Failure analysis of a composite wind turbine blade at the adhesive joint of the trailing edge

Engineering Failure Analysis 121 (2021) 105148

Contents lists available at ScienceDirect

Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal

Failure analysis of a composite wind turbine blade at the adhesive joint of the trailing edge Roham Rafiee *, Mohammad Reza Hashemi-Taheri Composites Research Laboratory, Faculty of New Science and Technologies, University of Tehran, Tehran 1439955171, Iran

A R T I C L E I N F O

A B S T R A C T

Keywords: Wind turbine blade Finite element modeling Trailing edge Adhesive joint failure Cohesive zone modeling

The trailing edge (TE) failure of a commercial wind turbine blade (WBT) is investigated in this article. The effect of adhesive thickness and band width on the TE failure is one of the most important issues in WTB production governing the integrity and soundness of the WTB structure. Different static load cases of a WTB are identified under various operating conditions as a case study. Static analysis is performed on a full 3-D finite element model of the blade and the critical region of its TE is determined. Then, sub-part modeling is performed focusing on the critical region of the TE. Adhesive joint in the constructed model is modeled using cohesive zone modeling (CZM). The failure of the adhesive joint with different dimensions of adhesive is examined under normal wind condition and gust condition. Taking into account the production limitations, the proper dimensions of the adhesive withstanding different conditions of the WTB are extracted.

1. Introduction The first patented invention for the use of wind energy in windmills dates back to 644 CE by an Iranian. The oldest evidence for the existence of windmills in Sistan, Iran, dates back to 947 CE [1]. Wind energy is currently used to generate electricity by wind turbines. Wind turbines convert wind kinetic energy into mechanical energy and then into electrical energy. About 19.5% of the failure of the wind turbine components occurs in the blade [2]. Today the increase in the blade length to more than 100 m, has caused major concerns about the blade resistance to damage over a period of 20–25 years. Examination of accidents and damages on the WTBs shows that blade damage may occur in various parts under static or fatigue loading. Most of these damages include delamination, local buckling, global buckling, and adhesive joint separation. Separation of adhesive joints happens mostly in the connection of the leading edge, spar cap, shear web and TE. The soundness of the TEs in the WTBs is very important in the stability of the whole WTB structure from aerodynamic point of view. Existence of combined loads on the TE, complex geometry, production process and blade design are some of the reasons rendering the failure of WTB trailing edge an important and challenging issue. Meanwhile, adhesive dimensions used in the TE is one of the most important causes of damage to the WTB and is one of the major concerns of large manufacturing companies. Therefore, the extraction of the geometrical dimensions of the adhesive used at the TE is being received more attention than before. Philip et al. [3] studied the geometrical non-linear buckling effect of the TE under combined loading and analyzed how it affects the ultimate strength of a blade in a TE failure. Zujin et al. [4] exposed the full-scale blade to fatigue loading in the direction of the

* Corresponding author. E-mail address: [email protected] (R. Rafiee). https://doi.org/10.1016/j.engfailanal.2020.105148 Received 25 August 2020; Received in revised form 11 November 2020; Accepted 27 November 2020 Available online 7 December 2020 1350-6307/© 2020 Elsevier Ltd. All rights reserved.

Engineering Failure Analysis 121 (2021) 105148

R. Rafiee and M.R. Hashemi-Taheri

edgewise and extracted the damage created on the TE. Zuo et al. [5] conducted a numerical study on the progressive damage of large composite rotor blade, considering the co-effects of matrix micro-cracking damage and joint adhesive debonding under both quasistatic and cyclic loadings. Also, Chen et al. [6] performed the failure sequence of TE sections cut from a full-scale taking into ac­ count multiple structural non-linearities associated with buckling deformation, contact conditions and progressive damages of different materials. Zhang et al. [7] performed an experimental test on a 52.5 m blade under combined loading and then analyzed the critical failure modes of the TE. Sayer et al. [8] performed 3-point bending experiments on sub-component at the spar cap adhesive joint to the shear web under fatigue loading and obtained the initiation and propagation crack and migration from adhesive to ma­ terial. Papadopoulos et al. [9] performed 3-point pull-out experimental experiment on sub-component t-beam and box-beam under quasi-static loading and numerical experimentation using CZM method to investigate the separation of adhesive joints of these specimens. Ji et al. [10] performed a numerical study on skin-web adhesive joint and by using CZM method the place of initiation and propagation of the crack was investigated. Using the virtual crack closure technique, Eder et al. [11] studied the initiation and propagation of the crack at the TE joint and were able to adapt the local buckling damage from the experimental test with the nu­ merical method. Also, Mishnaevsky et al. performed experimental and numerical tests on I-beam WTB specimens and studied shear strength adhesive [12]. Using polyvinylidene fluoride film sensor, Huh et al. were able to study the damage caused near the joint between the shear web and spar cap of the TE components [13]. Shohag et al. [14] Performed experimental tests in the form of Double cantilever beam and Three-point bend end notched flexure for sample adhesive joints and extracted its strength properties. Zarouchas et al. [15] have also conducted a similar study. Lahuerta et al. [16] performed an experimental test on the sub-component of the TE of the WTB under static loading. They examined three stages of the TE failure: pre-buckling stage, TE post-buckling stage, ultimate failure stage. Al-Khudairi et al. [17] performed an experimental test on a complete blade with the length of 45.7 m, by making an initial crack between the shear web and the spar cap. They studied modal analysis and static and fatigue test to evaluate blade debondidng with crack. Fernandez et al. [18] and Chen et al. [19] have also conducted similar investigations. Wu et al. [20] studied the on-strain response and fatigue life of adhesively bonded extended composite WTB undergoing unsteady aerodynamic loads. The main objective of this study is to investigate the failure initiation in an adhesive joint of the TE. It is also intended to determine the dimensions of the adhesive required for the joint – without considering the initial crack – in the operational and the gust conditions. In this research, a strategy for extracting the thickness of the adhesive in the TE is provided. Employing the strategy, the required adhesive properties is determined based on a certain thickness. Moreover, it enables calculating the minimum required thickness for the any arbitrary adhesives utilized in the TE of the WTB. 2. Adhesive joint failure Recently, with the increase in blade length, concerns about its strength to failure – over the life of 20–25 years – have increased. Therefore, investigating some failure factors that were ignored in the past, has turned into an important issue. The high cost of ad­ hesive, the influence of the consumed adhesive on the total weight of the WTB, the high cost and cumbersome process of repair and maintenance of the TE and also the necessity of integrity in the unitized structure of the WTB have caused the adhesive joint failure a major concern in recent years. The occurrence of failure in the TE not only is the main origin of more catastrophic failure in the blade structure, but also can reduce the performance of the blade seriously from aerodynamic point of view. The use of adhesives for composite joints will be much more convenient due to the lack of stress concentration and its distribution across the bond line without damaging the fibers. Moreover, this technology avoids increasing the weight of the blade. Adhesive joints vary depending on the type of use, the applied force and the geometrical specifications of the structure. The strength of adhesive joints depends on several important properties, from which the adhesive thickness and the adherend thickness are the most important. There is a critical size for the adherend thickness that depends on the type of loading to determine whether adherend failure occurs or not [21]. For the thickness of the adhesive, based on the results of the experiments, it is recom­ mended that the thickness is chosen between 0.1 and 0.2 mm based the maximum stress criterion [22]. In general, an adhesive joint can experience various failures categorized under three modes: (i) cohesive failure, (ii) adhesive failure and (iii) adherend failure, even though the growth path of the crack can migrate between these cases. Cohesive failure is often caused by shear stress and/or peel stress in the adhesive. Poor joint design, especially along the joint length, is one of the main reasons for this type of failure. Excessive porosity in the adhesive can expedite cohesive failure. Adhesive failure happens due to the weakness of production process and also improper surface preparation during gluing. Adhesive failure is associated with the separation of composite layers from the adhesive materials. Adherend failure may occur anywhere on the material in adhesive joints. In the WTB, these materials are the composite layers. The main reason for this type of failure is considered as the inappropriate bonding between composite layers or the improper production and incomplete curing. Analyzing the failure in adhesive joints can be accomplished using continuum mechanics, fracture mechanics or damage me­ chanics. The complex geometry and also complicated procedure enforce investigators to implement one of the abovementioned ap­ proaches through numerical simulations. Numerical simulations are widely used for this purpose through finite element (FE) analysis or extended finite element (XFEM) methods. Adams et al. [23] were among the first to use the finite element method for the numerical analysis of adhesive joints. Each of aforementioned approaches has its own special features making the method of calculation different depending on the type of material and geometrical dimensions. Whether the location of the crack tip in the material is known, or whether the input data is sufficient for the method and also the level of expected accuracy in conjunction with required computational efforts have significant impact on the selection of approach. Generally, the advantages and disadvantages of each approach for investigating the failure of adhesive joints are summarized in 2

Engineering Failure Analysis 121 (2021) 105148

R. Rafiee and M.R. Hashemi-Taheri

Table 1 [24]. Since the location of the initial crack tip is not known, damage mechanics approach is preferred and implemented using commercial FE package for the purpose of this research. Although XFEM seems very promising for the purpose of this study, the limitations of commercial software for performing the analysis on the adhesive joints is the main motivation for choosing conventional FE method. In this research, CZM is used to analyze the adhesive failure and adherend failure in the context of damage mechanics. Mohr–Coulomb stress criterion is also used to examine the cohesive failure based on the continuum mechanics [25]. 3. Conditions and loading on the WTB The source of all load cases applied to the blade is originated from external conditions and operating conditions on the wind turbine. External conditions are caused by environmental, electrical and soil parameters. The environmental conditions are further divided into wind conditions and other environmental conditions. The electrical conditions refer to the electrical power network conditions. Soil properties are relevant to the design of wind turbine foundations. Operating conditions, also called design conditions, are originated from the operation of the wind turbine. These conditions are usually studied independently. They are then combined and applied to the blade [26]. The wind regime on the WTB is divided into two states: normal and extreme conditions as external conditions. Normal conditions are usually considered in the long-term structural and functional conditions on the blade, while the extreme conditions occur at the certain times and creates a critical state. Both natural wind conditions as well as gust condition are considered in this research. Operating conditions are divided into four events as: normal operating conditions, fault, after the occurrence of the fault and transport, erection and maintenance [27]. The combination of wind as the main source of external condition (in both normal and extreme cases) and also normal operating condition is considered in this research. The other combinations are not taken into account, since they occur so rarely. Normal operating conditions are also divided into four stages as: stand-by, start-up, power production and normal shut-down. The other combinations are not taken into account, since among four aforementioned normal operating conditions, the highest level of load is experiences by WTB during the power production. The investigated blade as a case study in this research is a 23-m composite blade belonging to V470-660 wind turbine manufactured by Vestas Co. Detailed specifications of this blade are presented in Table 2. During the power production, different load cases are applied to WTB consisting of inertial and gravitational forces, aerodynamic forces, and gyroscopic forces caused by the WTB control. The employed coordinate system for purpose of calculating loads on the blade is also shown in Fig. 1. The aforementioned load cases are calculated using the procedures reflected under Germanschiner Lloyd [28] based on the distance from the root. The mathematical formulations for calculating the load cases are presented under Appendix A. The total summation of forces at root of the blade are presented in Table 3. As it can be seen from the presented results in Table 3, the moments generated at the root of the blade about Y-axis are greater than the moments about X-axis. The former represents the bending in the flap wise of the blade, while the later cause edge-wise bending. 4. FE model of the blade The investigated blade consists of three main parts: the upper and lower shell and spar. The cross section of the modeled blade has three types of airfoil, including FFA-W3 developed by Risoa national laboratory, MIX developed by Vestas and NACA 63 series. The airfoils of blade cross sections with certain dimensions are placed at certain distance from the root in accordance with the sketch drawing of the blade. Then, surfaces are passed between them and the outer shell of the blade is formed. The shear webs of the spar are located in quarter of the chord lines. The cross section of the blade twists about 15◦ due to the aerodynamic reasons and also it

Table 1 The pros and cons of different techniques for investigating the failure of adhesive joints. Continuum mechanics

Fracture mechanics

Damage mechanics

Advantages

• Simple and easy to use criteria • Very suitable for brittle materials • Less information needed

• Can be used for brittle and ductile materials • Suitable for multiple material structures like joints

• • • •

Disadvantages

• Not suitable for discontinuous structures • Not suitable for ductile materials • Sensitive to stress singularities • Can be inaccurate and give overestimation • Mesh sensitive • In need of initial crack tip

• Limited by joint geometry • Mesh sensitive • Multiple singular sources can cause difficulty • Difficult to implement • Requires initial crack tip

3

Can be used for brittle and ductile materials Good accuracy No need for an initial crack tip Can monitor crack propagation progressively for any element • Can be applied as bulk or interface elements • Mesh sensitive • dependent on material types and failure modes

Engineering Failure Analysis 121 (2021) 105148

R. Rafiee and M.R. Hashemi-Taheri

Table 2 Specification of investigated blade. length Maximum chord line Minimum chord line Twisting airfoils Location of the center of mass Mass The distance from the tip to the tower Swept area Cut-in wind speed Rated speed Cut-out wind speed Rotational speed of the rotor Rotational direction

22900 mm 2088 mm 300 mm 15.17◦ 8700 mm from the root 1250 kg 4500 mm 1735 m2 4 m/s 15 m/s 25 m/s 28.5 rpm Clockwise (front view)

Fig. 1. Blade coordinate system. Table 3 Forces on the root of blade. Normal operating loads *

Loads influenced by gust

Loads from oblique wind flow

Loads from wind gradients

Loads from gyroscopic forces

FXB = 72FYB = 5MXB = 82MYB = 1133

FXB = 200FYB = 10.7MXB = 169MYB = 3132

FXB = 51FYB = 5MXB = 84MYB = 799

FXB = 72FYB = 5MXB = 84MYB = 1128

FXBStartYawing = 0.08FZBStartYawing = 0.06FXBCteAngular = 1FYBCteAngular = 0.5FZBCteAngular = 0.5

*

All calculated loads and moments are presented in [KN] and [KN.m], respectively.

is tapered from the root to the tip. The geometrical model of the blade is presented in Fig. 2. Once the geometrical model is created, FE model is constructed in ANSYS commercial FE package. For this purpose, three types of elements including SOLID46, SHELL91 and SHELL99 are used [29]. Shell element is used for two halved of blade shell, while solid element is used for shear webs of the spar. In the manual meshing process, all elements have quadratic shape and their dimensions are chosen in a manner to have aspect ratios close to unity for all of them. Finally, the model become independent from the mesh. Finally, the total number of elements in this model is 33,832. The constructed FE model of the blade is also shown in Fig. 2. Ply configurations of the blade shell and spars are entered into the model. The blade has several different lay-up configurations not only in shell and spar but also along the length of the blade. Namely, the lay-up configuration differs from the root to the tip. The investigated blade consists of three main types of pre-preg glass/epoxy composites: Uni-Directional (U-D), bi-axial and tri-axial plies. Bi-axial and tri-axial plies contain two and three same U-D fabrics, respectively, which are stitched together and thus they are not woven. Tri-axial and bi-axial fabrics are used in the shell structure and U-D and bi-axial are used in the spar structure. The configu­ ration of the bi-axial ply is [0/90]T and the configuration of the tri-axial laminate is [0/+45/− 45]T. It should be reminded that the biaxial ply can be also viewed in the form of [±45]T. Two kinds of foam (PMI and PVC) are used respectively at the spar and shell locations, respectively, in order to construct the sandwich panel. Mechanical properties of utilized material in the structure of the blade are inserted in Table 4. After creating the model, the load distributions are applied to the model. It should be pointed out that inserted values in Table 3 are 4

Engineering Failure Analysis 121 (2021) 105148

R. Rafiee and M.R. Hashemi-Taheri

Fig. 2. FE model of the full-scale blade.

the calculated forces and moments are the total summation of loads and moments at the root of the blade. The distributions of the loads along the blade length based on the distance from the root are presented in Appendix A. After applying the load distribution, both normal condition load cases and also extreme condition load cases are applied to the models. As another boundary condition, all 6 degrees of freedom of the nodes placed at the root of the blade are fixed. Largedeformation feature of FE package is also activated to capture this source of non-linearity. Deformed shapes of the blade undergo­ ing normal condition load cases and extreme condition load cases are presented in Fig. 3. As it is shown in Fig. 3, the maximum amount of bending in normal wind condition is 1584 mm and in gust condition is 4221 mm. It can be seen that the maximum tip deflection of the blade under extreme condition is less than the tip-to-tower distance. It is worth mentioning that the convergence study is performed on the constructed model and the trends of tip deflection is monitored versus number of elements. It is observed that employing more than 25,000 elements, does not induce any fluctuation in the results. Moreover, both first flap-wise and edge-wise bending natural frequencies are obtained through free vibrations analysis and compared with experimental data. The former is obtained as 1.85 and the later is obtained as 1.1 which are in an excellent agreement with experimental measured values as 1.97 and 1.09, respectively. It can be understood from aforementioned examinations that the 3D FE model of the blade is properly constructed. Thus, the stress distribution along the TE is evaluated to find the most critical region. For extracting the critical region of the TE, the out-of-plane stress Table 4 Mechanical properties of constituents materials in the investigated blade [30]. Material

Ex [GPa]

Ey [GPa]

υxy

Es [GPa]

U-D Biax Triax PVC PMI

43 16.7 17.6 0.05 0.066

9.77 16.7 7.01 0.05 0.066

0.32 0.06 0.53 0.32 0.32

3.31 2.01 5.075 0.02 0.025

5

Engineering Failure Analysis 121 (2021) 105148

R. Rafiee and M.R. Hashemi-Taheri

Fig. 3. Deformed shape of the blade under normal condition and gust condition.

distribution along the whole length of TE is examined when the WTB is subjected to extreme condition. The location of the critical region in the full-scale model and also the out-of-plane stress distribution in comparison with other regions are presented in Fig. 4. It can be seen the maximum out-of-plane stress components occur between the root and 5 m from the root.

Fig. 4. Out-of-plane stress distributions along the TE in normal (left) and gust (right) conditions (σy stands for peeling stress, since y axis is defined as the out-of-plane axis). 6

Engineering Failure Analysis 121 (2021) 105148

R. Rafiee and M.R. Hashemi-Taheri

5. Sub-part FE modeling It would be really formidable to model the adhesive along the whole length of the TE and also the FE analysis is in need of huge runtime to get accomplished. Therefore, a sub-part FE modeling is performed focusing on the identified critical region in the previous section. Employing this strategy, the failure in the TE can be investigated more carefully and in details focusing on local behavior rather than the global behavior. A sub-part of the full blade with the length of about 300 mm is separated and examined. Analyzing the results of stress distribution in previous section (Fig. 4), the highest equivalent stress is experienced in that 300 mm shown in Fig. 5. Once the sub-part model is identified for failure analysis, the loads on the main blade must be correctly transferred and applied to this part. The load transferring is done by creating a connection between the results obtained from the full-scale model to the sub-part model. The solution output of an upstream (source) analysis is imported to downstream (target) system as the boundary conditions employing sub-modeling feature in ANSYS in order to more closely analyze a region of interest. Reanalyzing the entire model using a higher level of mesh refinement in order to obtain more accurate results in the particular region of interest is time-consuming and costly. Instead, sub-modeling technique is employed to generate an independent model with fine mesh density in that region of interest (sub-model). The software uses the results for full-scale model as the imported data and interpolates the data for the geometry region specified as the sub model [29]. The six red edges of the sub-part model shown in Fig. 5 are considered as boundary conditions in the sub-part model. The applied boundary conditions exported from the full-scale model using sub-part modeling feature is shown in Fig. 6 on the sub-part model. 5.1. Modeling adhesive After the sub-part model was identified and the load transferred, the adhesive must be added to the model. Recalling from section (2), all three possible failures, i.e. cohesive failure, adhesive failure and adherend failure are simultaneously taken into account. Generally, epoxy adhesive is commonly used in adhesive joints. this is due to its high shear strength compared to other adhesives. Hardness, tensile strength and flexural strength increase with suitable conditions for high temperature curing. Polyurethane adhesives, on the other hand, are known for their fracture toughness and high flexural strength at low temperatures. In addition, polyurethane adhesives are suitable for a wide range of structures with medium shear strength [31]. In the investigated blade, epoxy resin is used for composite layers and polyurethane adhesive called Arathane 3427 PU is used for adhesive joint of TE and properties are presented in Table 5. The following assumptions are considered for modeling the adhesive: • The adhesive has no bubbles or porosity, • No crack is predefined in the model, • The contact surface between the adhesive and the blade shell is completely clean and free of contamination. Solid element (SOLID185) is used for modeling the adhesive [29]. The element dimensions of the adhesive is defined in complete compatibility with the dimensions of the elements in the blade shell structure. Mohr-Columbus stress criterion is used to examine the occurrence of failure in adhesive known as cohesive failure. For investigating the adhesive failure, the contact method with the penalty formula and the cohesive element are used. Contac element (CONTAC174) and Target element (TARGE170) is used for this method [29].

Fig. 5. Sub-part FE modeling of the critical region on TE. 7

Engineering Failure Analysis 121 (2021) 105148

R. Rafiee and M.R. Hashemi-Taheri

Fig. 6. Applying boundary conditions from the full-scale model to the sub-part model. Table 5 Property of Arathane 3427 PU. Density (gr/cm3 )

Tensile strength at 23 ◦ C (MPa)

Tensile modulus (GPa)

Shear modulus (GPa)

Elongation at tensile break (%)

1.35

30

2.5

1.5

5

The cohesive zone is defined between each layer of the blade to investigate the delamination in the structural layers of the blade shell capturing possible adherend failure. The glass/epoxy layers of the blade shell are in the form of stitched tri-axial, bi-axial ma­ terials. Thus, the defined cohesive behavior between the blade layers is governed by adhesion properties of the utilized epoxy matrix. The constitutive laws for the cohesive zone between glass/epoxy layers of the blade and PU resin between adhesive and composite shell are presented in Fig. 7. 36 different models with different dimensions of adhesive are constructed. The thickness (t) is considered as 0.1, 1, 2, 3, 4 and 5 mm and the band width of the adhesive (b) is considered as 10, 20, 30, 40, 50 and 60 mm. It should be pointed out that each case is hereinafter referred to as t-b code. For instance, 60-5 code means a case wherein the adhesive band width is 60 mm and its thickness is 5 mm. The sub-part FE model of the TE containing adhesive is shown in Fig. 8.

Fig. 7. Constitutive laws for the cohesive zone between glass/epoxy layers of the blade and PU resin between adhesive and composite shell [32]. 8

Engineering Failure Analysis 121 (2021) 105148

R. Rafiee and M.R. Hashemi-Taheri

6. Results and discussion The results of FEA on 36 models introduced in the previous section are presented in this section. The FEA is performed for two cases where blade undergoes normal wind flow and also gust. The output of the FE analyses categorized by experienced failure mode is reported in Table 6. 6.1. Cohesive failure Under normal operating conditions of the wind turbine, cohesive failure occurs only in the case of 5–60. In other cases, the minimum safety factor of the adhesive is more than 4, except for case 4–60. This means that under normal operating conditions, the utilized adhesive will not fail. Under gust condition, cohesive failure is the most common type of failure in different dimensions. The highest safety factor is obtained as 3.25 for the case of 0.1–30. Apparently, considering the minimum safety factor more than 3.25, all existing dimensions will be led to adhesive failure. For adhesive joints, in cases where the joint has to last a long time and is always affected by different loads, the safety factor should not be less than 2 [33]. Since, the blade is expected to have a minimum life of 20 years, it is recommended to choose those cases where safety factor is greater than or equal to 4 to stay in the safe side. Fig. 9 shows the adhesive safety factor for different dimensions exposed to both conditions. The red dots in Fig. 9 indicate that adhesive failure occurs assuming the safety factor as 2. It can be realized that for the band widths between 10 mm and 20 mm, most cases are resulted in adhesive failure. Also, the maximum safety factor in each group of thicknesses is obtained for band widths of 30, 40 and 50 mm. As the thickness and band width reach their uppermost dimensions, the adhesive is more prone to experience failure. Also, the closer the band width is to 10 and 20 mm, the more the failure has been observed in the adhesive. These numbers also confirm the company’s existing instructions that the band width should neither exceed 60 mm nor fall below 30 mm. It can be understood that increasing adhesive thickness the possibility of cohesive failure is reduced, but instead, another failure mode becomes pertinent. These cases can be seen in all the peaks and valleys shown in Fig. 9. Fig. 10 shows an example of the range of safety factors of the adhesive under normal conditions and the presence of a gust for the case of 3–60. It can be seen that the minimum safety factor is 3.65, but the same case is failed under extreme conditions when gust is experience. 6.2. Adhesive failure This type of damage is observed only in two cases of 2–60 and 5–40 and only in the presence of a gust. The adhesive failure for these two cases is shown in Fig. 11 in the form of separation between the blade shell and adhesive. For the case of 2–60, the separation occurs in the maximum tension and its amount has become 0.051 mm. This value is associated with the peak in the constitutive law of CZM for PU adhesive and thus implies on beginning of the crack propagation on the contact surface. For the case of 5–40, the separation value is 0.1 mm. This value is associated with the full separation point in the constitutive law of the CZM describing PU adhesive. Therefore, complete separation between adhesive and shell layers is expected at this moment.

Fig. 8. Sub-part FE model and adhesive variable dimensions. 9

Engineering Failure Analysis 121 (2021) 105148

R. Rafiee and M.R. Hashemi-Taheri

Table 6 Failure results in TE under normal and gust condition. Normal condition t [mm] b [mm] 0.1 1 2 3 4 5

10

20

30

40

50

60

– – – – – –

– – – – – –

– – – – – –

– – – – – –

– – – – delamination delamination

– – delamination delamination delamination delamination, cohesive

Cohesive Cohesive Cohesive Cohesive – –

– – – – – –

– – – – Cohesive Delamination, Adhesive

– – – – Delamination Delamination, Cohesive

Delamination Delamination, Cohesive Delamination, Cohesive, Adhesive Delamination, Cohesive Delamination, Cohesive Delamination, Cohesive

Gust condition 0.1 Cohesive 1 Cohesive 2 Cohesive 3 Cohesive 4 Cohesive 5 Cohesive failure

14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

Gust condition

0.1-10 0.1-20 0.1-30 0.1-40 0.1-50 0.1-60 1-10 1-20 1-30 1-40 1-50 1-60 2-10 2-20 2-30 2-40 2-50 2-60 3-10 3-20 3-30 3-40 3-50 3-60 4-10 4-20 4-30 4-40 4-50 4-60 5-10 5-20 5-30 5-40 5-50 5-60

Safety Factor

Cohesive Failure Normal condition

Geometrical specifications

Fig. 9. Safety factors for different dimensions.

6.3. Adherend failure Adherend failure in the TE of the WTB is recognized as delamination of composite layers in the blade shell. Delamination takes place between the first and second inner most layer in the upper and lower shell of the investigated blade. It can be seen from the presented results in Table 5, delamination occurs for almost all cases when the band width of adhesive in the TE is 60 mm. This in­ dicates that the delamination is mostly triggered by the band width rather than adhesive thickness. Hence, increasing the band width of the adhesive will not necessarily be resulted in stronger joining of the WTB trailing edge. The failure in the TE of WTB can be shifted from adhesive failure toward adherend failure by excessive increase in the band width of the adhesive. Fig. 12 shows the crack that occurs in a composite shell with adhesive dimensions of 5–50. The adherend failure should be avoided by proper adjustment of the adhesive dimension, since this failure cannot be repaired practically. 7. Conclusion The adhesive joint failure in the TE of a commercial WTB is studied. Three types of failure can occur in the adhesive joints comprising of cohesive failure, adhesive failure and adherend failure. All cohesive, adhesive and adherend failures are simultaneously taken into account. Numerical modeling is conducted in the context of continuum mechanics using commercial FE package. Mohr–Coulomb criteria based on continuum mechanic approach is used to investigate the cohesive failure and the cohesive zone model is used to investigate the adhesive and adherend failure based on damage mechanics. Natural wind and gust conditions are considered as external conditions and power production under normal conditions is considered as the operating conditions for wind turbines. Different load cases on the blade are identified and calculated. The loading is applied to the full-scale 3-D FE model of the blade and the most critical region of the TE is identified. The, sub-part modeling is performed focusing on the most critical region of the TE. A Parametric study is carried out to analyze the influence of the adhesive dimensions on the failure of TE. It is revealed that the band width of the adhesive plays a key role in the failure of TE and thus its careful adjustment during the production process is of great importance. In the presence of natural wind, it is observed that applying adhesive with the band width of 60 mm will be led to delamination of layers. The majority of failures occurs when the WTB is exposed to gust condition as the ultimate static loading scenario. for the band 10

Engineering Failure Analysis 121 (2021) 105148

R. Rafiee and M.R. Hashemi-Taheri

Fig. 10. Safety factors variations for the case of 3–60 in (a) gust condition and (b) normal condition.

widths of 10 and 20 mm, only the cohesive failure is observed. In addition to the cohesive failure, adherend failure is also observed in the form of delamination in all cases for the band width of 60 mm. Adhesive failure is just observed for the two cases of 2–60 and 5–40. Finally, band widths of 30–50 mm with thicknesses of 0.1–3 mm are identified as the safe cases under normal and gust conditions, respectively. Therefore, aforementioned dimensions are the preferred dimensions of adhesive with safety factor of 2.2 for the TE of the investigated WTB under static loading. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

11

Engineering Failure Analysis 121 (2021) 105148

R. Rafiee and M.R. Hashemi-Taheri

Fig. 11. Adhesive failure for the cases (a): 2–60 and (b): 5–40 under gust condition.

Appendix A. Calculating of load cases on the WTB A.1. Normal operating loads A.1.1. Aerodynamic loads on the rotor For calculation purposes a mean pressure PN is used on the rotor swept area [28]:

ρ

(A1)

PN = CFB ⋅ ⋅ VR2 2 where ρ represents air density, CFB = 8/9 according to Betz and VR is rated wind speed [28]: PN =

VR2 kN/m2 1800

PN =

225 = 0.125 kN/m2 1800

(A2)

The coordinate system that is considered to extract the aerodynamic forces on the rotor is called the hub coordinate system. As shown in Fig. A.1 the hub coordinate system has its origin in the center of the hub and the coordinate system does not rotate when the blade is rotated. Thus, the force acting on the head of the tower is [28]: 12

Engineering Failure Analysis 121 (2021) 105148

R. Rafiee and M.R. Hashemi-Taheri

Fig. 12. Delamination in composite shell for the case of 5–50.

Fig. A.1. Hub coordinate system.

(A3)

FXN = PN ⋅ A = 0.125 × 1735 = 217 KN where A is swept area. The moment MXN , is equal to [28]: MXN =

Pel

(A4)

ω⋅η

where ω the angular velocity of the rotor, Pel the maximum electrical power output,η the overall efficiency of generator and gear box. If no actual values are available for electrical output or overall efficiency, η = 0.88 is to be assumed: MXN =

660 = 251 KN ⋅ m 28.5 × 2π /60 × 0.88

(A5)

A.1.2. Aerodynamic loads on one blade The aerodynamic forces acting on a blade are regarded as triangular line loads based on the blade coordinate system as shown in Fig. A.1 and thus we have:

13

Engineering Failure Analysis 121 (2021) 105148

R. Rafiee and M.R. Hashemi-Taheri

∫

∫r 2 ⋅ FXN ⋅ r 2 × 217 r2 dr = rdr = 262 N 2 2 2 Z ⋅ R 3 × 23.5 0 0 ∫r ∫r 3 ⋅ MXN ⋅ r 3 × 251 r2 fYB (r) = − dr = − rdr = − 19 N; 0 < r < 23.5 3 3 2 Z ⋅ R 3 × 23.5 0 0 ∫r 3 3 × 251 2 r mXB (r) = ⋅ r dr = − 19 ⋅ N.m 3 3 0 3 × 23.5 ∫r 2 × 217 2 r3 mYB (r) = ⋅ r dr = 262 ⋅ N.m 2 3 0 3 × 23.5 r

fXB (r) =

(A6)

where r is the distance from the rotor axis, Z is the number of blades and R is the radius of wind turbine. A.2. Loads influenced by gusts Loads influenced by gusts are treated like the normal loads, but with the wind speed increased by the gust factor kb [28]: kb = 5/3

(A7)

5 Vb = kb ⋅ VR = × 15 = 25 m/s 3 Therefore, the pressure on the tower caused by the gust is equal to: Pb =

VR2 225 = 0.346 KN/m2 = 650 650

(A8)

Thus, the force acting on the head of the tower influenced by gust is: (A9)

FXN = Pb ⋅ A = 0.346 × 1735 = 600 KN The moment MXN , is twice the value of normal case and equal to: MXb = 2 × MXb = 2 ×

Pel

ω⋅η

(A10)

660 = 502 KN ⋅ m 28.5 × 2π/60 × 0.88

Therefore, the distribution of forces on the blade due to gust is as follows: ∫r ∫r 2 ⋅ FXN ⋅ r 2 × 600 r2 fXB Gust (r) = dr = rdr = 724 N 2 2 2 Z⋅R 0 0 3 × 23.5 ∫r ∫r 3 ⋅ MXN ⋅ r 3 × 502 r2 fYB Gust (r) = − dr = − rdr = − 39 N 3 3 2 Z⋅R 0 0 3 × 23.5 ∫r 3 3 × 502 2 r mXB (r) = ⋅ r dr = − 39 ⋅ N.m 3 3 0 3 × 23.5 ∫r 2 × 600 2 r3 mYB (r) = ⋅ r dr = 724 ⋅ N.m; 0 < r < 23.5 2 3 3 × 23.5 0

(A11)

A.3. Loads from oblique wind flow or wind gradients When the wind flow is oblique to the axis of WTB, the following loads on rotor are to be assumed [28]: 1 1 FXN Oblique = √̅̅̅ ⋅ PN ⋅ A = √̅̅̅ × 0.125 × 1735 = 153 KN 2 2 1 1 FYN Oblique = ±√̅̅̅ ⋅ PN ⋅ A = ±√̅̅̅ × 0.125 × 1735 = ±153 KN 2 2 FXN Gradient = PN ⋅ A = 0.125 × 1735 = 217 KN FYN Gradient = ±PN ⋅ A = ±0.125 × 1735 = ±217 KN The aerodynamic loads on the rotor blades are to be calculated as under Eqs. (A6) and (A11).

14

(A12)

Engineering Failure Analysis 121 (2021) 105148

R. Rafiee and M.R. Hashemi-Taheri

A.4. Loads from gyroscopic forces The rotor loads are to be entered as [28]: FXN = ±Pb ⋅ A = ±0.346 × 1735 = ±600 KN FYN = ±Pb ⋅ A = ±0.346 × 1735 = ±600 KN

(A13)

˙ = dΩ/dt and the angular velocity Ω of the yaw moment are decisive for the force arising. generally, two The angular acceleration Ω cases “start of yawing” and “yawing with constant angular velocity” are considered. A.4.1. Start of yawing The following maximum line loads to be taken into account then act the individual rotor blade due to the yaw movement [28]: ˙ fXBStartyawing (r) = − μ(r) ⋅ r ⋅ Ω ˙ fZBStartyawing (r) = − μ(r) ⋅ e0 ⋅ Ω

(A14)

where e0 is the distance of the rotor’s center of gravity from the tower axis and μ(r) is the mass per unit length as follow [28]:

μ(r) =

2500 (23.5 − r) 23.52

(A15)

0 < r < 23.5

And if the integral is taken from the above relation, the blade mass is obtained: ∫ 23.5 mB = μ(r) = 1250Kg 0

(A16)

A.4.2. Yaw movement with constant angular velocity For a cantilever rotor blade, the section forces at the blade rotor due to yawing are: FXBcteangular = − mB ⋅ (e0 ⋅ (Ω2 + 2) ⋅ rS ⋅ ω ⋅ Ω) = − 1250 × 8 × 2.25 × 3 × 28.5 × 0.5 ≅ − 1KN FYBcteangular = − mB ⋅ Ω2 ⋅ rs /2 = − 1250 × 0.25 × 1.5 = − 525N FZBcteangular = mB ⋅ Ω2 ⋅ rs /2 = 1250 × 0.25 × 1.5 = 525N

(A17)

where rs is the distance of the blade’s center of gravity from the rotor axis.

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]

T.M. Letcher, Wind Energy Engineering, A Handbook for Onshore and Offshore Wind Turbines, Academic Press, London, 2017. J. Chou, C. Chiu, I. Huang, K. Chi, Failure analysis of wind turbine blade under critical wind loads, Eng. Fail. Anal. 27 (2013) 99–118. P. Haselbach, K. Branner, Initiation of trailing edge failure in full-scale wind turbine blade test, Eng. Fract. Mech. 162 (2016) 136–154. P. Zujin, W. Jianzhong, L. Jian, Z. Xinhua, Fatigue failure of a composite wind turbine blade at the trailing edge, Defect and Diffusion Forum 382 (2018) 191–195. Y. Zuo, J. Montesano, C. Singh, Assessing progressive failure in long wind turbine blades under quasi-static and cyclic loads, Renew. Energy 119 (2018) 754–766. X. Chen, P. Berring, S. Madsen, K. Branner, S. Semenov, Understanding progressive failure mechanisms of a wind turbine blade trailing edge section through subcomponent tests and nonlinear FE analysis, Compos. Struct. 214 (2019) 422–438. L. Zhang, Y. Guo, J. Wang, X. Huang, X. Wei, W. Liu, Structural failure test of a 52.5 m wind turbine blade under combined loading, Eng. Fail. Anal. 103 (2019) 286–293. F. Sayer, A. Antoniou, A. Wingerde, Investigation of structural bond lines in wind turbine blades by sub-component tests, Int. J. Adhes. Adhes. 37 (2012) 129–135. F. Papadopoulos, D. Aiyappa, R. Shapriya, E. Sotirchos, H. Ghasemnejad, R. Benhadj-Djilali, Advanced natural stitched composite materials in skin-stiffener of wind, Key Eng. Mater. 525 (2013) 45–48. Y.M. Ji, K.S. Han, Fracture mechanics approach for failure of adhesive joints in wind turbine blades, Renew. Energy 65 (2014) 23–28. M.A. Eder, R.D. Bitsche, Fracture analysis of adhesive joints in wind turbine blades, Wind Energy 18 (2015) 1007–1022. L. Mishnaevsky Jr, P. Brøndsted, Rogier Nijssen, D.J. Lekou, T.P. Philippidis, Materials of large wind turbine blades: recent results in testing and modeling, Wind Energy 15 (2012) 83–97. Y.-H. Huh, J.I. Kim, J.H. Lee, S.G. Hong, J.H. Park, Application of PVDF film sensor to detect early damage in wind turbine blade components, Procedia Eng. 10 (2011) 3304–3309. M. Shohag, T. Ndebele, O. Okoli, Real-time damage monitoring in trailing edge bondlines of wind turbine blades with triboluminescent sensors, Struct. Health Monit. 18 (2018) 1–12. D. Zarouchas, D. Hemelrijck, Mechanical characterization and damage assessment of thick adhesives for wind turbine blades using acoustic emission and digital image correlation techniques, J. Adhes. Sci. Technol. 28 (2014) 1500–1516. F. Lahuerta, N. Koorn, D. Smissaert, Wind turbine blade trailing edge failure assessment with sub-component test on static and fatigue load conditions, Compos. Struct. 204 (2018) 755–766. O. Al-Khudairi, H. Hadavinia, C. Little, G. Gillmore, P. Greaves, K. Dyer, Full-scale fatigue testing of a wind turbine blade in flapwise direction and examining the effect of crack propagation on the blade performance, Materials 10 (2017) 1152.

15

Engineering Failure Analysis 121 (2021) 105148

R. Rafiee and M.R. Hashemi-Taheri

[18] G. Fernandez, H. Usabiaga, D. Vandepitte, Subcomponent development for sandwich composite wind turbine blade bonded joints analysis, Compos. Struct. 180 (2017) 41–62. [19] X. Chen, J. Tang, K. Yang, Modeling multiple failures of composite box beams used in wind turbine blades, Compos. Struct. 217 (2019) 130–142. [20] G. Wu, Z. Qin, L. Zhang, K. Yang, Strain response analysis of adhesively bonded extended composite wind turbine blade suffering unsteady aerodynamic loads, Eng. Fail. Anal. 85 (2018) 36–49. [21] C.H. Wang, A. Gunnion, Design Methodology for Scarf Repairs to Composite Structures, Defense Science and Technology Organization, Victoria, 2006. [22] D.M. Gleich, M.J.L. Van Tooren, A. Bbukers, Analysis and evaluation of bondline thickness effects on failure load in adhesively bonded structures, J. Adhes. Sci. Technol. 15 (9) (2012) 1091–1101. [23] R.D. Adams, N.A. Peppiatt, Stress analysis of adhesive-bonded lap joints, J. Strain Anal. Eng. Des. 9 (1974) 185–196. [24] L.F.M. da Silva, R.D.S.G. Campilho, Advances in Numerical Modelling of Adhesive Joints, Springer Briefs in Computational Mechanics, London, 2012. [25] H.C. Biscaia, C. Chastre, M.A.G. Silva, Modelling GFRP-to-concrete joints with interface finite elements with rupture based on the Mohr-Coulomb criterion, Constr. Build. Mater. 47 (2013) 261–273. [26] IEC 61400-1, Wind turbine generator systems- part 1: design requirements, 2014. [27] IEC 61400-13, Wind turbine generator systems- part 13: measurement of mechanical loads, 2014. [28] Germanschier Lloyd, “Rules and Regulations”, IV– Non-Marine Technology, Part, Regulation for the certification of Wind Energy Conversion System, Appendix 4.2: Simplified Design Loads, Ed. 1993. [29] ANSYS Help, Engineering simulation & 3D design software. Release 2020 R2, January 2019. [30] R. Rafiee, M. Tahani, M. Moradi, Simulation of aeroelastic behavior in a composite wind turbine blade, J. Wind Eng. Ind. Aerodyn. 151 (2016) 60–69. [31] Jorgensen, Jeppe Bjorn. Adhesive joints in wind turbine blades. Thesis. DTU Wind Energy, Denmark, 2017. [32] K. Dastjerdi, E. Tan, F. Barthelat, Direct measurement of the cohesive law of adhesives using a rigid double cantilever beam technique, Exp. Mech. 53 (2013) 1763–1772. [33] D. MeQuillan, The Structural Use of Aadhesives, The Institution of Structural Engineers. SETO, London, 1999.

16