Research On The Compaction Behaviors Of Unidirectional Warp-knitted Fabrics For Wind Blades

At present,in the situation of energy saving and emission reduction,wind energy,as a natural clean energy,has become the development target by governments.Fiber reinforced composites are common materials for wind turbine blades which accounts for over 95%due to their light weight,high strength,low cost and excellent performances.Wind turbine blades are usually manufactured using the vacuum assisted RTM process(VARTM),in which vacuum draws the resin through the preforms.However,the VARTM process has a rigid mold on one side and a flexible vacuum bag on the other side,which makes the thickness of composites uncontrollable,affecting the permeability of the preforms,the final thickness and mechanical properties of composites.The thickness of composites is mainly related to the compression properties of preforms,thus,exploring the compaction behaviors of fabric is the key to controlling the thickness and fiber volume fraction of composites.In this paper,four types of glass fiber reinforced unidirectional warp knitted fabrics,commonly used in wind power blades,are used to explore the compaction behaviors of fabrics from the view of experiment,geometrical modeling,finite element simulation and compaction deformation,which provides the practical and theoretical basis for the experiment and production.The thickness of composites is closely related to the compression properties of dry preforms.This paper mainly focuses on exploring compression properties of dry fabric from the view of experiment firstly,variances such as the vacuum pressure,thickness uniformity,number of layers,stacking sequence,and the cyclic loading were studied to investigate the compaction behaviors of fabrics under the VARTM process.It is found that the fiber volume fraction (V_f) of preforms increased with the increase of pressure which could be fitted by the second-order exponential function,that is,V_f changed obviously at the initial compaction phase,as the pressure reached at a certain level,V_f tended to be gentle.However,the slope of compression tangent was opposite to the law of V_f,as the pressure was larger than 0.02 MPa,four types of fabrics had similar tangent slopes,which could be explained via the compression mechanism.In order to evaluate the uniformity of the fabric thickness,four test points of different locations of fabric were evaluated in this study,and the discrete values of the thickness and the coefficient of dispersion were analyzed.Results showed the discrete values of thickness of four test points were less than0.05mm,while the thickness dispersion coefficient was less than 5%throughout the compaction process,which indicated the thickness for each test point kept almost uniform.It is found that the preform could achieve a higher V_f as the layer number of preform increased,a new nesting calculation method was proposed to explain the nesting phenomenon of fabrics,the nesting values of four fabrics exhibited:Fetc>Hengshi>Newf>Taishan.This paper also explored the effect of cyclic loading and unloading on the compaction behaviors of fabric.It is found that the number of loading cycles could increase the final V_f of fabrics to a certain extent,however,the effect of increasing V_f was not obvious while the cycling number reached at a value.At the same time,this paper proposed the compression work,the recovery work and the compression recovery work and respectively evaluated the compression-rebound elastic energy,rebound resilience and the plastic deformation of fabrics under different cycles.It is found that the fabric increased the compression difficulty with the increase of the number of loading cycles.The elastic capacity increased,and the plastic deformation was exponentially reduced until it did not change at the end.It also found that the fabric unloading had rebound lag,and the fabric began to rebound until the pressure was less than 0.03MPa for four fabrics under multiple unloading cases.Meso-structure of yarns in the composite reinforcement determines the forming process and the mechanical properties of composites,the meso-scale model of unidirectional warp knitted fabric based on the real geometry was developed and analyzed.Firstly,the cross-section parameters of warp and weft yarns in the unidirectional warp knitted fabric were obtained by the metallographic method,and parametric modeling has been attempted.Then,the fiber tracer method was employed,the differences between the refractive index of the polyester stitching yarn and the glass fiber,resin was used to highlight the stitching yarn profile,and Nurbs spline curve was used to fit the three-dimensional space path of the stitching yarn.Besides,the trace of stitching yarn could be expressed via a series of mathematical equations.Finally,the unit cell model was assembled according to the location of yarns and extended to the global model,checking the penetration phenomenon among the yarn components,no penetration was found.The meso-scale model of unidirectional warp knitted fabric was constructed which truly reflected the relationship between the shape and relationship of yarn components,it also provided an accurate model for the finite element simulation for exploring the compression properties.Due to its low cost and high efficiency,finite element simulation was widely used in the process optimization and mechanical property prediction for textile structure composites.In this part,the pores change mechanism of fabrics was firstly analyzed by the experimental method,it was found that the compaction process of fabrics mainly featured the decrease of macroscopic pores and microscopic pores.A constitutive model(porous elastic model)was proposed to characterize the compression properties of unidirectional warp knitted fabric,and the finite element simulation method was attempted to simulate the compaction behavior of fabrics.The macroscopic model was firstly built to study the compaction characteristics of the fabric,and the simulation results were in good agreement with the experiment.Then the mesoscopic model simulation was performed,in order to guaranteeing the reliability of the meso-scale model,the influence of model structure parameters and boundary conditions were investigated.Results revealed that the model size and compaction platen location had little effect on the compaction behaviors,while the cross-sectional shape of the warp yarn had a certain influence on the compaction behavior,the displacement deviation of runway and ellipse cross-sectional warps from FEM calculation were 3.0%,2.6%respectly.Finally,compared the deformation of yarns in the simulation and experimental data,the warp yarn height and width in the model were higher 1.45%,1.70%than the experimental values,while the yarn area were 2.92%higher than the experimental result.Finally,this paper studied the compression deformation from the fabric and yarn deformation.Firstly,two methods including the online(dry method)and offline(composite method)methods were used to obtain the different compression states of fabrics.According to the yarn stacking sequences,two special cases of yarn stacking were considered,the yarn alignment and nesting,the fabric cross sections and yarn geometric parameters were analyzed under different thickness.It is found that the fabric porosity,yarn micro-porosity and macro-porosity curves from the dry and composite method were almost same,indicating the accuracy of dry method for obtaining yarn parameters was feasible.This work revealed the compression of fabric was characterized by the reduction of macroscopic pores and microscopic pores.During the compression process,the macro-porosity decreased linearly,the initial falling speed was fast when the fabric height was lower than3.6mm,the decrease speed became slow,while the micro-porosity changed in the opposite law,the decrease changed slow firstly and then fast.It is indicated that the initial stage was mainly the reduction of macroscopic pores,in the latter stage,the fabric thickness was mainly related to the microscopic pores.Finally,the compression deformation process of the warp yarn profile was studied,the spline curve was used to simplify the yarn profile at different thickness and the warp yarn profile was unified for various bundles which represented the warp yanrn under different compaction state.Analysed the cross section of warp yarns,the height and cross-sectional area were decreased during the compacion process,while the yarn width changed little.Compared the fiber distribution in the case of 0MPa pressure and the 0.1MPa pressure,pressure inceased the fiber number of local area and fiber contact,which also increased the possibility of fiber hexagonal packing.