Main Spar Research Articles

Static Testing Using OFDR-based Quasi Continuous Fiber Optic Strain Sensing on the WiValdi Wind Turbine Rotor Blade

The German research wind farm WiValdi was taken into service in 2023 and enables researchers and industry to conduct full-scale experiments. It currently consists of two highly instrumented wind turbines and multiple wind measurement masts. In one of the six wind turbine rotor blades, four Fiber Optic Strain Sensors (FOSS) are installed [1].The strain which a wind turbine rotor blade exhibits under load is a key factor in the design, the certification and the assessment of its remaining service life as well as its further development and optimization. The aerodynamic optimization of wind turbine rotor blades leads to longer, and increasingly slender rotor blades and therefore increasingly relevant static and dynamic structural mechanical challenges.Usually the strain of wind turbine rotor blades during static deflection tests is measured with electrical strain gauges (SGs) or Fiber-Bragg-Gratings (FBGs). In the present study, quasi-continuous Fiber-Optic-Strain-Sensors (FOSS) are used in parallel with FBGs. The underlying technology is called optical frequency domain reflectometry (OFDR) and since only glass-fibers are used, it is inherently lightning protected [2]. Besides that, it is commercially available and delivers strain results at a spatial resolution of 2.6 mm. For this study, there are four 50-meter-long glass fibers bonded to the wind turbine rotor blade on suction and pressure side of each main spar of a 57 m rotor blade. This contribution shows how the FOSS measurements are post-processed and validated. The validated data is then used to obtain the relation between strain and introduced force over the length of each FOSS. With the accurate prediction of root-bending-moments, one of many possible use-cases of the FOSS results, is presented.Besides the computational model updating of the corresponding Finite-Element model in future investigations, the results will also be used to judge the current structural mechanical state of the wind turbine rotor blade at recurring service inspections. In that, results of the then to be acquired state at a given time and the shown base-line state will be compared. This will lead to high resolution insight into the deterioration of the structural integrity of the wind turbine rotor blade over time.Considering that standard telecommunication glass fibers can be used as sensors, the cost per sensor (i.e. fiber) is relatively low (circa 1500 € for 50 m). Despite the high cost of the required measurement system, the low prices of the fibers make FOSS measurements not only a scientifically interesting but also a potentially economically viable solution for measuring strain on wind turbine rotor blades.

Estimation of Stress Concentration Factor for a plate with hole under Axial Tension Loading

The purpose of this study is to investigate the effects of hole/holes on stress concentrations in a plate using a current industry standard software - Finite Element Analysis (FEA). Plate with hole/holes is a common engineering application such as in automobile, marine, aerospace and mechanical structures. Hole/Holes can be seen in many thin-walled mechanical and automobile structures and components. For example, hole/holes are found in residential/commercial buildings’ steel structural studs to allow plumbing installation, web or flange of steel box girders in bridges is furnished with holes to ease inspection works, electrical and heating conduits in the walls or ceilings and ribs attached to the main spar of an aeroplane’s wing are frequently come with holes. These hole/holes are one type of discontinuities within the structure (e.g. thin plate) that leads to changes in elastic stiffness and may tend to failures. Proper knowledge of stresses, strains, deflection and stress concentration (SC) are required to design any structures. In the geometry of the plate, under different loading, stress concentration rises from any abrupt change. And due to this, throughout the cross-section, the uniform stress distribution does not occur. At a point of stress concentration, as a results failures like fatigue cracking and plastic deformation commonly occur. Hence, it is very important in any engineering structural design to know about the stress concentration on plates with holes. In the present study stress concentration in a plate with a circular central hole and offset hole subjected to uniaxial loading (axial tension) is calculated using Finite Element of Analysis (FEA). Equations of SCF given by Peterson are dependent on three parameters namely, hole radius (a), distance from the bottom edge to the centre of hole (c) and distance from top edge to centre of hole (e). By varying either ‘c’ or ‘e’ or the ratio of ‘e/c’, an effort is made to study stress concentration factors for the determination of the edge effect by varying the hole to edge distance. The results show that von Mises stresses increased with increasing ‘e/c’ or ‘a/c’, which in turn increases SCF and finally leads to failure.

Optimizing of Turbine blade spar using Ansys program

The current work involved optimizing the spars of wind turbine blades while taking into account the wind speed quantities that affected the blade structure. The objective was to determine the optimal dimensions of turbine blade spar configurations using the finite element method under the influence of the maximum pressure associated with the first mode shape while maintaining the Von Misses stresses within the assumed safety factor (1.5). (200-230 MPa). The blade was stiffened with a main box spar and two auxillary spars on each side. Appropriate spar locations were specified for poisons with a high natural frequency first mode. The blade parts' dimensions were discretized to allow for greater flexibility and precision in dimension assignment. By utilizing the ANSYS program, the optimization process required a certain number of iterations to modify the blade structure's dimensions. Optimized iteration was considered in order to increase the thickness in areas of high stress and decrease the thickness in areas of low stress. Additionally, a comparison between a blade structure with optimal dimensions and one with non-optimal dimensions was included.

A Polymorphing Wing Capable of Span Extension and Variable Pitch

This paper presents the development of a novel polymorphing wing capable of Active Span morphing And Passive Pitching (ASAPP) for small UAVs. The span of an ASAPP wing can be actively extended by up to 25% to enhance aerodynamic efficiency, whilst its pitch near the wingtip can be passively adjusted to alleviate gust loads. To integrate these two morphing mechanisms into one single wing design, each side of the wing is split into two segments (e.g., inboard and outboard segments). The inboard segment is used for span extension whilst the outboard segment is used for passive pitch. The inboard segment consists of a main spar that can translate in the spanwise direction. Flexible skin is used to cover the inboard segment and maintain its aerodynamic shape. The skin transfers the aerodynamic loads to the main spar through a number of ribs that can slide on the main spar through linear plain bearings. A linear actuator located within the fuselage is used for span morphing. The inboard and outboard segments are connected by an overlapping spar surrounded by a torsional spring. The overlapping spar is located ahead of the aerodynamic center of the outboard segment to facilitate passive pitch. The aero-structural design, analysis, and sizing of the ASAPP wing are detailed here. The study shows that the ASAPP wing can be superior to the baseline wing (without morphing) in terms of aerodynamic efficiency, especially when the deformation of the flexible skin is minimal. Moreover, the passive pitching near the wingtip can reduce the root loads significantly, minimizing the weight penalty usually associated with morphing.

Experimental Aeroelastic Benchmark of a Very Flexible Wing

The paper presents the design, analyses, and testing of the Pazy wing, a very flexible wing model that was designed to study aeroelastic phenomena associated with geometrically nonlinear deflections and to provide data for validation of nonlinear aeroelastic simulation models. The Pazy wing is made of a main thin aluminum spar and a Nylon 12 printed ribs chassis providing the NACA0018 aerodynamic airfoil shape. The wing was covered entirely with a polyester film that serves as the skin. Static loading tests were used for model adaptation and to assure that the wing can deform to over 50% of its span without failing. Ground vibration tests were also used for model adaptation. Because there are uncertainties in the characteristics of the polyester skin under loads, both the static and ground vibration tests were used to asses the contribution of the skin to the wing’s stiffness and determine the stiffness bound with and without the skin. Wind-tunnel tests were conducted at conditions that lead to static and unsteady structural responses. The wing deformations were tracked by a motion recovery system, and the strains over the wingspan were recorded with a Bragg-grating fiber-optics system. In the static wind-tunnel tests, the aerodynamic loads were recorded by a force balance. The wind-tunnel tests included angle-of-attack sweeps in constant airspeed and velocity sweeps in fixed angle of attack. The largest wing-tip static deformation recorded in test was of 50% span. In the velocity-sweep tests, the wing showed strong oscillations at a velocity at which the wing-tip deformation was approximately 25% of the span and became stable again at higher speeds. The wing models and test data are publicly available through the 3rd Aeroelastic Prediction Workshop (https://nescacademy.nasa.gov/workshops/AePW3/public).

Fatigue life prediction for the main spar with wrinkle defects of a wind turbine blade

AbstractA practical method for predicting fatigue life of the main spar with wrinkle defects in a wind turbine blade is presented in this paper. First, the effect of different wrinkle geometry on stiffness in a composite laminate is quantified. Then, the model of intrinsic dissipated energy is proposed to determine the fatigue limit rapidly. A fatigue life prediction model containing unknown parameters is developed by introducing intrinsic dissipated energy to characterize stiffness degradation of the main spar laminate with wrinkle defects. The specimens with different wrinkle geometry are subjected to fatigue tests, monitored with an infrared thermography system to obtain experimental data. After determining the unknown parameters of the developed model, the S‐N curves of specimens are evaluated in a relatively short time. The predicted fatigue life is within the 95% confidence interval of lifetime obtained by the traditional tests, which suggests the developed model is accurate. This work is suitable for fatigue life prediction of main spar with different wrinkle defects.

Damage tolerance design of transport aircraft

In a transport aircraft, there are normally two spars to withstand bending loads. A large part of this bending moment is drawn from the main spar. The main spar in large transport wings is an integrally machined part that is physically connected to the skin and ribs. At some fastener openings, the mechanical fastening contributes to extreme tension concentration where a fatigue crack can originate. This crack will form first in the flange under service loading and then expand into the spar network. If not detected during service and repaired, this crack growth can lead to catastrophic failure. This investigation aims at examining the main spar’s alternate structural design to make it damage tolerant. With one more intermediate flange and at a height of one-third from the bottom flange, the spar construction will be in two separate parts. The bottom flange and web can fail in the case of fatigue cracking, but the top flange, web, and intermediate flange will remain intact and can be designed to bear the requisite load limit design. An approach to finite element modelling and analysis is also used to analyze all forms of spar constriction and test the principle of harm tolerance nature.

Strength analysis of the conceptual model of a main rotor blade spar with actuators

This paper concerns the conceptual design of a main rotor blade spar with actuators. A three-dimensional model of a main rotor blade is based on the preliminary geometric blade setting angle to increase the aerodynamic performance caused by different linear velocities of individual cross-sections. A modification of the blade setting angle results in a change of the angle of attack and influences the value of the lifting force in a given blade cross-section. The geometric setting angle was applied in designing both the main rotor blade spars and the blade. The overall dimensions of the blade depended on the available surface of the outmost blade airfoils. The important element of the designed spars was a cut in the form of a notch enabling the upper and lower crossbars of the designed spar to move during the operation of actuators. The FEM analysis was carried out on the parts of the designed blade spars. Three types of materials were compared in the finite element analysis. The internal stresses and displacements of the outmost edges of the crossbars were analyzed. The Catia V5 software was used for the designing work. The sections of the spars were analyzed using the Finite Element Method. The Pre/Post module of NX 12 was used for the FEM analysis. The displacement results were analyzed, which allowed us to determine their twist angle.

Detecting defects in the main spar of a wind turbine blade

Various defects produced during the manufacture of a wind turbine blade can affect the damage evolution under fatigue loads. In this paper, a calculation model based on energy dissipation was used to predict damage evolution. Based on the structure of the unidirectional laminates in the main spar of a 1.5-MW blade, specimens with voids or delamination defects of different dimensions were fabricated. Fatigue tests of the specimens were monitored with an infrared thermography system. The energy dissipation density was calculated from the surface temperature. A damage parameter D is proposed to represent the degree of damage evolution. By analyzing the energy dissipated during cyclic loading, it is possible to evaluate the critical moment when a defect evolves into initial critical damage. The calculation model agrees well with the observed damage evolution. This innovative method could be used to detect the initial damage in a main spar with defects during a fatigue test.

Static testing for composite wing of a two-seater seaplane

The paper studied the strength and deformation characteristics of a composite wing of a two-seater seaplane to get the certificate of airworthiness which static testing is indispensable. The primary wing structure component includes upper and lower skins, leading edge, trailing edge, a root rib and main spar. The main purpose of static testing is to examine the bending strength of the wing. The testing results show good agreement with the FE analysis results, and the bending strength of wing is strong enough to support the limit loads (the maximum loads to the expected in service) without detrimental and permanent deformation, and without failure for at least 3s under the ultimate loads (limit loads multiplied by prescribed factors of safety), which meets the requirement of ASTM F2245-16c (Standard Specification for Design and Performance of a Light Sport Airplane).

Wind Tunnel Test for Videogrammetric Deformation Measurement of UAV for Mars Airplane Balloon Experiment-1 (MABE-1)

This paper reports the results of the aerodynamic deformation measurements of the meter-scale, entire shape, actual UAV in the wind tunnel using a video grammetry technique. The measured airplane was the airplane for Mars exploration being developed by Japan Aerospace Exploration Agency (JAXA) and Japanese universities. Its main wing span length was 2.4 m. The video grammetry measurement was performed using VICON’s system. Retroactive markers and stickers were put on the airplane. JAXA’s 6.5 m × 5.5 m Low-Speed Wind Tunnel was used. The airplane was mounted on the strut support with pitch-free or pitch-locked conditions. The deformations of the main wing bending, the main wing twisting, the tail boom bending, and the elevator deflection angle change were revealed quantitatively. The bending stiffness of a main wing spar that was designed as a safety factor of 2.8 at load factor of 5 was sufficient. The main wing spar was located around a center of pressure of an airfoil and it showed enough stiffness for twisting at nominal condition. The effects of the main wing bending and twisting, and the tail boom bending on the aerodynamic performance were estimated but they were in an acceptable range from the standpoint of the controllability of the aerodynamic performance using control surfaces. Even though the servo motor was located near the elevator and the linkage between the servo motor and the elevator was short, the measured elevator deflection angle was at most 4% smaller than the angle at no-wind condition. The obtained results and presented method are useful for control, flight data analysis, and design of lightweight airplanes.

Effect of main spar ply Parameters on performance of composite wind turbine blades

The change of layer parameters of composite material will affect the performance of wind turbine blades. In this paper, in order to study the regularity of the variation of the ply parameters of the main spar on the structural performance of the large-scale wind turbine blade, a 5 MW wind turbine blade was taken as the research object. By changing the angle of the main spar ply and the ply ratio at different ply angles, the different wind turbine blades were established which had different ply plans. The numerical simulation was conducted to get the pressure on the blade surface through FLUENT software at rated wind speed. After this, the static analysis and modal analysis were been performed to the blades which applied load. The results show that the maximum stress the blade bears is the smallest when the main spar layup angle of fibre was 0°, illustrating that 0° layup angle could withstand bending load better. So the angle 0° should be the main layup angle of the main spar. And the modal frequency and structural performance of blades is best when the angle 0° accounts for layup angle 49.333%.

Cross-sectional Optimization of a Human-Powered Aircraft Main Spar using SQP and Geometrically Exact Beam Model

Cross-sectional Optimization of a Human-Powered Aircraft Main Spar using SQP and Geometrically Exact Beam Model

Post-processing of ultrasonic signals for the analysis of defects in wind turbine blade using guided waves

In this work, the most promising ultrasonic signal processing methods—discrete wavelet transform, variational mode decomposition and Hilbert transform—are applied for the analysis of disbond-type defects in the segment of wind turbine blade. Two disbond-type artificial defects having diameters of 81 and 25 mm were located on the main spar of wind turbine blade. The low-frequency ultrasonic system developed by Ultrasound Research Institute of the Kaunas University of Technology was used for the experimental investigation of wind turbine blade using guided waves and only one side of the blade segment was accessed. Two contact type ultrasonic transducers separated by 50 mm distance and fixed on a movable mechanical panel were used as a transmitter–receiver pair during the experiment for the ultrasonic signals recording up to the scanning distance of 250 mm with the scanning step of 1 mm. Both types of defects were marginally detected from the conventional experimental B-scan and therefore appropriate signal processing techniques were used to improve the accuracy of the analysis of defects. The discrete wavelet transform was combined with the amplitude detection method for estimating the size and location of defects. Finally, the variational mode decomposition is combined with the Hilbert transform to compare the instantaneous frequencies and amplitudes of the defect-free and defective signals as well as for the measurement of time-delays between the defect-free and defective signals.

Refinement of defect detection in the contact and non-contact ultrasonic non-destructive testing of wind turbine blade using guided waves

The guided waves are widely used for the inspection of many composite structures as they can travel up to long distance along the thickness of the structure. Most of the times, the experimental investigations standalone is not able to locate and size the damages or defects due to dispersive, superimposed and scattered guided wave modes. Hence signal refinement of ultrasonic guided wave signals is required for identifying and characterizing the defects. In this work, disbond type defects presented on different locations of the segment of the wind turbine blade are estimated by applying the signal refinement techniques after experimental analysis. The experiment was carried out on a 1005 x 870 mm segment of wind turbine blade manufactured using a composite glass fiber reinforced plastic material. Two defects on the trailing edge (with diameter 15 and 25 mm) and three defects on the main spar (with diameters 25, 51 and 81 mm) of the WTB segment were investigated. The combination of macro fiber composite transducer, contact type and air-coupled transducers were used to transmit and receive the ultrasonic guided waves. The signal processing techniques are applied to the experimental signals for the estimation and characterization of defects.

Static Testing and Analysis of Composite Wing of a Two-Seater Aircraft Powered by Li-Ion Battery Electric Propulsion

The present paper studied the strength and deformation characteristics of a composite wing of a two-seater aircraft powered by Li-ion battery electric propulsion. In order to get the airworthiness certificate, the static testing is indispensable. The primary wing structure component includes upper and lower skins, leading edge, trailing edge, a root rib and main spar. The main purpose of static testing is to examine the bending strength of the wing. The testing results are in good agreement with the FE analysis results and the bending strength of wing is strong enough to support the limit loads (the maximum loads to the expected in service) without detrimental and permanent deformation, and without failure for at least 3s under the ultimate loads (limit loads multiplied by prescribed factors of safety), which meets ASTM F2245-11(Standard Specification for Design and Performance of a Light Sport Airplane) enough.

기준신호 데이터를 이용한 보강된 복합재 구조물에서의 저속 충격위치 탐색

Low-speed impact was localized on a stiffened composite structure, using 4 FBG sensors with 100 kHz-sampling rate interrogator and devised localization algorithm. The composite specimen consists of a main spar and several stringers, and the overall size of the specimen`s surface is about . Pre-stored reference data for 247 grid locations and 36 stiffener locations are gathered and used as comparison target for a random impact signal. The proposed algorithm uses the normalized cross-correlation method to compare the similarities of the two signals; the correlation results for each sensor`s signal are multiplied by others, enabling mutual compensation. 20 verification points were successfully localized with a maximum error of 43.4 mm and an average error of 17.0 mm. For the same experimental setup, the performance of the proposed method is evaluated by reducing the number of sensors. It is revealed that the mutual compensation between the sensors is most effective in the case of a two sensor combination. For the sensor combination of FBG #1 and #2, the maximum localization error was 42.5 mm, with average error of 17.4 mm.

The effect of delaminations on local buckling in wind turbine blades

In this article the effect of delaminations on the load carrying capacity of a large wind turbine blade is studied numerically. For this purpose an 8.65 m long blade section with different initial delaminations in the main spar was subjected to a flapwise dominated bending moment. The model was setup in Abaqus and cohesive elements were chosen for modelling delamination growth.For initial delaminations with a width of 30–50% of the cap width the study showed that delamination close to the surface started to grow in load ranges of normal operation conditions and led to local buckling modes. The local buckling caused high strains and stresses in the surrounding of the delamination, which exceeded the material design properties and therefore should be considered as dangerous.Delaminations placed near the mid-surface of the cap did not have a significant effect on the blade response under normal operation conditions. In the simulations the static load exceeded the design load by more than 40% before delamination growth or cap buckling occurred.It could be concluded that delamination induced near-surface buckling modes have to be considered critical due to an onset of local sublaminate buckling below the design load level.

Online Process Monitoring and Control by Dielectric Sensors for a Composite Main Spar

An online monitoring scheme to determine viscosity and permeability for a composite main spar infusion is presented. During the manufacturing process, defects such as dry spots and pores can occur and lead to poor part quality. In this study, dielectric sensors are used to create the information needed to model the process. Several experiments have been carried out to investigate the correlation between the sensors signal and the viscosity and to derivate constitutive equations. The provided data and the derived permeabilities are integrated into a simulation model of a cross section of the composite main spar. As a result, the real permeability can be identified. The knowledge of these parameters is one of the key aspects for a prediction of the resin flow and therefore for an improved process control.

고고도 장기체공무인기 주익 Spar 비선형 구조 해석

고고도 장기체공무인기 주익 Spar 비선형 구조 해석