Spar Cap Research Articles

Design and analysis of hundred-meter wind turbine blade trailing edge based on finite element method (FEM)

Abstract The trailing edge region of composite wind turbine blades usually has a large deformation under wind load and the accidents caused by the damage of blade trailing edge accounts for a large proportion among blade accidents. With the development of large blade size, higher requirements are put forward for the stability of wind turbine blade trailing edge. Therefore, the trailing edge region is critical in the design of blade. A detailed study of the design and analysis for wind turbine blade trailing edge region was carried out in which the structure stability was regarded as the key. The influence of auxiliary spar cap and auxiliary spar web on the stiffness distribution and stability of the trailing edge were studied based on focus and finite element method (FEM). The respiratory effect of different structures due to deformation were analyzed and compared. The results showed that the structure of the auxiliary spar cap and auxiliary spar web can significantly increase stiffness, stability and decrease respiratory deformation at the trailing edge of blade compared to the traditional three webs structure. The structure of auxiliary spar and auxiliary spar web is the best design scheme for hundred-meter composite blade trailing edge.

Study on the delamination damage prediction of wind turbine blade spar cap based on morphology feature recognition

In order to study the degradation of local mechanical properties caused by delamination defects of wind turbine blades, a progressive damage prediction method of blade spar cap based on quantitative identification of delamination morphology features by infrared thermal image was proposed in this paper. Firstly, based on the thermal conduction theory of laminate plates, infrared thermal imaging technology was used to realize quantitatively identify delamination morphology features. Then, based on the cohesive zone model (CZM) and the classical laminate theory (CLT), a numerical analysis model of unidirectional laminate with delamination defects was established, and the accuracy of the numerical model was verified by comparison with the experimental results of axial tensile and compressive displacement loads. Finally, a 2 MW-45.3 blade spar cap with maximum chord cross-section was used as a prediction model, and the effect of different delamination sizes and depths on the failure mode, failure strength and damage evolution of the blade spar cap was successfully predicted by using the equivalent fatigue load. The research shows that the delamination depth has a more profound influence on the damage of spar cap compared with the delamination size. With the increase of delamination depth, the delamination failure modes undergo the evolution process of non-buckling, local buckling, delamination expansion and strength failure. Shallow delamination causes local buckling and strength failure of blade spar cap. When the delamination depth reaches half of the spar cap thickness, the delamination extends to intralaminar and interlaminar, the local failure changes to global failure.

Parametric investigation on stepped-lap scarf repair of wind turbine blade spar cap

Parametric investigation on stepped-lap scarf repair of wind turbine blade spar cap

Finite Element Analysis of Delamination Initiation in Wind Turbine Blade Spar Caps: Role of Compression, Strain Energy, and Principal Stresses

In wind turbine blades, utilizing the finite element method (FEM) to identify potential failure modes before production provides considerable value and cost savings in contrast to conventional structural tests. This paper focuses on examining the factors leading to delamination in spar caps and exploring the initiation of this delamination using the finite element method. Compression, total strain energy density, and principal stresses are significant among the variables examined to determine the effects on model deformation and crack formation. Examining these variables may contribute to understanding the mechanisms involved in initiating and progressing delamination. This study examined the effects of compression, total strain energy density, and principal stresses on model deformation and crack formation. This study focuses on the effects of principal stresses on displacement, the effects of compression on delamination and crack formation, and the effects of total strain energy density on fracture tendency and failure modes due to delamination.

Performance and application of pultrusion-processed fibre-reinforced composites in large-scale wind turbine blades

Pultruded composite materials have broad application prospects in the field of wind power generation. In this study, we explored the application of three different pultruded composites for the spar caps of wind turbine blades. Through comprehensive morphological, thermal, and mechanical analyses, as well as static and fatigue loading tests, we assessed the properties of these composites. Notably, the pultruded composites demonstrated high modulus, adequate strength, and excellent fatigue resistance. Among them, the optimal composite had an elastic modulus that was 46.4% higher than that of an E-glass vacuum-assisted resin-infused laminate. To further investigate their viability, we constructed a finite element model for a wind turbine blade with a pultruded composite spar cap using ANSYS. The results indicated that in a 94-m blade, replacing the E-glass vacuum-assisted resin-infused blade spar cap with the pultruded composite would result in a substantial 32.5% weight reduction. Finally, a 94-m blade was prototyped and tested, confirming its compliance with static and fatigue loading requirements. This study contributes to the advancement of composite materials and manufacturing methods for large-scale wind turbine blades.

Characterization of Oscillatory Response of Light-Weight Wind Turbine Rotors under Controlled Gust Pulses

Given the industry-wide trend of continual increases in the size of utility-scale wind turbines, a point will come where reductions will need to be made in terms of the weight of the turbine’s blades to ensure they can be as long as needed without sacrificing structural stability. One such technique that may be considered is to decrease the material used for the shell and spar cap. While this will solve the weight issue, it creates a new one entirely—less material for the shell and spar cap will in turn create blades that are more flexible than what is currently used. This article aims to investigate how the oscillatory response of light-weight wind turbine rotors is affected by these flexibility changes. The object of our study is the Sandia National Lab National Rotor Testbed (SNL-NRT) wind turbine, which the authors investigated in the course of a research project supported by SNL. Using a reduced-order characterization (ROC) technique based on controlled gust pulses, introduced by the authors in a previous work, the aeroelastic dynamics of the NRT’s original baseline blade design and several of its flexible variations were studied via numerical simulations employing the CODEF multiphysics suite. Results for this characterization are presented and analyzed, including a generalization of the ROC of the SNL-NRT oscillatory dynamics to larger machines with geometrical similarity. The latter will prove to be valuable in terms of extrapolating results from the present investigation and other ongoing studies to the scale of current and future commercial machines.

Impact of Process Technology on Properties of Large-Scale Wind Turbine Blade Composite Spar Cap

As wind turbine blade length increases, reconciling lightweight design with strength necessitates continuous advancements in process technology. The impact of three different process technologies–vacuum-assisted resin transfer moulding (VARTM), prepreg, and pultrusion–on the properties of wind turbine blade composite spar caps was investigated using scanning electron microscopy, dynamic mechanical analysis, differential scanning calorimetry, thermogravimetric analysis, and static and fatigue testing. The results demonstrated that the fibre weight content and 0° tensile modulus of the VARTM and pultrusion composites increased as compared to those of the prepreg samples. Subsequently, the properties of a 94-m blade were analysed using the Ansys Composite PrepPost (ACP) and static structure modules in Ansys simulations, and the weights of the spar cap were compared with test data of materials under different process technologies. The results showed that the masses of the spar cap of a 94-m blade in the pultrusion, VARTM, and prepreg processes were 7965, 9170, and 9942 kg, respectively. The quantitative influence rules on the weight of the wind turbine blade spar cap prepared through different process technologies were formulated. The findings of this study are promising and are expected to aid the development of wind turbine blade process technologies.

A Structural Health Monitoring System for Bond Line Flaws Detection on a Full-Scale Wingbox Section Demonstrator

In recent years, there has been a significant increase in the use of structural health monitoring (SHM) technologies as systems for monitoring the integrity of aircraft’s structures. The use of compact and embeddable sensor networks, like the ones based on fibre optics (FO), is particularly attractive from the perspective of releasing an integrated structural system with intrinsic sensing capacity. Usually, an SHM system architecture is completed by a dedicated algorithm that processes the data gathered from the sensors to elaborate on the level of damage currently suffered by the structure, with the further possibility of providing information to the relevant specialists involved with its supervision. One of the main SHM applications that is attracting major interest is related to the inspection and detection of anomalies in bonded joints, which is extremely relevant in many composite realizations. Aeronautical regulations allow the use of bonded joints on an aircraft’s primary structure but require the implementation of a means to ensure their absolute safety, such as the introduction of further mechanical links aimed at stopping the propagation of a possible flaw or the availability of Non-Destructive Inspection (NDI) systems to prove the absence of relevant damaged areas. Generally, the main typical defects occurring during the manufacturing of bonded joints include adhesive curing, kissing bonds, poor porosity, and poor surface preparation. The current NDI systems more widely used and available to detect defects are still inaccurate due to the lack of standard procedures for the creation of representative defects in a controlled manner, which would allow for the development of reliable methodologies and tools able to ensure the safety of a bonded joint, as required by safety regulations. This paper shows the results relative to the implementation of an SHM system developed by the Italian Aerospace Research Centre (CIRA) aimed at monitoring the bonding lines between spar caps and panels of a typical composite wingbox section and detecting faults in location and length. The work was performed during typical ground static tests by using a fibre optical sensing network embedded within relevant adhesive paste layers during the manufacturing process of the structure. In the reported investigation, the SHM system assumed the function of an NDI system tool. The results show that the developed SHM system has good reliability for the detection of both the position and size of damage areas that were artificially inserted within the test article during the bonding phase, showing its potential as a candidate to be used as a tool to verify the conditions of a bonded joint, as required by aviation authorities’ regulations.

Monitoring fatigue delamination growth in a wind turbine blade using passive thermography and acoustic emission

Damage monitoring is an essential step to understand fatigue damage growth in composite wind turbine blades and for reliable lifetime prediction. The current study is an experimental investigation of fatigue delamination induced by an intentionally embedded defect in the spar cap of a 31 m wind turbine blade. A constant cyclic flap-wise bending was applied to the blade for 320,000 cycles. Delamination growth is identified and measured by visual inspection, acoustic emission (AE), and infrared thermography of the blade surface. It was observed that the area of delaminated regions grew faster during the early cycles, then their growth rates decreased, and the interlayer cracks reached stable growth. The experiment shows fatigue delamination develops gradually in the spar cap and allows monitoring of the damage before reaching a critical stage. The ability of AE and thermography methods to detect subsurface damage is demonstrated by the identification of steady delamination growth during cyclic load. Damage localization by both methods is in good agreement with the delamination location. Most acoustic activities are spotted in the boundaries of delaminated regions, and the position of a significant number of acoustic activities with the highest energy content correlates with the location of delamination crack fronts. It is shown that the surface temperature distribution contour indicates the shape of the largest delaminated region, and the growth of the hot region area correlates with damage propagation. However, multiple delaminations through the thickness cannot be discriminated from the thermal images.

3D multiscale dynamic analysis of offshore wind turbine blade under fully coupled loads

Wind turbine blades are typically sophisticated structures with complex geometry and composite layup. The realistic loads acting on blades serviced in harsh offshore environments are fully coupled dynamic loads involving wind, wave, current, servo-control, gravity and other inertial loads. This work presents an easy-to-implement methodology for the time-domain detailed analysis of three-dimensional (3D) blade structure, by combining fully coupled dynamic simulator and general finite element program. As a practical example subjected to multiple marine environmental loadings, one of the rotating composite blades atop a 5-Megawatt (MW) monopile-supported offshore wind turbine (OWT) in soft clay is used for verifying this method step by step, and the effect of rotation and foundation flexibility on blade behaviours is explored. Good agreement between OpenFAST and proposed method in tip deflections and root reaction loads is visible from validation results, and the recommendations on how to further improve this method in future iterations are made. Subsequently, based on stress-time history of blade composite layups, time-domain 3D multiscale analysis including the failure evaluation for layers in outer shell and shear webs using Tsai-Wu strength criteria, as well as the fatigue damage assessment of carbon spar caps was performed.

Defect detection in carbon fiber-reinforced composites using directional dark-field imaging and tomography

This paper proposes the use of circular X-ray grating interferometry as an effective technique for defect detection with potential applications for in-line inspection of carbon fiber-reinforced pultruded profiles used inside the load-carrying spar caps of wind turbine blades. A fuzzball defect in the pultruded profile is characterized as a demonstration. The method allows for large field-of-view quantification of local fiber alignment and relative fiber volume fraction. A two-dimensional through the thickness averaged distribution of the fiber orientation, the mean scattering, and fractional anisotropy are determined. Based on this, it is possible to determine the size of the defect as well as quantify the severity of the defect.

Residual stresses in thick composites considering through-thickness temperature gradients during manufacturing

In the composite materials of wind turbine rotor blades, residual stresses form during manufacturing due to the different thermo-chemo-mechanical behaviors of the fibers and the matrix. These residual stresses decrease the ultimate and fatigue strength of the blade because the failure stress and cycles to failure are reached at lower levels of external loading. It is thus of utmost importance to consider residual stresses in the design stage to obtain reliable blade designs.This work deals with the prediction of residual stresses in a thick composite representative for a spar cap in a wind turbine rotor blade. A model is formulated for the thermo-chemo-mechanically coupled problem. An emphasis is given on the non-uniform and time-variable temperature distribution across the thickness of the composite. The different sources of residual stresses (e.g., chemical shrinkage, non-uniform distribution of different thermal expansion coefficients of fibers and matrix, exotherm, etc.) are analyzed, quantified, and compared. The impact of the time-dependent temperature distribution on the formation of residual stresses is highlighted by means of a representative numerical example.

Design of an element test specimen for fatigue delamination growth of a thick laminate

Following the observation of multiple delaminations growing from a tunnelling crack in the spar cap of a wind turbine rotor blade subjected to cyclic loading, an element test specimen is proposed. Design of the element test specimen to study fatigue delamination of a thick laminate is presented. Complexities inherent to a full structure e.g., curvatures, ply drops, manufacturing defects, etc. are removed to study the fatigue delamination damage mechanism. The element test specimen is made of unidirectional layers with two embedded artificial delamination cracks connected by a tunnelling crack. In this paper, analytical and numerical (2D) models are used to design the element test specimen.

Constraining effect of load frames during full-scale rotor blade fatigue testing

In rotor blade fatigue tests used for the certification process, load frames are used to introduce the loads to the blade. These load frames have a constraining effect on the cross-sectional deformation. This work investigates these constraining effects using a finite element contact analysis on a current-generation commercial rotor blade. Two different load frame variants are considered: a conventional load frame covering most of the cross-section and a reduced load frame covering only the main spar caps. The interaction between blade and load frame is implemented via contact formulations, which allows pretensioning in combination with a flap and a lead-lag load case. Strains on the outer surface of the blade are evaluated and compared to an artificial reference loading. The area of influence of the load application through the load frame, in which the strains deviate significantly due to clamping effects, mostly corresponds to the 0.75 times the chord length assumed for the certification [1]. The strains in the trailing edge are significantly less affected by the reduced load frame variant than by the conventional one, thus potentially making it possible to consider the trailing edge at the position of the load frame as being properly tested for certification purposes.

Buckling behavior of wind turbine blade

The use of composite materials is growing up compared to the traditional materials, in basically all the industrial domains, where we need powerful, lightweight and cost effective structures. An example of a challenging lightweight structure made of composite materials are wind turbine blades. Indeed, the modern wind turbine blades are becoming dramatically bigger, and nowadays a length of a standard one is over 60 m. The wind turbine blades are often facing extremely complex loads due to environmental conditions they are working in. One of the main loading acting on the wind turbine bale are pressure load, which can engender flap wise and edgewise bending. On the other hand, the consequence of optimum design strategy, the wind turbine blades are becoming thin-walled structures, which raise the risk of buckling phenomena. Therefore, the understanding of the wind turbine blade behaviour under this kind of aforementioned load is very crucial. In this study, the buckling behaviour of the spar cap of a wind turbine blade using the finite element method is investigated. For so doing, a generic NACA 634- 421 airfoil section is considered. The spar cap is taken as a multilayer curved cylindrical shell. A parametric study is made in order to study the effect of material choice, stacking sequence and number of layers on the buckling resistance of the spar cap. The results showed that the buckling load is very sensitive to the considered parameters.

Passive aeroelastic study of large and flexible wind turbine blades for load reduction

In this paper, passive aeroelastic coupling of wind turbine blades for load reduction is realized by introducing bending-twist coupling (BTC) and geometric sweep coupling (GSC). The BTC is implemented by composite fiber off-axis of the blade spar cap, and the GSC is achieved by defining sweep curves of the blade reference axis. To investigate the passive coupling effect and load reduction of the large and flexible blades, the IEA 15 MW blades are considered. BTC are introduced into the three-dimensional finite element (FE) models and the residual stiffness coefficient is obtained. Meanwhile, geometrically exact beam theory (GEBT) is used to build the flexible blade. Both BTC and GSC are introduced into the beam model, and the effect elastic deformation on blade load is also considered. Multi-objective optimization (MOO) process of rotating blades for load reduction is proposed under steady-state rated conditions. The results show that the highest coupling efficiency can be obtained when the fiber off-axis angle is between 12° and 15°. The BTC of the blade can reduce the load of blade root with a small fiber off-axis angle. Meanwhile, the GSC can further reduce the axis torque. Compared with the initial design of IEA 15 MW steady-state rotating blade, the maximum thrust, flapwise moment and axis torque of the blade can be reduced by 8.8 %, 13.4 % and 22.9 % respectively.

Structural testing and numerical modelling of a glass fibre-reinforced composite demonstrator for turbine blades

Tidal energy, a clean, predictable and reliable renewable energy source, can play an important role in creating a carbon-free energy system in Europe. The cumulative tidal stream technology deployed in Europe was 27.9 MW in 2020, which contributes to 77% of the global total tidal energy device installations. Under a high growth scenario, about 2388 MW of tidal energy capacities will be deployed in Europe by 2030. The structural performance of a tidal turbine blade is vital as it guarantees the safe operation of a turbine within its lifespan in marine environments. Experimental testing is a reliable way of investigating the structural performance of a tidal turbine blade. In this research, the structural performance of a composite demonstrator is carried out. The 5-m long demonstrator represents a tidal current turbine’s spar cap, the strongest region of a typical rotor blade. The demonstrator consists of two spar cap and two webs, which are manufactured with glass-fibre reinforced composite materials. Steel inserts are drilled into the root of the spar cap to connect to the support frame. A hydraulic actuator is used to apply loading to the tip region of the demonstrator to simulate the operation loading. Instrumentations, including strain gauges, accelerometers and displacement transducers, are installed to monitor the demonstrator responses. Besides testing the demonstrator, the mechanical properties of the glass-fibre reinforced composite material are also obtained through coupon tests. The test results are utilised to develop a finite element model, which will be used in the blade design and optimisation case study in the future.

Economic competitiveness of pultruded fiber composites for wind turbine applications

Pultrusion manufacturing of fiber reinforced polymers has been shown to yield some of the highest mechanical properties for unidirectional composites, having a high degree of fiber alignment with consistent performance. Pultrusions offer a low-cost manufacturing approach for producing unidirectional composites with a constant cross-section and are used in many applications, including spar caps of wind turbine blades. However, as an intermediate processing step for wind blades, the additional cost of manufacturing pultrusions must be accompanied by sufficient increases in mechanical performance and system benefits. Wind turbine blades are manufactured using vacuum-assisted resin transfer molding with infused unidirectional fiberglass or carbon pultrusions for the spar cap. Infused fiberglass composites are among the most cost-effective structural materials available and replacing this material in the cost-driven wind industry has proven challenging, where infused fiberglass spar caps are still the predominant material system in use. To evaluate alternative material systems in a pultruded composite form, it is necessary to understand the costs for this additional manufacturing step which are shown to add 33%–55% on top of the material costs. A pultrusion cost model has been developed and used to quantify cost sensitivities to various processing parameters. The mechanical performance for pultruded composites is improved versus resin-infusion manufacturing with a 17% increase in design strength at a constant fiber volume fraction, but also enables higher achievable fiber volume fractions. The cost-specific mechanical performance is compared as a function of processing parameters for pultruded composites to identify the opportunities for alternative material and manufacturing approaches for wind turbine spar caps. Four materials are compared in a representative wind turbine blade model to assess the performance of pultruded carbon fiber systems and pultruded fiberglass relative to infused fiberglass, where the pultruded systems produce lower weight blades with various cost distinctions.

Fatigue and post-fatigue static crack characterisation of a wrinkled thick glass fibre laminate in a composite wind turbine blade

In this study, we present the fatigue and post-fatigue static crack characterisation of a wrinkled thick glass fibre/polyester composite laminate, which is cut out from a spar cap of a wind turbine blade after full-scale fatigue loading. Fatigue cracks in the laminate were characterised with X-ray computed tomography. The fracture mode and fracture path under fatigue loading were identified. A subsequent static compression test was conducted on the laminate to induce a further propagation of the existing fatigue cracks. The laminate was scanned again with X-ray computed tomography after static compression to identify the crack growth path. The fracture features induced by the fatigue and static compression were examined using scanning electron microscope analysis. It is found that the fractures of the wrinkled thick laminate under fatigue loading is characterised by Mode-II dominated fracture. Prominent and frequent matrix rollers and striations were observed on the fatigue fracture surfaces. The fractures of the laminate under static compression show mixed mode. The effects of existing fatigue cracks on the post-fatigue static cracks were investigated. The fracture features characterised in this work provide a reference for the root cause analysis of blade failure in the field.

The Impact of Bend–Twist Coupling on Structural Characteristics and Flutter Limit of Ultra-Long Flexible Wind Turbine Composite Blades

Flutter is an instability phenomenon that can occur in wind turbine blades due to fluid–structure interaction, particularly for longer and more flexible blades. Aeroelastic tailoring through bend–twist coupling is an effective method to enhance the aeroelastic performance of blades. In this study, we investigate the impact of bend–twist coupling on the structural performance and flutter limit of the IEA 15 MW blade, which is currently the longest reference wind turbine blade, and determine the optimal layup configuration that maximizes the flutter speed. The blade is modeled by NuMAD and iVABS, and the cross-section properties are obtained by PreComb and VABS. The accuracy of the blade model is verified in terms of stiffness and frequency. The bend–twist coupling is implemented by changing the fiber angle of the skin and spar cap considering symmetric and asymmetric layups. The flutter limits of both the baseline and the bend–twist coupled blade are evaluated based on HAWC2. The results show that the angle of spar cap carbon fiber has a greater effect on the blade’s structural properties and flutter speed than the skin fiber. Varying the spar cap carbon fiber angle increases the flutter speed, with the effect being more significant for the symmetric layup, up to 9.66% at a fiber angle of 25 degrees. In contrast, the variation in skin fiber angle has a relatively small impact on flutter speed—within ±3%.