No full-text available
To read the full-text of this research,
you can request a copy directly from the author.
Fatigue Life Prediction and Strength Degradation of Wind Turbine Rotor Blade Composites
Abstract
Wind turbine rotor blades are subjected to a large number of highly variable loads, but life predictions are typically based on constant amplitude fatigue behaviour. Therefore, it is important to determine how service life under variable amplitude fatigue can be estimated from constant amplitude fatigue behaviour. A life prediction contains different, partly independent, elements: · the counting method, used for describing variable amplitude signals as a collection of constant amplitude cycles · formulations for describing S-N curves which relate the stresses to the number of cycles to failure · constant life diagrams which are made up of S-N curves for different stress ratios · damage rules, which relate the life expectancy of a specimen to the stress history For the description of damage, two models were investigated and compared, viz. the Miner's sum method and strength-based life prediction. In the Miner's sum method, the results of a counting method and constant amplitude fatigue behaviour description are converted into a damage parameter, "Miner's sum". Potential effects of load order are not taken into account. Moreover, the value of the damage parameter only indicates whether or not failure occurred: it does not relate to a physically quantifiable damage. These are limitations to the model which suspectedly might cause inaccurate predictions. In the strength-based method, life is predicted by calculating the effect of each load cycle on strength, until the load exceeds the remaining strength. An expected advantage of this cycle-by-cycle method is, that sequence effects can be implicitly included. Moreover, the damage parameter is at all times related to a physically quantifiable parameter (viz. strength). The successful application of the strength-based method requires a description of the post-fatigue strength, which entails considerable experimental effort. In addition, a strength-based life prediction is much more computationally intensive than Miner's sum and can not always utilise the same counting methods. In the comparison of the Miner's sum and the strength-based method, the influence and significance of the other life prediction elements, such as counting methods and description of constant amplitude fatigue behaviour on life prediction are included. The experimental research involved a considerable amount of material tests. The material tests give a detailed image of static strength, constant and variable amplitude fatigue behaviour (both block tests and (variants of) the WISPER spectrum were used), as well as strength degradation for different glass-fibre reinforced laminates. By selecting a single coupon geometry for all material tests on a single material, and the definition and use of standard test conditions, a consistent database was created. The block-test experiments confirm the existence of sequence effects on life, although more data are required to fully quantify them. The residual strength tests show the strength degradation after fatigue for a range of fatigue load conditions. Significant tensile strength degradation is observed in R=0.1 and R=-1 fatigue experiments. Generally, compressive strength remains within the boundaries of the initial static strength distribution. This behaviour was observed for different laminates. The significance of an adequate description of the constant amplitude behaviour is evident from the various life predictions. Commonly used simplifications, such as the Linear Goodman Diagram, result in highly non-conservative predictions. The residual strength model yields more conservative predictions than Miner's sum for the investigated tension-dominated load sequences. The experimental effort required for the determination of the strength degradation, and the computational effort do not justify this relatively small advantage. For future research, it is recommended to further improve the description of the constant life diagram. This work is focussed on fatigue of composites for wind turbine rotor blades. Nevertheless, the results are relevant for other composite structures as well.
... The prediction of the life of composites and structures under variable amplitude fatigue loading has been recognized as an important problem for more than three decades [14,15]. A number of distinguishing characteristics of this problem make it different from the fatigue modeling and life prediction of their isotropic counterparts: their anisotropic and inhomogeneous nature leads to various damage mechanisms and models during the entire fatigue life stage. ...
... A number of distinguishing characteristics of this problem make it different from the fatigue modeling and life prediction of their isotropic counterparts: their anisotropic and inhomogeneous nature leads to various damage mechanisms and models during the entire fatigue life stage. In addition to the material, the loading characteristics, such as the amplitude, the ratio of cyclic stress, and the load sequence, have an effect on damage mechanisms and models [15,16]. This complexity is the cause of no methodology being consistent in robustly and accurately predicting life of various material systems. ...
... For methodologies based on phenomenological modeling to predict the fatigue life and the deterioration of structural strength under variable amplitude loads, the most widely used damage metric that does not always lead to accurate results is the simple empirical linear Palmgren-Miner rule, which cannot take into account the effect of load sequence [15]. Alternatively, the advantages of residual strength or static strength degradation as a damage metric make it promising in a scheme of fatigue life and deterioration of structural strength [17,18]. ...
A study was undertaken to develop a methodology for assessing the residual strength of C/SiC ceramic matrix composite panels subjected to combined thermal-acoustic loadings. A 2D plain-woven C/SiC ceramic matrix composite panel subjected to spatially uniform thermal loading and band-limited Gaussian white noise is chosen as the computational test article, with its geometric nonlinear response determined via numerical simulation. As the input, the material properties (static strength, residual strength, and fatigue life) of this material are fully characterized under tensile and compression loads, for fiber direction at elevated temperature in static and fatigue loading conditions. Based on the methodology, a computer code is developed that simulates the cycle-by-cycle behavior of composite panels under fatigue loadings. The methodology is validated with the residual strength test of 2D plain-woven C/SiC composite panel subjected to combined thermal-acoustic loadings. It has been shown that the results of residual strength predicted by the methodology are well correlated with the experimental results.
... Fatigue lifetimes can be calculated for variable loading conditions by combining the S-N curves with the Miner sum approach and Goodman diagrams, as is well known from standard materials textbooks, e.g., [6,7]. This regular approach has been used successfully for composite laminates for many years, as documented, for example, in references [8][9][10][11][12][13][14]. Alternative lifetime calculation methods have also been proposed for composites; good overviews are given in [11,15]. ...
... This regular approach has been used successfully for composite laminates for many years, as documented, for example, in references [8][9][10][11][12][13][14]. Alternative lifetime calculation methods have also been proposed for composites; good overviews are given in [11,15]. It is beyond the scope of this article to evaluate the alternative methods, but it seems that the regular approach is by far the most widely used in industrial applications. ...
... The authors of this article have shown previously that a relatively simple and accurate method for tolerance bounds can be found for some special cases [26]. Nijssen [11] shows a similar case in his in-depth review of fatigue prediction methods to characterize S-N curves. This article takes these developments one step further. ...
Fatigue S–N curves provide the number of stress cycles that result in fatigue failure at stress range S and need to be measured for new engineering materials where data are not as readily available as they are for well-characterized and widely used metals. A simple statistical method for the estimation of characteristic fatigue curves defined in terms of lower-tail quantiles in probability distributions of dependent variables is presented. The method allows for the estimation of such quantiles with a specified confidence level, taking account of the statistical uncertainty caused by a limited number of experimental test results available for the estimation. The traditional general approach for estimating characteristic S–N curves by tolerance bounds is complicated and is not much used by engineers. The presented approach allows for calculating the curves with a simple spreadsheet. The only requirement is that the experimental log S data for the S–N curve are fairly uniformly distributed over a finite logS interval, where S denotes the stress range. Experimental fatigue test programs are often designed such that test data fulfil this assumption. Although developed with fatigue of composite laminates in mind, the presented statistical procedure and the presented associated charts are valid for fatigue curve estimation for any material.
... To the authors' best knowledge, one single research effort in the literature has focused on fatigue testing for a full-scale WTB, while performing modal analysis at different stages to investigate and track the variation of the natural frequency and damping ratio associated with the fundamental mode as damage progressed [26]. In that study, the number of cycles that the test specimen was subjected (2 × 10 6 ) does not correlate properly with what is stated for composite materials of WTBs, where fatigue failure is produced throughout tens of million cycles, with an estimated life between 10 × 10 6 and 100 × 10 6 cycles [27]. Hence, the expected variation of Content courtesy of Springer Nature, terms of use apply. ...
... Both types of repetitive tests, representing short-term resonancetype and long-term fatigue-type conditions, progressively induced loss of effective stiffness in the tested WTBs, which in turn carried a decrease in the natural frequencies of the flapwise modes (direction of excitation) identified with the state-of-the-art identification algorithm using free vibration data. Similar observations related to the loss of stiffness as consequence of damage in fiber-reinforced composites [45] and WTBs [22,27] have been previously reported in the literature. It is noted that in [46] the natural frequencies identified in this paper were used within a Bayesian approach for damage diagnosis and prognosis of WTBs, confirming their ability and robustness as damagesensitive features. ...
Modal properties of dynamically tested wind turbine blades (WTBs) of a utility-scale wind turbine are identified. A comprehensive experimental program including free vibration and short- and long-term forced vibrations representing resonance and simplified fatigue conditions was carried out to investigate vibration-based features for damage diagnosis and prognosis. A set of 12 undamaged WTBs were tested to study the variability of the identified modal parameters. Results indicate that the variability of the natural frequencies was rather low, while the obtained damping ratios exhibited significant differences. Forced vibration tests were then conducted. To reach the failure of the blades, approximately 1.9 × 10⁴ and 4.2 × 10⁷ cycles were induced in the short- and long-term tests, respectively. Modal properties identified during testing protocols suggest that natural frequencies correlate well with damage. A linear finite element model was also developed, and its modal properties are compared to the identified modal parameters of the undamaged blades.
... These can cause changes in structural properties like degradation of the stiffness and the load-carrying capacity over a period of time and can eventually result in blade failure. Damage progression under cyclic loading is one of the primary issues to deal with in the design of wind turbine rotor blades [2]. It is of great impor- 10 tance to uncover the factors that drive the fatigue damage growth, in particular, the evolution of a new damage mode from the existing damage. ...
... Multidirectional laminates are used inside the cross-section of a rotor blade in shear webs 3 J o u r n a l P r e -p r o o f and in sandwich panel [2]. During flapwise bending, the laminates on the suction side of the blade mainly experience cyclic compressive loading while on the pressure side experience cyclic tensile loading. ...
Microscale fatigue damage evolution of off-axis tunnelling cracks was investigated for non-crimp fabric composites used in wind turbine blades. Test specimens fabricated from a multidirectional laminate of the layup sequence [0b/60b/0b/−60b]s (with subscript ’b’ representing backing) with glass fibres and epoxy resin, were subjected to cyclic tension-tension loads. Following the testing, small-sized off-axis samples with sides parallel to the major −60° off-axis fibre bundle were cut from the gauge zone of the tested specimens and were investigated by X-ray computed tomography using a dye penetrant. The height of the tunnelling cracks was found to vary in both the one-bundle + 60° and the two-bundle − 60°2 plies of the laminate. In the presence of the resin rich pockets above or below crack-tips in the thickness direction, the cracks were observed to penetrate with twisted crack planes aligned perpendicular to the overall loading direction. In the presence of backing, the crack-tips either continued to penetrate between fibres along fibre/matrix interface or deflected at near right angles on the fibre, depending on the orientation of backing fibres. The experimental observations of crack penetration and crack deflection mechanisms were analysed with respect to a crack deflection criterion.
... In the field of wind energy, it is essential to increase the yield per turbine by reducing the turn-off times and increasing the rotor diameter. With the rotor blades among the most fatigue-stressed technical components in terms of fatigue life time, load variability and ambient conditions [1,2], there is a need for stronger and more damage-resistant light-weight materials [3]. In the design process of wind turbine rotor blades, fatigue is one of the most important issues [4]. ...
... In the design process of wind turbine rotor blades, fatigue is one of the most important issues [4]. Therefore, excessive experimental characterisation of wind turbine composite materials [5,6] and rotor blade damage behaviour [5,[7][8][9][10][11] has been conducted over the last decades in addition to improving design and fatigue life prediction methods [1,4,12]. An improvement of the fatigue characteristics of the glass fibre-reinforced plastics (GFRP) not only increases the reliability of the rotor blades but also enables their size to be enlarged through a mass-reduced design. ...
Advanced nanoparticle-reinforced glass fibre composites represent a promising approach to improving the service life of fatigue-loaded structures such as wind turbine rotor blades. However, processing particle-reinforced resins using advanced infusion techniques is problematic due to, for example, higher viscosity as well as filtering effects. In this work, the effects of boehmite nanoparticles on viscosity, static properties and fatigue life are investigated experimentally. Whereas rheological analysis reveals a significant increase of viscosity in the case of pristine boehmite particles, an additional taurine surface modification of the particles can effectively reduce viscosity increase. As regards mechanical properties, significant improvements of both static as well as fatigue properties are found. The addition of 15 wt.% of boehmite particles increases fatigue life by a maximum of 270% compared to the unmodified fibre-reinforced epoxy. Transmitted light-based investigation of the damage mechanisms shows delayed initiation and smaller growth rates for laminates containing boehmite particles. At the same time, the observed mechanisms and their accumulation along the relative cycle number do not change significantly. In addition, by characterising autonomous heating, the so-called Risitano fatigue limit is determined. The results reveal that with increasing particle content there is an increase in the fatigue limit.
... For wind turbine blades, the composite material matrix is typically made of a thermoset material such as epoxy, polyester, and vinyl-Esther, and is rarely made of thermoplastic material [20]. Thermoset-based matrices constitute 80% of the market share of reinforced polymers, and they are preferable and dominating the market due to some advantages in their properties compared to thermoplastics [24]. For example, room/low cure temperature and low viscosity ease the infusion for manufacturing processes and increase production speed [20]. ...
... Also, due to much higher viscosity in thermoplastics than thermosets, it is difficult to manufacture parts longer than 5 m and thicker than 5 mm [20]. When comparing fatigue behavior, thermosets generally demonstrate better behavior than thermoplastics when glass or carbon fibers are used [24]. ...
Harsh environments such as desert and arid environments are showing promising potential for energy production from wind turbine blades. However, when wind turbines are erected in such environments, unavoidable damage due to sand particle impingement during sandstorms, especially on the blade's leading edge, is expected. The leading edge erosion of wind turbine blades has a detrimental effect on the aerodynamic performance of the turbine. This investigation aims to experimentally study the erosion behavior of wind turbine blades when subjected to abrasive silica particles. The effect of multiple testing parameters on the intensity of erosion was investigated, such as the angle of impingement, erosion duration, and air pressure. The effect of protective coatings on reducing the intensity of erosion was also studied. Three types of coatings were used either as a single layer or in a multilayer coating system: graphene H-146, graphene IA-700, and polyurethane. It has been found that graphene has great potential as a protective coating against erosion since a single layer of H-146 provided up to 19% material removal reduction compared to an uncoated specimen. Moreover, combining a polyurethane layer on top of a graphene IA-700 layer showed a significant erosion reduction reaching up to 60% when protecting the leading edge of a wind turbine blade. The effect of these coatings on the aerodynamic performance of wind turbine blades was also studied. It has been found that the leading erosion causes an increase in drag coefficient, a decrease in lift coefficient, and a decrease in the amount of power generated. Protective coatings were able to reduce aerodynamic loss. In general, a combination of protective coatings made of graphene and polyurethane showed an excellent erosion resistance in terms of reduction in the amount of material removed, reduction in the depth of erosion scars, and reduction in the aerodynamic loss. In light of the results presented in this investigation, polyurethane/graphene IA-700 coating can be used as a protective measure for wind turbine blades in erosion-prone environments and should be further investigated in terms of actual field applications.
... During its twenty years in service, a wind turbine blade (WTRB) undergoes 10 8 to 10 9 fatigue cycles [3,4] and is exposed to temperatures ranging from − 20 • C to +60 • C. At the composite scale, these loadings can lead to a significant deterioration of the material mechanical properties, due to damage such as fibre-matrix debonding, cracking and delamination [5][6][7][8]. These different types of damage occur at different scales, they can interact with each other, and lead to premature fibre breakage [9,10]. ...
... The issue remains concerning the identification of b and a parameters. Since decreasing them to 1.00 and 1.20, respectively, worsens the model prediction for UD [90] 4 lamination (Fig. 15), a compromise has to be found for the b and a values so that the chosen damage model gives satisfactory predictions simultaneously for UD [90] 4 Fig. 19 shows the model predictions of the shear fatigue strength at 15 • C and 40 • C. Keeping in mind that fatigue tests generally exhibit scattered results, the prediction seems to agree with the experimental results and to succeeds in representing the effect of temperature. However, supplementary fatigue tests at both 15 and 40 • C, and above all at 15 • C, reaching between 10 5 and 10 6 cycles, will be useful to confirm this capability. ...
In this study, a model formulated at the ply scale was considered to describe the mechanical behaviour of an acrylic-thermoplastic-matrix and glass-fibre-reinforced laminated composite under monotonic tensile and fatigue loadings within a temperature range of -20°C to +60°C. This study showed that the temperature dependency of the composite transverse and off-axis mechanical behaviour could be determined concisely, by considering only the temperature dependency of the matrix initial elastic properties and keeping the parameters of the damage evolution laws constant. This is a relevant result for designing composite structures with a limited set of experimental data.
... Approximately 95% of the modern wind turbine blades are made of fiber-reinforced composites because of their good mechanical characteristics: high stiffness, low density, and long fatigue life [1]. Compared to alternative materials, fiber-reinforced composites have other advantages in terms of weight, cost, quality, technical feasibility, market expectation, environmental impact, and health and safety. ...
... Here, u and v are random values from centered Gaussian distributions; β is the scale parameter, and its recommended range is [1,2]. In the process of the cuckoo search algorithm, n randomly chosen nests come into being and the ith nest is set nest i = (x i1 , x i2 , . . . ...
Moisture and temperature are the most important environmental factors that affect the degradation of wind turbine blades, and their influence must be considered in the design process. They will first affect the resin matrix and then, possibly, the interface with the fibers. This work is the first to use a series of metaheuristic approaches to analyze the most recent experimental results database and to identify which resins are the most robust to moisture/temperature in terms of fatigue life. Four types of resins are compared, representing the most common types used for wind turbine blades manufacturing. Thermoset polymer resins, including polyesters and vinyl esters, were machined as coupons and tested for the fatigue in air temperatures of 20 °C and 50 °C under "dry" and "wet" conditions. The experimental fatigue data available from Sandia National Laboratories (SNL) for wind turbine-related materials have been used to build, train and validate an Artificial Neural Network (ANN) to predict fatigue life under different environmental conditions. The performances of three algorithms (Backpropagation BP, Particle Swarm Optimization PSO, and Cuckoo Search CS) are compared for adjusting the synaptic weights of the ANN, and evaluating the efficiency in predicting the fatigue life of the materials studied, under the conditions mentioned above. For accuracy evaluation, Mean Square Error (MSE) is used as an objective function to be optimized by the three algorithms.
... This approach is still the one most commonly used in guidelines for the design of wind turbine (WT) blades [12] because it provides a more conservative life estimate than other models that consider the strength degradation [44], for example. Suresh [45] suggested that the simplest way to invoke stress-based approaches to fatigue in the presence of multi-axial stresses which involve a nonzero mean stress is to combine the equivalent stress amplitude σ a e and the equivalent mean stress σ m e on the basis of a suitable failure criterion, such as the one according to Beltrami (Eq. ...
Tunneling cracks in the adhesive layer of the trailing-edge (TE) joint can be initiated early in the lifetime of a wind turbine blade. Two major types of loads affect the crack initiation: thermal residual stresses that develop during manufacture, and mechanical stresses due to operating loads. Although giving consideration solely to the longitudinal stress component, which contributes mainly to the fatigue, is state-of-the-art, the current design guideline encourages designers to also take account of other stress components, i.e., peel and shear stresses.
Hence, this research investigates the impact of the multi-axial stress state due to cyclic loading on the tunneling crack initiation in the TE adhesive joint in order to develop an engineering approach for predicting the number of load cycles toward crack initiation. First, this research analyzes the stress-life of a polymeric neat adhesive material. Second, this research analyzes the crack initiation in the thick adhesive sandwich joint. To this end, a two-dimensional (2D) finite element (FE) model of the adhesive joint to approximate the asymptotic stress field at the bi-material corner is introduced. Third, this research analyzes the crack initiation in the TE adhesive joint of a full-scale rotor blade. To this end, the 2D FE model of the joint is extended to take account of the adhesive layer’s free-edge geometry, as well as the boundary conditions, and the multi-axial thermal and mechanical internal loads in a blade. Fourth, this research analyzes the crack initiation in the TE joint of a rotor blade subjected to field loads.
The Stüssi-Boerstra stress-life (S-N) model derived was found to reliably predict the fatigue life in the high-cycle fatigue regime with the lowest standard deviation. The present 2D FE model together with a calibration approach with an S-N model gave a better prediction of the crack initiation than classical laminated plate theory in a test campaign of a structural detail, namely a generic thick adhesive sandwich joint. The validity of the approach was proved in a cyclic full-scale test through the good agreement between the prediction of the approach parametrized with simulated test loads and observations on crack initiation made during the cyclic test. The thermal residual stress at the inner adhesive edge dominated the crack initiation. The stress level at the bi-material corner reacted very sensitively to fillet angle and adhesive thickness.
... Despite important advances on the use of computational techniques such as genetic programming for fatigue assessment of composite materials, 7 the process remains largely based on macroscopic empirical results and many intense experimental programs conducted in recent years confirms this trend. [8][9][10] Some fatigue approaches developed for metals were adapted for its use in FRCs. There are, however, substantial differences in the fatigue behavior between these materials. ...
The fatigue strength curves for fiber reinforced laminates are normally available at certain combinations of load ranges and minimum-to-maximum load ratios. The data are alternatively presented in the form of a constant life diagram. Real-world signals contain, however, a finite number of load ranges and mean values pairs which rarely coincide with those experimentally measured. For fatigue assessment purposes, an interpolation in the constant life diagram, seeking for fatigue strength curves not originally measured, is needed. It is the aim of this paper to present and apply a new way for interpolating in the constant life diagram. The interpolated fatigue strength curve is then used in a simulated fatigue life prediction case study. The source for fatigue loads comes from two normalized load signals of variable amplitude that can be scaled to any desirable (and reasonable) maximum stress maximum stress value. In this way, two predicted maximum stress versus cycles to failure can be computed. The relative position of these two curves agrees with the severity of each load signal which confirms the consistency of the proposed algorithm. Furthermore, the variable amplitude curves fall below the experimental ones (at a load ratio [Formula: see text]), that is, disregarding the interactions between large and small cycles implies in unacceptable (non-conservative) predictions in fatigue design.
... shells) while reducing the mathematical description down to a few degrees of freedom, is that it provides a computationally efficient platform to perform the intensive and iterative calculations involved in damage tracking, while still being sufficiently accurate for most design purposes. The new method taps into the significant progress made in the recent past with the modeling of thin-walled structures [7,21,23] and progressive failure analysis [26,30], the capabilities of which have been already validated against shell-based FE models in the previous works by Cardenas et al. mentioned above [20,22], while drawing on powerful yet simple fatigue damage and degradation modeling tools for composites [9,10,25]. The proposed methodology opens the way to applications such as fatigue damage progression modeling of rotary machinery under realistic operating conditions (as opposed to studies of design cases only), with previously unattained computational efficiency. ...
A thin-walled beam (TWB) model for integrated progressive failure analysis in composite materials under fatigue loading is presented for the first time. The model is computationally lean and capable of tracking the spatial distribution and further propagation of fatigue-induced damage in beam-like thin-walled composite structures (TWCS) by accounting for both sudden and gradual degradation of strength and stiffness moduli of material cells located at arbitrary positions. An integrated TWB model of a cylindrical symmetrical 8-layer test structure was studied to obtain insights into damage progression patterns and interactions between different damage mechanisms. It was shown that it is possible to localize and quantify the extent of damage at individual layers while providing explanations for damage initiation and propagation in engineering terms. The proposed model provides a computationally efficient platform for performing fatigue-induced progressive failure analysis in TWCS such as rotating blades of wind turbines or helicopters, allowing the assessment of a number of what-if scenarios with current off-the-shelve computing power.
... This creeping deformation can cause sorption-induced aging phenomena [16]. Indeed, the phenomenon of aging in the structures is a theme that finds application in various fields of engineering (e.g., [47]), in addition to civil engineering structures, and therefore, is a problem of wide interest. For the type of complex coupled diffusiondeformation-damage phenomena being considered in this work, the beam model [12; 24; 32; 43; 59] can serve as a feasible first step before considering the analysis of the more general 3-D problems. ...
... 9 For all materials, fatigue life prediction can be roughly divided into three categories: cumulative or progressive damage prediction, phenomenological/empirical methods, and micromechanics or mechanics-based models. 10 In 1934, Palmgren proposed his linear damage accumulation hypothesis to predict fatigue life under variable amplitude loading, which was tested by Miner in 1945 with data from fatigue experiments. 11 This well-known Palmgren-Miner (PM) rule uses a simple linear cumulative rule as a common damage metric. ...
Residual strength models are widely used to predict the fatigue life of composite laminates under highly variable loads. However, they often require considerable experimental effort to accurately determine model parameters. This paper introduces a new approach for predicting the residual strength of composite materials with less experimental data. The method is based on two common strength-based wearout models, the Sendeckyj model and Schaff and Davidson model. The Sendeckyj model consists of an equation with two model parameters, which describes the shape of the S-N curve by fitting fatigue test data. The Schaff and Davidson model is a single-parameter function which calculates the residual strength based on the number of fatigue cycles. Using a novel mathematical algorithm, the residual strength model parameter in the Schaff and Davidson model is estimated directly from the S–N curve without running any residual strength tests. Five data sets from the literature are used to validate the new methodology. The results show that the residual strength model parameter is dependent on both the stress level and the number of loading cycles experienced at this stress level, both of which are considered in the new strategy. In addition, if the fatigue data are well distributed, the residual strength model parameter estimated by the new strategy is close to the experimental data.
... On one hand, the study explained that fatigue degradation was rapid in the starting phase due to matrix cracking. On the other hand, while the degradation became gradual and linear in the centre phase due to interfacial debonding and matrix cracking, degradation again become rapid near the end of fatigue life as result of fibre breakage [127][128][129]. ...
The energy transition is growly rapidly. Yet, energy security and sustainability are still global concerns. The transition from fossil based, e.g., gas, to renewables, e.g., wind, hence, require reliable equipment and accurate lifetime predictions. Therefore, this review study is focused on turbine blades failures analysis with respect to their applications, materials, and operational conditions. Several cases relating the damage mechanisms associated with blades failures, e.g., corrosion-erosion, carbides precipitation, oxidation, coating degradation, high and low cycle fatigue, and creep, are discussed. To converge the topic, the work focuses on gas and wind turbine blades only. In addition, it sheds lights on several lifetime and failure prediction models and outlines recent trends in the additive manufacturing of turbine blades, e.g., core and microstructural grading. Lastly, it highlights several future research gaps that can aid in preventing similar failures.
... The higher slope of S-N curve of rPET-UPR matrix-based composite indicates the fatigue life degradation rate of the rPET-UPR matrix-based composite was higher as compared to the virgin polyester matrix composite, as slope of the S-N curve characterizes degradation rate in fatigue life [52]. Nijssen [53] reported that the composite materials of low S-N curve slope showed better fatigue behavior. This was probably due to the weak interface or poor adhesion between fiber and rPET-UPR and higher degradation of the recycled matrix. ...
Viability of recycling polyethylene terephthalate (PET) can be enhanced by increasing its usage as a matrix material in manufacturing of composite materials. Structural applications of composite materials almost always involve fatigue loading and evaluation of fatigue behaviour is essential to explore the full potential of composite materials based on recycled PET. With an aim to increase the acceptability of recycled PET based resins, fatigue performance of glass fiber reinforced composite materials based on unsaturated polyester resin derived from recycled PET (rPET-UPR) has been evaluated in the present research. Glass fiber composites laminates of stacking sequence [0/(±45)2/0]T have been fabricated using vacuum infusion process. Fatigue tests were performed at the stress ratio of 0.1, where the stress level varied from 40-80% of the ultimate tensile strength (UTS). The results of fatigue tests show that the fatigue lives of composites based on rPET-UPR were lower as compared to virgin polyester matrix composites. However, Statistical analysis of the fatigue life data using two parameter Weibull distribution, established that there is no deleterious effect on the scatter observed in fatigue lives of these composites, as compared to composites based on virgin polyester resin. Due to higher degradation of rPET-UPR matrix as well as weak interfacial properties, 13% higher self generated temperature was observed during fatigue loading in comparison to virgin resin based composite materials. A three phase stiffness degradation curve has been observed for these composite materials indicating stiffness loss in the range of 20-35 % till the time of failure, which correlates well with the experimental damage observations. A nonuniform and rapid damage growth was observed at high-stress levels, whereas a more uniform damage zone was observed at the low-stress levels.
... The advantages of thermosets are that they can be cured at room temperature; and have low viscosity, so their processing is easier. Polyester was used a lot in the past, but in recent years, manufacturers have moved to the use of epoxy due to the manufacturing size of the blades, since it is easier to work with this resin due to its rapid curing and better adhesion conditions with the fiber [23]. Thermoplastics are beginning to emerge for a simple reason: they are recyclable. ...
Wind turbines obtain clean energy from the wind, however, there is a significant environmental impact due to the use of some of their materials. This article analyzes the manufacturing, life cycle, and dismantling of these machines, to under-stand new opportunities to improve these negative aspects, through the review of various articles. The search was focused on SCOPUS articles, using the word "wind turbine" in titles, abstracts, and keywords, obtaining 68,362 results. Subsequently, these results were filtered only articles, reviews, and research theses, reducing the search to 3,663 results, the search was limited to only 10 years, counting from 2020 to 2010, reaching 2,189 documents. The analysis of 2,189 documents obtained is carried out, reducing the literary base to 185 documents with information on manufacturing processes, life cycle analysis, and advances in some countries in the implementation of improvements in the manufacture of wind turbines, to reduce environmental impact. The use of thermosetting materials in wind turbine blades is a reality that must be modified by the environmental problems that these are causing, new materials for blades must be developed by the principles of the circular economy.
... The findings have shown considerable weight reduction using CFRP while load bearing and deformation of steel suspensions are lower than composite suspensions. Nijssen, R.P.L et al [2] has investigated helical coil suspensions with varying fiber volume percentage from 30% to 75% . The findings have shown increase in tensile strength with increase in fiber contents and vice versa. ...
Helical coil suspensions are used to absorb vibrations as vehicle moves on rough roads. As automobile industry demands for increased fuel efficiency, the designers emphasize on weight reduction of automobile components to the maximum extent possible without much compromise in strength. The current research analyzes the Hero Honda suspension subjected to bumps on roads and causing deformation and stresses on the component. Using Taguchi Response surface optimization, response surface plots and sensitivity plots are generated for stress and strain energy. From the response surface optimization, it is evident that nearly 23% of mass reduction is possible. The CAD modelling and analysis is conducted using ANSYS 18.1 software which is based on Finite Element Analysis and response surface optimization is also conducted on the same platform.
... In addition, at present, most commercial large-scale wind turbine blades are made of glass fiber reinforced materials, which are mainly glass fiber reinforced polyester or epoxy resin [76]. Through fatigue campaign, fatigue properties of composites are found to be superior to those of many other materials, by virtue of their 'flat' S-N curve; however, the scatter in fatigue life is typically higher than for metals, and can only partly be explained by variations in test conditions [77]. ...
Wind, as a sustainable and affordable energy source, represents a strong alternative to traditional energy sources. However, wind power is only one of the options, together with other renewable energy sources. Consequently, the core concerns for wind turbine manufacturers and operators are to increase its reliability and decrease costs, therefore enhancing commercial competitiveness. Among typical failure modes of wind turbines, fatigue is a common and critical source. In view of the significance of fatigue reliability in wind turbine structural integrity, reliable probabilistic fatigue theories are necessary for design scheme optimization. By reducing the expenses on manufacturing, operation, and maintenance in reliability- and cost-optimal ways, the cost of energy can be significantly reduced. This study systematically reviews the state-of-the-art technology for fatigue reliability of wind turbines, and elaborates on the evolution of methodology in wind load uncertainty modelling. In addition, fatigue reliability assessment techniques on four typical components are summarized. Finally, discussions and conclusions are presented, intending to provide direct insights into future theoretical development and methodological innovation in this field.
... This indicates that the fatigue life degradation of the rPET-UPR matrix-based composite was higher as compared to virgin polyester matrix composite. Nijssen [42] reported that the composite materials of low S-N curve slope showed better fatigue behavior. Therefore, the fatigue performance of rPET-UPR based glass ber composite was lower as compared to virgin polyester-based glass ber composite. ...
Waste polyethylene terephthalate (PET) in the atmosphere creates an environmental concern. The use of waste PET as a matrix in a composite lowers the cost and environmental impact. In this study, the unsaturated polyester resin (rPET-UPR) is extracted from waste PET through the chemical recycling route (glycolysis) for the fabrication of four-ply glass fiber composites laminate of stacking sequence [0/(± 45) 2 /0] T . fatigue tests were performed at the stress ratio of 0.1, where the stress level varied from 40–80% of the ultimate tensile strength (UTS). The results of fatigue tests showed that the fatigue life of composites based on rPET-UPR was lower as compared to virgin polyester matrix composite probably due to weak interfacial properties and higher degradation of rPET-UPR matrix. Moreover rPET-UPR glass fiber composite had comparable fatigue performance. With increasing the fatigue loading cycle nonuniform and rapid damage growth were observed at high-stress levels however more uniform damage zone was observed at the low-stress level. From the fatigue fracture surface the fiber pullout, fiber pullout and fiber breakage, and fiber breakage failure mode was observed at 80%, 60%, and 45% stress level respectively while fiber pullout from intermediate ply had been observed might be due to weak interface and greater load transfer to intermediate ply after the failure of outer ply (UD). FE-SEM results revealed that excessive matrix damage was observed at low-stress level. The magnitude of stiffness degradation increased with decreasing the stress level might be due to excessive matrix damage.
... Repurposing options including small-scale, medium-scale, and large-scale applications have been presented by various authors, including Goodman (2010), Jensen and Skelton (2018), Bank et al. (2018), Suhail et al. (2019), Alshannaq et al. (2021a, b), André et al. (2020), Joustra et al. (2021), and Anmet/GP Renewables (2021). The results presented in the literature regarding strength and stiffness retention of FRP composites provide further support for proposed end-of-life applications, where it is expected that the previously used wind blade composite material will retain 80%-90% of its original properties (Nijssen 2006;Post et al. 2008;Lian and Yao 2010). ...
E-glass fiber-reinforced polymer (FRP) composite wind turbine blades are nonbiodegradable, and their end-of-life recycling solutions are limited. Research on reusing and repurposing applications, where minimal amounts of refabrication are needed, is being conducted to address this issue. To design new structures from decommissioned blades, their as-received mechanical and physical properties are needed. Even though some long-term property data for FRP composites exist in the literature, very little actual data for the as-received residual properties of decommissioned blades have been reported. The current work is aimed at developing a methodology to obtain as-received material property data for decommissioned wind turbine blades that are being proposed for use as second-life structural components. In this paper, details of the methods used and the test results for the key physical and mechanical properties of glass FRP material specimens extracted from the spar cap of a decommissioned 1.5-MW GE37 wind turbine blade are reported (the blade is from a General Electric 1.5 MW turbine which is known as a GE37 blade), including burnout testing for constituents' weight and volume fractions as well as fiber architecture and tension, compression, and shear testing in the longitudinal and transverse material directions. Comparisons between test results of other investigators and the experimental data obtained show promising strength and stiffness retention levels of the material for different properties. The results show that structural integrity still exists for the tested composite materials and no deterioration, crack propagation, or delamination was observed in the materials due to the cyclic loading levels experienced in their first life.
... A large number of accidents and research results show that fatigue is one of the main failure mechanisms for blades [1,2]. So, fatigue testing of a wind turbine blade is the most effective and reliable method to find weaknesses in a blade's design [3,4]. At present, the existing full-scale fatigue loading method is mainly composed of centrifugal pendulum resonance loading and hydraulic forced loading, which is almost applied to onshore wind power blades [5][6][7][8]. ...
A new dual-actuator fatigue loading system of wind turbine blades was designed. Compared with the traditional pendulum loading mode, the masses in this system only moved linearly along the loading direction to increase the exciting force. However, the two actuators and the blade constituted a complicated non-linear energy transferring system, which led to the non-synchronization of actuators. On-site test results showed that the virtual spindle synchronous strategy commonly used in synchronous control was undesirable and caused the instability of the blade’s amplitude eventually. A cross-coupled control strategy based on the active disturbance rejection algorithm was proposed. Firstly, a control system model was built according to the synchronization error and tracking error. Furthermore, based on arranging the transition process, estimating the system state and error feedback, and compensating disturbance, an active disturbance rejection controller was designed by adopting the optimal control function. Finally, on-site test results showed that the cross-coupled control strategy based on the active disturbance rejection algorithm could ensure the synchronization of two actuators. The maximum speed synchronization error of the two motors was less than 16 RPM, the displacement synchronization error of the two actuators was less than 0.25 mm and approaching zero after 4 seconds, and the peak value of vibration of the blade was less than 5 mm, which satisfied the fatigue test requirement.
... On the other hand, a flat S-N curve (S < 1) is often considered to show superior fatigue characteristics over the sloping S-N curve (S > 1) [26]. Therefore, due to the good fatigue behaviour of many composite materials, this slope in the composite materials is typically very low and a positive value of less than one should be considered. ...
S-N curves are used in the design of composite structures to estimate their fatigue behaviour. In this work, an empirical fatigue model has been developed based on one first proposed by Sendeckyj. To reduce the complexity of the nonlinear optimization in the Sendeckyj wearout model, a new constraint is derived based on the material S-N fatigue curve and a mathematical curve fitting. To improve the Sendeckyj model’s accuracy in all fatigue regions, a novel estimation method is used. In this method, an exponential model is used to determine the Sendeckyj model parameter for the low cycle fatigue region and a power-law model is used to determine the Sendeckyj model parameter for the high cycle fatigue region. The new model was compared against the original Sendeckyj model for glass fibre laminates with various polymer resins. The novel wearout method leads to more accurate parameters and more accurate S-N curve.
... Wind turbine structures are generally designed with a target fatigue life of 20 years [22,23]. Some studies have evaluated the fatigue life of turbines using the finite element method. ...
Vertical-axis wind turbines (VAWTs) are being reconsidered as a complementary technology to the more commercially used horizontal-axis wind turbines (HAWTs) because of their economical installation and maintenance. The selection of the blade numbers is one of the crucial concerns for VAWTs. This study focuses on the effects of the blade numbers on the fatigue lives of VAWT tower bases subjected to wind loading. Three straight-bladed VAWTs, with the same solidity ratios but different blade numbers, varying from two to four, were designed. The aerodynamic loading incurred by the VAWTs was computed using the corrected double-disk multistreamtube (DMS) model. The dynamic equations of the turbine systems were solved using the explicit central difference method. Then, a fatigue assessment model, including the crack-initiation and crack-propagation stages, was developed for the turbine tower bases. The results indicate that the three- and four-bladed VAWTs always presented better performances than the two-bladed VAWT in terms of the fatigue life. Moreover, increasing the number of blades from two to three improves the fatigue life of the tower base more than increasing it from three to four at lower wind speeds, while the latter is the more effective way to improve the tower-base fatigue life at higher wind speeds.
... The state-of-the-art procedure, in contrast, requires the characterization of each FRP fabric, cf. [12]. ...
One of the dominating factors in the fatigue of structures made from fiber-reinforced polymers, for example, wind turbine blades, is the polymer matrix. Traditionally, experimental stress–life data of polymers are approximated via a linear double-log Basquin model. Recently, the nonlinear stress–life formulation by Stüssi was found to provide a better fit of the experimental data with a substantially reduced standard deviation. Moreover, a nonlinear constant–life formulation, as proposed by Boerstra, can enhance the representation of the mean stress effect compared with state-of-the-art linear models given by the modified Goodman relation. To this end, Stüssi’s model was incorporated into the Boerstra relation to take account of the mean stress effects of an epoxy. This stress–life formulation was then enhanced with the Weibull probability function. The probabilistic–stress–life model provided a good approximation of the fatigue performance as a function of the stress ratio on the basis of an experimental data set. Finally, a stepwise engineering approach was suggested to derive the permissible stress–life with a view to practical design purposes. The procedure increased the reliability of the fatigue design evaluation compared with the state-of-the-art methodologies.
... The fatigue analysis approach used in this work is typically employed in the wind turbine industry for blade design, see e.g. [3]. Offset is taken in variable amplitude loading, which is quantified by Rainflow counting yielding a set of scaling factors for determining amplitude and mean stress. ...
... These materials are considered a good replacement for thermosetting matrices [51]. They can be recycled, which is very advantageous. ...
To meet the increasing energy demand, renewable energy is considered the best option. Its patronage is being encouraged by both the research and industrial community. The main driving force for most renewable systems is solar energy. It is abundant and pollutant free compared to fossil products. Wind energy is also considered an abundant medium of energy generation and often goes hand in hand with solar energy. The last few decades have seen a sudden surge in wind energy compared to solar energy due to most wind energy systems being cost effective compared to solar energy. Wind turbines are often categorised as large or small depending on their application and energy generation output. Sustainable materials for construction of different parts of wind turbines are being encouraged to lower the cost of the system. The turbine blades and generators perform crucial roles in the overall operation of the turbines; hence, their material composition is very critical. Today, most turbine blades are made up of natural fiber-reinforced polymer (NFRP) as well as glass fiber-reinforced polymer (GFRP). Others are also made from wood and some metallic materials. Each of the materials introduced has specific characteristics that affect the system’s efficiency. This investigation explores the influence of these materials on turbine efficiency. Observations have shown that composites reinforced with nanomaterials have excellent mechanical characteristics. Carbon nanotubes have unique characteristics that may make them valuable in wind turbine blades in the future. It is possible to strengthen carbon nanotubes with various kinds of resins to get a variety of different characteristics. Similarly, the end-of-life treatment methods for composite materials is also presented.
... This interest is driven by the application of carbon fiber plastics in structures subjected to vibrations. It is enough to list such high-load critical products as the an airplane wing, a fan blade of aircraft engine or a rotor blade of a wind turbine [3][4][5][6]. ...
It is necessary to achieve high fatigue strength in order to apply carbon reinforced fiber plastics (CRFP) for critical elements subjected to vibration. Gradual accumulation of fatigue damage is accompanied by changes in the material stiffness and natural frequencies. The purpose of this work is to found experimental data on the change of the elastic characteristics of layered CRFP as fatigue damage accumulates. The object of the study is standard samples made of unidirectional carbon/epoxy fiber with different layering schemes. The samples were subjected to fatigue tests under cyclic tension with constant amplitude. The test of each sample was stopped several times at different loading stages to execute experimental modal analysis and non-destructive inspection of the appeared defects. The found natural frequencies were used to solve the inverse problem of identifying the elastic parameters of the laminate monolayer: two Young’s modulus, shear modulus and Poisson’s ratio. As a result, the dependences of these parameters on the relative fatigue life were obtained. These dependences, together with the results of non-destructive testing, can be used to describe the process of fatigue damage accumulation and for the subsequent development of methods for the fatigue life prediction.
... It is important to note that the strength and modulus data presented herein are for wind blades that are newly manufactured. Trends in the data from the literature showed that a slight decline (10%-20%) in the strength and stiffness is observed in the life of fatigued composite materials (forming the wind blades), while a sudden drop is attained near their full design life (Post 2005;Nijssen 2006;Post et al. 2008). Materials testing and full-scale structural testing is being conducted on used wind blades by the research team to validate these findings for wind blade composites. ...
This paper focuses on the conceptual use of a fiber-reinforced polymer (FRP) wind turbine blade that is repurposed for a second life as an electrical transmission pole. Thousands of tons of fiber-reinforced polymer composite wind turbine blades are currently coming out of service globally and are being landfilled or incinerated. These are not environmentally preferable disposal methods. This paper presents a detailed structural analysis of a Clipper C96, 46.7-m-long turbine blade used as an electrical pole. The analytical procedure needed to characterize the wind turbine blade for repurposing includes determining the external and internal geometry of the blade, identifying the types of materials and laminates used throughout the blade, and calculating effective moduli and section properties for structural analysis. Code-specified load combinations are then used to analyze the transmission line BladePole to determine internal forces and deformations and stresses. Maximum stresses were compared to those obtained from theoretical models. The results indicate that wind turbine blades can safely be used as electrical transmission poles.
... A significant body of work on PHM for wind turbine subsystems exists. The key subsystems that the majority of this work focuses on includes: blades and rotor (Hameeda et al., 2009;Hyers, Mcgowan, Sullivan, Manwell, and Syrett, 2006;Nijssen, 2006;Tchakoua, Wamkeue, Ouhrouche, Slaoui-Hasnaoui, Tameghe, and Ekemb, 2014;Tchakoua, Wamkeue, Tameghe, and Ekemb, 2013), gearbox and bearings (Hameeda et al., 2009;Hussain & Gabbar, 2013;Hyers et al., 2006;Niknam, Thomas, Hines, and Sawhney, 2013;Plumley, Wilson, Kenyon, Andrew, Quail, and Athena, 2012;Qu, Bechhoefer, He, and Zhu, 2013;Tamilselvan, Wang, Sheng, and Twomey, 2013;Tchakoua et al., 2014;Tchakoua et al., 2013), generator (Hameeda et al., 2009;Hyers et al., 2006;Tchakoua et al., 2014;Tchakoua et al., 2013;Yang, Sheng, and Court, 2012) and tower (Adams, White, Rumsey, and Farrar, 2011;Chase, Danai, Lackner, and Manwell, 2013;Ciang, Lee, and Bang, 2008;Hameeda et al., 2009;Hyers et al., 2006;Tchakoua et al., 2014;Tchakoua et al., 2013). These works use the data from the supervisory control and data acquisition (SCADA) and other sensors. ...
A simulation-based real options analysis (ROA) approach is used to determine the optimum predictive maintenance opportunity for a wind turbine with a remaining useful life (RUL) prediction. When an RUL is predicted for a subsystem in a single turbine using PHM, a predictive maintenance option is triggered that the decision-maker has the flexibility to decide if and when to exercise before the subsystem or turbine fails. The predictive maintenance value paths are simulated by considering the uncertainties in the RUL prediction and wind speed (that govern the turbine’s revenue earning potential). By valuating a series of European options expiring on all possible predictive maintenance opportunities, a series of option values can be obtained, and the optimum predictive maintenance opportunity can be determined. A case study is presented in which the ROA approach is applied to a single turbine.
... Several possible approaches are available for estimating . Although a multiple R-value constant-life diagram (Nijssen, 2006) would potentially offer the highest accuracy, it would require the definition of several material parameters. For the sake of simplicity, in this investigation a (shifted) Goodman diagram is therefore used. ...
In the design and calculation of rotor blades for wind turbines, a sector representation of the loads is often used. The investigation proposed in this paper indicates that, depending on the loading direction, the evaluated sector and the considered location on the rotor blade, the sector approach can result in a significant overestimation of the acting stresses. This translates in a higher calculated fatigue stress exposure (up to 101% in the considered example). However, under particular conditions, the stresses and therefore the stress exposure are correctly assessed. Based on the observation, an improvement of the current method is suggested. The improved method, called invariant-sector method, relies on the appropriate selection of the sector(s) to be evaluated. The method allows for an accuracy equal to the one of the direct method but with a computational effort that is orders of magnitude lower.
... En effet, l'étude de [Vauthier et al., 1998] sur les effets de vieillissement hygrothermique sur les propriétés de fatigue d'un unidirectionnel verre/époxy composite montre que la durée de vie du composite diminue après vieillissement dans l'eau. Concernant l'effet de la température, [Nijssen, 2006] rapporte que les propriétés mécaniques en quasi-statique diminuent avec l'augmentation de la température, notamment les propriétés en cisaillement. En outre, la fatigueà températureélevée conduità des durées de vie plus courtes. ...
L'allègement des véhicules est un enjeu majeur de l'industrie automobile pour participer, avec l'évolution des motorisations, à la maîtrise des consommations énergétiques et la réduction des émissions de gaz à effet de serre. Dans ce travail, nous nous intéressons à l'introduction des matériaux composites dans les pièces de structure et particulièrement dans le périmètre de la liaison au sol composé d'organes de sécurité active, sujets au phénomène de fatigue multi axiale à grand nombre de cycles.Les matériaux composites présentent une solution séduisante en raison de leurs propriétés mécaniques intéressantes combinées à une faible densité. Toutefois, la fatigue des matériaux composites reste un sujet complexe relativement peu abordé. C'est dans ce cadre que s'inscrit cette thèse qui vise à mettre en place une méthodologie de dimensionnement des composants automobiles de structure, à partir d'un composite tissé verre/époxy. Cette méthodologie s'attache à être facile d'utilisation et adaptable au calcul de structure pour être applicable en Bureau d'Études. La première étape de cette étude est la caractérisation de la tenue en service du matériau sous chargements monotones et cycliques et l'identification des cinétiques d'endommagement. Au vu des résultats expérimentaux obtenus et à partir des approches de dimensionnement existantes, un critère de fatigue multi axiale est proposé. Ensuite, une optimisation du protocole d'identification des paramètres est effectuée afin de réduire au minimum le volume des campagnes d'essais. Enfin, le critère mis en place pour évaluer la durée de vie en fatigue du matériau composite tissé est validé sur des éprouvettes trouées et sur le train avant à lame composite.
... Although the impact of moisture on mechanical properties of composites has been investigated for a long time, still to date, the interactions have not yet been fully described. In the past, various studies have been published reaching from short term up to 10 or 15-years in-and outdoor conditioning [2][3][4][5][6]. Therefore, it is known, that diffusion of water within composites affects the matrix [7], the fibers [8] and the interphase region [9][10][11] in different manners. ...
The growing success of fiber reinforced polymers (FRP) as material for the construction of high-performance lightweight structures used under maritime environmental conditions, requires foremost the knowledge about their long term durability. As the supplier market is still growing fast, methods which allow manufacturers to distinguish between suitable and less suitable composites are needed. In this study, the effects of moisture on the mechanical properties of glass fiber reinforced polymers (GFRP) using different glass fiber fabrics are investigated under several ageing and testing conditions. Focusing on the fabrics and introducing an ageing method prior to composite manufacturing, allows to describe the proportions of fiber, matrix, sizing and interphase damage to the composites durability in more detail. Absorption quantity related testing after ageing at temperatures between 8°C and 50 °C highlights the resulting effects on the tensile strength of mostly unidirectional GFRPs. The strength of composites based on fabrics with high resistance to moisture degradation decreases steady but moderate during absorption. This effect is mainly associated with changes of matrix properties. However, less durable composites show a two-stage behavior. In this case, severe interphase damage and cracking leads to an additional drastic strength decrease when exceeding a defined amount of water absorption.
The use of thermal indexes assessed by monitoring rapid fatigue tests with infrared detectors as reliable damage parameters to reconstruct the S/N curve is currently a topic still presenting open points. First of all, the selection of the proper thermal index representing damage is a topic to be explored, and also the relationship between the damage from rapid stepwise tests and constant amplitude tests is another point of discussion.
In the present work, we deal with such an issue by investigating the first amplitude harmonic (FAH) of thermal signal related to thermoelastic phenomena and dissipative effects too. It has been demonstrated that FAH is related to stiffness degradation and stress-induced effects. Moreover, it provides a local analysis of specific effect related to the material fatigue damage without artefacts.
The results show that due to the relationship between stiffness degradation and FAH and specific material properties, it is possible to reconstruct the S/N curve by carrying out just one constant amplitude test and a stepwise rapid test. Moreover, the capability of temperature FAH to study fatigue behaviour and detect damage during any loading procedure is also presented.
Multiphysics of Wind Turbines in Extreme Loading Conditions addresses the extreme transient loading of wind turbines through a multiphysics modeling approach, notably by considering the dynamic effects and the nonlinearities of the physics involved in such situations. The book forms the basis for understanding multiphysic numerical simulations conducted on onshore and offshore wind turbines and subjected to extreme loading conditions, including storms, earthquakes, blasts, impacts, and tsunamis. The multiphysics approaches used in this book are explained in each chapter, with algorithms then turned into numerical codes to attain a realistic picture of the dynamic response in each scenario. With numerical methods and loading data explained, the complexity of potential problems encountered when extreme dynamic loads are discussed, along with loading types and their effects. The book fills a specific niche in wind power, namely extreme transient loading of wind turbine, offering information and industrial practices as wind energy makes it useful to practice engineers, designers, undergraduate and graduate students.
Laminated composite structures have a distinct inherent potential for optimization due to their tailorability and their associated complex failure mechanisms that makes intuitive design remarkably difficult. Optimization of such is a maturing technology with many criteria and manufacturing constraints having been successfully demonstrated. An approach for high-cycle fatigue is however yet to be developed in a gradient-based context. Thus, the objective of this work is to introduce a novel framework that allows for effective high-cycle fatigue optimization of laminated composite structures.Offset is taken in the Discrete Material and Thickness Optimization parametrization, which allows for simultaneous material and thickness selection for each layer that constitute a laminate. The fatigue analysis approach is based on accumulating damage from all variable-amplitude cycles in an arbitrary spectrum. As high-cycle fatigue behavior is highly nonlinear, it is difficult to handle in optimization. To stabilize the problem, damage is scaled using an inverse P-mean norm formulation that reduces the nonlinearity and provides an accurate measure of the damage. These scaled damages are then aggregated using P-norm functions to reduce the number of constraints. This is convenient, as it allows sensitivities to be efficiently calculated using analytical adjoint design sensitivity analysis. The effectiveness of this approach will be demonstrated on both benchmark examples and a more complicated main spar structure.
The size of wind turbine blades is increasing rapidly, and they are being installed in remote offshore locations. Consequently, it is essential to focus on improving the design and maintenance procedures in the blade industry to meet the growing demand. Of particular concern is the long-term operational performance of the wind turbine blade trailing edge. In this paper, we discuss the application of durability and damage tolerance analysis (DADTA) approaches to trailing edge service life prediction. DADTA is mandated in the aerospace sector to support airworthiness certification and to provide an updated life prediction of the structure based on the different stages of their service life. The DADTA framework has two main parts: durability and damage tolerance analysis. The durability part uses a structural fatigue approach based on a damage accumulation method during the initial design phase to predict the lifespan of a structure without defects. On the other hand, the damage tolerance analysis part uses a fracture mechanics approach and a damage growth method to update the lifespan prediction of a structure during the operation stages. This is achieved by utilizing sensors and inspection data as inputs while the structure is in service. Both these methods are comprehensive and have merits; however, their broad adoption in the wind turbine blade industry is still lacking. The current paper provides an extensive review of these methods and shows how these can be applied to the wind turbine blade industry, specifically for predicting the structural design life of the trailing edge of composite wind turbine blades. The review includes (a) defining wind turbine trailing edge failure modes, (b) trailing edge design procedures, and (c) a detailed discussion of the application of durability and damage tolerance analysis for trailing edge life prediction. Overall, this review paper would be of special interest to blade designers and would guide researchers and engineers interested in life prediction methodologies based on DADTA approaches for wind turbine blades.
Compression moulding is a well-established technique for processing polymer composite materials, using either thermoset or thermoplastic matrices. It is sometimes referred to as matched-die moulding, where the fibre reinforced composite material is forced to deform or flow within the mould cavity. Compression moulding is commonly associated with the forming of composite materials with discontinuous fibre reinforcements, at low (15%) to medium (50%) volume fractions.
Compression moulding is used to process pre-impregnated intermediate products to produce semi-structural and structural composite components, for example thermoset Sheet Moulding Compounds (SMC), Glass Mat Thermoplastics (GMT) or Long Fibre Thermoplastics (LFT). Most recently, compression moulding has been used to process platelet-like materials, i.e. fibre strands or bundles pre-impregnated with a thermoset or a thermoplastic matrix, which are similar in nature to waste thermoset prepregs or recycled thermoplastic materials. This process is attractive because it is easily automated and can be used to produce very complex geometries without any waste, where the shape of the component can be used to significantly enhance the stiffness of the structure (e.g. ribs). Molding cycle times are typically dependent on part thickness, but range from 60 to 300 seconds, offering one of the fastest moulding processes for thermoset materials. The aim of this chapter is to give an overview of the various moulding compounds and their associated compression moulding processes, and to discuss the effect of processing conditions on the induced microstructure.
The use of thermal indexes assessed by monitoring rapid fatigue tests with infrared detectors as reliable damage parameters to reconstruct S/N curve is a topic still presenting open points. First of all, the selection of the proper thermal index representing damage is a topic to be explored and also the relationship between the damage from rapid stepwise test and constant amplitude test is another a point of discussion.
In present work, we deal with such an issue by investigating the First Amplitude Harmonic (FAH) of thermal signal related to thermoelastic phenomena and dissipative effects too. It has been demonstrated that FAH is related to stiffness degradation and stress induced effects. Moreover, it provides a local analysis of the specific effect related to the material fatigue damage without artifacts.
The results show that due to relationship between stiffness degradation and FAH, and specific material properties it is possible to reconstruct S/N curve by carrying out just one constant amplitude test and stepwise rapid test. Moreover, the capability of temperature FAH to study fatigue behaviour and detect damage during any loading procedure, are also presented.
This study focuses on characterizing the fatigue damage accumulated in nonlinear aeroelastic systems subjected to stochastic inflows through both numerical simulations and wind tunnel experiments. In the mathematical model, nonlinearities are assumed to exist either in the structure (via a cubic hardening nonlinearity in the pitch stiffness), or in the flow (via dynamic stall condition), or simultaneously in both the structural and aerodynamic counterparts. The aerodynamic loads in the attached flow and dynamic stall conditions are estimated using Wagner’s formulation and semi-empirical Leishman-Beddoes model, respectively. To augment the findings to in-field flow conditions, the oncoming wind flow is considered to be randomly time-varying in nature. The stochastic input flow fluctuations are modeled using a Karhunen–Loeve Expansion formulation. The response dynamics and the associated fatigue damage of the aeroelastic system, possessing different sources of nonlinearities, are systematically investigated under isolated cases of deterministic and stochastic input flows. Specifically, the pertinent role of stochasticity in the input flow is brought out by presenting the response dynamics and the associated fatigue damage accumulation for different values of noise intensity and time scale of the input flow fluctuation. It is demonstrated that under fluctuating flow conditions, the dynamics intermittently switch between attached flow and the dynamic stall regimes even at low mean flow speeds. The intermittent nature of the response varies as the time scale and intensity of the oncoming flow are varied. The role of torsional stresses as the predominant component dictating the fatigue damage accumulation irrespective of the source of nonlinearity is illustrated. Using the rainflow counting method and Miner’s linear damage accumulation theory, it is shown that the accumulated fatigue damage is substantially higher under stochastic flow conditions as compared to deterministic input flows. Importantly, it is observed that different time scales and intensities of the oncoming flow fluctuation play a pivotal role in dictating the fatigue damage in aeroelastic systems. Finally, fatigue damage is observed to be significantly higher for torsionally dominant oscillations in the dynamical stall regime compared to the oscillations at the attached flow regime. The numerical findings are strengthened by drawing comparisons with the preliminary results obtained from wind tunnel experiments performed on a NACA 0012 airfoil undergoing dynamic stall. To the best of our knowledge, this is the first study that systematically bridges the dichotomy between the stall-induced dynamical signatures in stochastic aeroelastic systems and maps the same to the corresponding structural damage.
This study focuses on characterizing the fatigue damage accumulated in nonlinear aeroelastic systems subjected to stochastic inflows through both numerical simulations and wind tunnel experiments. In the mathematical model, nonlinearities are assumed to exist either in the structure (via a cubic hardening nonlinearity in the pitch stiffness), or in the flow (via dynamic stall condition), or simultaneously in both the structural and aerodynamic counterparts. The aerodynamic loads in the attached flow and dynamic stall conditions are estimated using Wagner's formulation and semi-empirical Leishman-Beddoes model, respectively. To augment the findings to in-field flow conditions , the oncoming wind flow is considered to be randomly time-varying in nature. The stochastic input flow fluctuations are modeled using a Karhunen-Loeve Expansion formulation. The response dynamics and the associated fatigue damage of the aeroelastic system, possessing different sources of nonlinearities, are systematically investigated under isolated cases of deterministic and stochastic input flows. Specifically, the pertinent role of stochasticity in the input flow is brought out by presenting the response dynamics and the associated fatigue damage accumulation for different values of noise intensity and time scale of the input flow fluctuation. It is demonstrated that under fluctuating flow conditions, the dynamics intermittently switch between attached flow and the dynamic stall regimes even at low mean flow speeds. The intermittent nature of the response varies as the time scale and intensity of the oncoming flow are varied. The role of torsional stresses as the predominant component dictating the fatigue damage accumulation irrespective of the source of nonlinearity is illustrated. Using the rainflow counting method and Miner's linear damage accumulation theory, it is shown that the accumulated fatigue damage is substantially higher under stochastic flow conditions as compared to deterministic input flows. Importantly, it is observed that different time scales and intensities of the oncoming flow fluctuation play a pivotal role in dictating the fatigue damage in aeroelastic systems. Finally, fatigue damage is observed to be significantly higher for torsionally dominant oscillations in the dynamical stall regime compared to the oscillations at the attached flow regime. The numerical findings are strengthened by drawing comparisons with the preliminary results obtained from wind tunnel experiments performed on a NACA 0012 airfoil undergoing dynamic stall. To the best of our knowledge, this is the first study that systematically bridges the dichotomy between the stall induced dynamical signatures in stochastic aeroelastic systems and maps the same to the corresponding structural damage.
Axial flow fan blades made of glass fibre reinforced plastics have a detrimental impact on the environment after their service life due to their non-biodegradability. To reduce the impact of blade materials on the environment and to improve biodegradability, it is necessary to reinforce the material with natural fibres without effecting the mechanical properties. This paper investigates the tension–tension fatigue life of three different hybrid composites. They are an 18-ft axial flow fan blade material (GFRP), the 8th layer of GFRP replaced with woven jute (G8J), and the 12th layer of GFRP replaced with woven jute (G12J). The number of cycles to failure was found at a stress ratio of 0.1, a test frequency of 10.5 Hz and at normalized peak stress ratios of 0.85, 0.7, 0.55, and 0.4 by using constant amplitude loading. As S-N curves were fitted into the experimental data, power function formulas were developed to estimate fatigue life at specified normalised peak stress ratios. It was interesting to note that all the three materials had reached their endurance limit (10⁶ cycles) at a normalized peak stress ratio of 0.4 and the fatigue life of woven jute reinforced composites is about 78% of GFRP. Thus, it was proposed that partial reinforcement of woven jute in fan blades is recommended at low normalised peak stresses when biodegradability after service life is the major concern. The manufacturing cost of the woven jute reinforced blade is also less than the conventional blade. The two-parameter Weibull distribution function was used to correlate the scattered fatigue life data at various normalized peak stress ratios. These curves have a lot of design value in real composite material applications because they anticipate the sample response at the time of service based on the degree of reliability.
This paper discusses the effect of loading rate on the static strength and fatigue behavior of glass/polyester and graphite/epoxy composites. It is shown that the rate-sensitivity of the static strength and the fatigue data can be analytically taken into account in the framework of a two-parameter “residual strength” model, inherently fulfilling the Strength-Life Equal-Rank Assumption (SLERA). Based on the static strength's loading rate sensitivity, a method to use the fatigue life data obtained at any fixed frequencies to predict the fatigue life under different frequencies is proposed. The method's reliability is verified by predicting different data sets of CA fatigue life and residual strength data obtained on glass/polyester composites under different loading rate conditions. The responses of angle-ply carbon/epoxy laminates to arbitrary two-block cyclic loadings in ascending and descending frequency sequences revealed the method's capability to discriminate between the applied loading sequences and the potential to develop a numerical routine accounting for the cycle-by-cycle variability during realistic loading conditions.
Composite materials offer many advantages during the use phase, but recovery at the end of a lifecycle remains a challenge. Structural reuse, where an end of life product is segmented into construction elements, may be a promising alternative. However, composites are often used in large, complex shaped products with optimised material compositions that complicate reuse. A systematic approach is needed to address these challenges and the scale of processing. We investigated structural reuse taking wind turbine blades as a case product. A new segmentation approach was developed and applied to a reference blade model. The recovered construction elements were found to comply to geometric construction standards and to outperform conventional construction materials on specific flexural stiffness and flexural strength. Finally, we explored the reuse of these construction elements in practice. Together, the segmentation approach, structural analysis and practical application provide insights into design aspects that enable structural reuse.
Wind turbine fatigue estimation is based on time‐consuming Monte Carlo simulations for various wind conditions, followed by cycle‐counting procedures and the application of engineering damage models. The outputs of the fatigue simulations are large in volume and of high dimensionality, as they typically consist of estimates on finite‐element computational meshes. The strain and stress tensor time series, which are the primary quantities of interest when considering the problem of fatigue estimation, are dictated by complex vibration characteristics due to the coupled effect of aerodynamics, structural dynamics, geometrically non‐linear mechanics, and control. A Variational Auto‐Encoder (VAE) is trained in order to model the probability distribution of the accumulated fatigue on the root cross‐section of a simulated wind turbine blade. The VAE is conditioned on historical data that correspond to coarse wind‐field measurement statistics, such as mean hub‐height wind speed, standard deviation of hub‐height wind speed and shear exponent. In the absence of direct measurements of structural loads, the proposed technique finds applications in making long‐term probabilistic deterioration predictions from historical Supervisory, Control, and Data Acquisition (SCADA) data, while capturing the inherent aleatoric uncertainty due to the incomplete information on strain time series of the wind turbine structure, when only SCADA data statistics are available.
The dominating factor in the fatigue of structures made from fiber reinforced polymers (FRP), for example wind turbine blades, is the polymer matrix. Traditionally, experimental stress-life data of polymers is approximated via a linear double-log Basquin model. Recently, the non-linear stress-life formulation by Stüssi was found to provide a better fit of the experimental data with a substantially reduced standard deviation. Moreover, a non-linear constant-life formulation, as proposed by Boerstra, for example, can enhance the representation of the mean stress effect compared to state-of-the-art linear models, i.e., the modified Goodman relation. To this end, we incorporated Stüssi's model into the Boerstra relation to take account of the mean stress effects of an epoxy. This stress-life formulation was then enhanced with the Weibull probability function. The probabilistic-stress-life model provided a good approximation of the fatigue performance as a function of the stress ratio on the basis of an experimental data set. Finally, we suggested a step-wise engineering approach to derive the permissible stress-life with a view to practical design purposes. The procedure increased the reliability of the fatigue design evaluation compared to the state-of-the-art methodologies.
The research was made to study the effect of low amplitude loads on the fatigue life of CFRP samples with impact damage in a cyclogram of irregular loading. The evaluation of the applicability of the linear summation hypothesis of damage (Palmgren’s Miner’s rule) at this level of loading was also carried out. The results have shown that in some cases correction factors need to be introduced.
A survey of Classical Fatigue Analysis predictions has been carried out for fibre reinforced composite laminate specimens subjected to spectrum loading. Results illustrate acceptable predictions for matrix dominated composites but unreliable and generally un-conservative predictions for fibre dominated composites. A need for cyclic damage event definition relating to composite behaviour has been identified. Load-cycle “Mix” damage events have been proposed based on block test results which show reduced fatigue life for increased load cycle mixing. A methodology has been proposed based on two-level block test base data to account for mix damage events. New counting methods have been investigated to account for possible mix damage events in addition to “Rainflow” hysteresis events. FALSTAFF, TWIST and variations of these spectra have been interrogated using mix counting algorithms to illustrate the level of load mixing that occurs in these standard spectra. Preliminary predictions have been made using simple multi-block tests to trial the proposed methodology accounting for mix-damage events. Results show improved predictions warranting further work on mix event definition and extrapolation methods.
A cumulative damage procedure is developed to predict the fatigue failure of engineering metals subjected to complicated stress-strain histories. Histories with plastic strainings and cycles not completely reversed in stress are considered. The relationship between stress-strain behavior and fatigue life is investigated for unnotched axially loaded specimens for which the stresses and strains can be measured for the duration of all tests. Since either the stress history or the strain history was known before each test was conducted, the other could be estimated and a life prediction made. A general cumulative damage procedure is used to make life predictions for a wide variety of complicated history tests on 2024-T4 aluminum.
▪ Abstract Renewable energy resources, of which wind energy is prominent, are part of the solution to the global energy problem. Wind turbine and the rotorblade concepts are reviewed, and loadings by wind and gravity as important factors for the fatigue performance of the materials are considered. Wood and composites are discussed as candidates for rotorblades. The fibers and matrices for composites are described, and their high stiffness, low density, and good fatigue performance are emphasized. Manufacturing technologies for composites are presented and evaluated with respect to advantages, problems, and industrial potential. The important technologies of today are prepreg (pre-impregnated) technology and resin infusion technology. The mechanical properties of fiber composite materials are discussed, with a focus on fatigue performance. Damage and materials degradation during fatigue are described. Testing procedures for documentation of properties are reviewed, and fatigue loading histories are discuss...
Degradation of the residual strength of a glass-fiber-reinforced polyester composite of a lay-up typical of the wind rotor
blade material is studied at low-cycle fatigue. A gradual reduction of the residual strength is observed as expected for GRP,
accompanied by an increasing scatter of strength. The residual strength model based on the strength-life equal rank assumption
yields an accurate approximation of experimental data. The strength reduction at a stress level corresponding to high-cycle
fatigue (N>10
6 cycles) appears to correlate well with the test results at higher stress levels, which indicates that the strength degradation
at the design stress level can be evaluated using low-cycle tests. Assuming that the parameters of the strength degradation
model do not depend on the applied stress level, the residual strength data obtained in low stress level tests of comparatively
short duration can be used to estimate the average fatigue life at the same stress thus reducing the total test time.
Two simple algorithms for performing rainflow counting are presented in this paper. The second algorithm is suitable for microcomputer devices that are placed in vehicles to record field data.
The combined Mode I (crack opening) and Mode II (forward shearing) fracture behaviour of unidirectional T300/1034C graphite/epoxy and graphite-reinforced APC-1 polyetheretherketone is characterized using the notched off-axis tensile test. The results and limitations of the test are discussed. The modified three rail shear test is used to obtain the in-plane Mode II fracture toughness, and results for the epoxy and thermoplastic system are compared. Scanning electron microscope images of the epoxy and thermoplastic at various mixed-mode ratios are presented and discussed.
Stress/number of cycles to failure relations were established for unidirectional, cross-plied and angle-plied laminates of APC-2 using three-point bend flexure tests. Results for constant stress ratio fatigue tests were modelled using both Weibull and fatigue modulus degradation analyses. Behaviour was compared with that of an established carbon/epoxy system.
Fatigue damage increases with applied load cycles in a cumulative manner. Cumulative fatigue damage analysis plays a key role in life prediction of components and structures subjected to field load histories. Since the introduction of damage accumulation concept by Palmgren about 70 years ago and ‘linear damage rule’ by Miner about 50 years ago, the treatment of cumulative fatigue damage has received increasingly more attention. As a result, many damage models have been developed. Even though early theories on cumulative fatigue damage have been reviewed by several researchers, no comprehensive report has appeared recently to review the considerable efforts made since the late 1970s. This article provides a comprehensive review of cumulative fatigue damage theories for metals and their alloys, emphasizing the approaches developed between the early 1970s to the early 1990s. These theories are grouped into six categories: linear damage rules; nonlinear damage curve and two-stage linearization approaches; life curve modification methods; approaches based on crack growth concepts; continuum damage mechanics models; and energy-based theories.