failure of box beams.The buckling mode maps are established to evaluate the influence of the geometric shape and component thickness on the structural failure of box beams.The optimal relationship between structural strength and weight reduction is obtained under the condition of the mixed-mode buckling development of the spar cap and the shear web.The results show that the larger curvature of spar caps can significantly improve the buckling strength due to better resistance to the cross-sectional flattening.The changes of cross-sectional aspect ratio affect the buckling strength more than the change of the weight or the material usage when the spar cap buckling dominates.The boundary condition is also investigated and it shows that the effect of load introduction on buckling response depends on loading conditions.This evaluation method only uses the modeling techniques readily available in common commercial finite element software and no in-house user subroutines are needed,thus allowing the failure evaluation to be performed efficiently for the preliminary design of wind turbine blades.In order to efficiently establish the high-fidelity three-dimensional(3D)finite element model of large blades,an automatic modeling technique for the airfoil segment is proposed to process all modelling steps by predefiniting parameters.The developed modeling program can generate nodes,create elements,input materials,define boundary and load conditions,which allows the model employing solid elements and cohesive elements to be parameterically created from the outer profile curve.Besides,different mesh densities,layer transition and internal bonding are detailed considered to output flexible to a large extent.The airfoil segment model are loaded under bending,and the simulation results of displacement and strain are in good agreement with the experimental results of full-scale blade,indicating that the modeling technique is reliable.This high-fidelity 3D modeling technique using solid elements could be applied to the global-local multiscale blade model which contributes to the fine design and analysis of the large wind turbine blade.