QUALIFICATION OF MATERIALS AND BLADES FOR WIND TURBINES. Jakob Wedel-Heinen and Josef Kryger Tadich

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1 Proceedings of the 27 th Risø International Symposium on Materials Science: Polymer Composite Materials for Wind Power Turbines Editors: H. Lilholt, B. Madsen, T.L. Andersen, L.P. Mikkelsen, A. Thygesen Risø National Laboratory, Roskilde, Denmark, 2006 QUALIFICATION OF MATERIALS AND BLADES FOR WIND TURBINES Jakob Wedel-Heinen and Josef Kryger Tadich Det Norske Veritas; Global Wind Energy, Tuborg Parkvej 8 2 nd, 2900 Hellerup, Denmark ABSTRACT The IEC series of standards for the design and testing of wind turbines has been established over the last decade and is now generally accepted in the international wind turbine industry. The series includes, among others, a general standard IEC (2005) for design requirements including loads and safety, and a specification for structural type testing wind turbine blades in IEC (2001). The design of new wind turbine blades requires the qualification of materials and type testing of structural strength. The present paper explains how this qualification of materials and type testing is carried out according to these IEC standards. It is explained that detailed testing of coupons in both tension and compression is required for ultimate strength design. The testing of coupons in fatigue shall consider the impact of the mean stress on the fatigue life. In full scale fatigue testing of wind turbine blades the 20 year fatigue load spectrum is translated to a typically constant amplitude equivalent fatigue test in two directions, with a total duration of around 4 months. Several considerations when determining the test loads are also discussed. The existing design and test methodology in the IEC series of standards has been based on the common methodology applied in the design of steel structures. However, composite materials are used almost exclusively in modern wind turbine blades now and for the foreseeable future. This paper addresses some of the challenges for further development of the IEC standards when they shall form basis for advanced wind turbine blade design in composite materials. Among these challenges are improved specifications of how temperature effects and mechanical loads are combined and a more rational approach in managing manufacturing defects and imperfections. 115

2 Wedel-Heinen and Tadich 1. INTRODUCTION The design of modern electricity generating wind turbines have from the start been subject to independent certification schemes. Initially, certification schemes were developed in Denmark, Germany and the Netherlands in parallel with these countries wind turbine development. Each of these local certification schemes had different load and safety specifications, as well as different detailed requirements for structural design and testing. The international IEC series of standards were established at the end of the 1990s to consolidate these different local schemes, and today these IEC standards are in the process of taking over as reference documents for design and testing in the certification in each country. IEC have also proposed an overall wind turbine certification scheme in IEC WT 01 (2001). The IEC standards relevant to the design and testing of wind turbine blades are IEC (2005) and IEC (2001). These standards cover general design requirements and type testing of blades. The use of the standards and critical issues in further development is described in the following sections. 2. IEC DESIGN REQUIREMENTS The IEC (2005) standard covers the design loads for the complete wind turbine. The design loads are divided into two groups: ultimate and fatigue loads. Both types of loads are specified in a way that allows for their calculation from the response of the wind turbine in time simulation of different scenarios with aeroelastic simulation. Both ultimate and fatigue loads depend on input from the turbine s control and safety system, and the modelling of this system is an important part of the aeroelastic simulation. The local reference coordinate system used for bending moments in blades typically follows the blade s chord line. As a result, edgewise bending moments result in strain in the leading and trailing edge, with flapwise bending moments resulting in strains the upper and lower sides of the blade. Figures 1 and 2 illustrate a typical time simulation of the flapwise and edgewise bending moment at the blade root for a multi- MW turbine during operation in turbulent wind flow. Fig. 1. Flapwise moment Mx at the root of a MW turbine in operation 116

3 Qualification of materials and blades for wind turbines Fig. 2. Edgewise moment My at the root of a MW turbine in operation The IEC (2005) defines a set of environmental conditions, namely wind distributions. Each set includes a 50 year extreme wind speed, a frequency distribution of the wind speed and reference turbulence. The IEC (2005) standard only specifies extreme temperatures for turbine classes and does not go into details on the combined action of temperatures and the loads from the aeroelastic response. Measurement of load on prototype turbines is used as a verification of the simulated extreme and fatigue loads for the turbine. A technical specification for load measurements, IEC (2001), is also a part of the IEC series of standards. The requirement to structural safety in IEC is specified in the traditional format of partial safety factors used in design of civil engineering structures. The requirement is specified on a general basis and allows for acknowledged steel design standards to be used for detailed design. For the detailed design of composite material structures the extent of the application of IEC (2005) is very limited. Subsequently, an interpretation of the IEC requirements for composite materials has been provided in a draft DNV standard for design and manufacturing of wind turbine blades DNV (2006). 2.1 Design for extreme loads. For simple static structures, such as chimneys, the ultimate loads are associated with the structure s response to extreme wind speeds. Typically 50 or 100 year gusts are used for the reference as characteristic values when calculating their design loads. For chimneys other extreme loads such as those from vortex shedding can be avoided by adding simple aerodynamic devices such as vortex spoilers. The effect of the air density on loads is straight forward to take into account when determining ultimate loads on simple structures. The wind pressure is calculated according to the measured wind velocity and the air density for the measured temperature and pressure. The wind pressure is statistically processed to derive 50 year exceedance values and the 50 year wind velocity is determined from the 50 year pressure at a reference air pressure and temperature. 117

4 Wedel-Heinen and Tadich For wind turbines the extreme wind speed is not the only important load scenario. Among the other important load cases are operation with high turbulence intensity in the wind flow and emergency braking of the rotor where the blades are turned quickly and the chord line is oriented parallel to the rotor axis. For wind turbine also conditions with error in the control or safety system are the basis for calculation of extreme loads. One such case could be braking where one blade by error does not turn. Braking with two blades will lead to large loads from the imbalance of aerodynamic loads on the rotor disc. The effect of air density on wind turbine loads with advanced control and safety systems cannot be determined with a simple correction of the wind speed as for the simple structures. The IEC does not go into details with this effect. Instead it is specified that all loads for the wind turbine classes refer to an air density of kg/m 3. The loads on the wind turbine blades were in the past transformed into strains in the blade by simple beam theory. For recent blade designs for MW turbines the elements in the blade structure are more slender. This implies that the blade cross section deforms during loading. Further, there is a significant sensitivity of strains to imperfections due to initial buckling. For this reason finite element analysis is often used to calculate the strains in the blade for the design loads. There are two types of extreme events for a wind turbine blade: the material strength being exceeded and the blade deflection being so large that the blade hits the tower. This means that both stiffness and strength of laminates are important for structural safety. Additionally, as numerous material types are often used (such as Carbon Fibre Reinforced Plastic [CFRP], fibreglass Fibre Reinforced Plastic [GFRP], balsa cores, and steel bolts) the various material stiffnesses as well as interface strength must be well understood to ensure a controlled load transfer through the structure. The measurement of laminate properties for blade design has recently been scrutinised in a joint industry programme OPTIMAT. The OPTIMAT programme included detailed measurements of material stiffness and strength of a few laminate types all manufactured from GFRP and one specific epoxy resin system. For further details see Janssen, Wingerde et al. (2006). The research in the OPTIMAT project was reviewed to identify relevant issues for standards and guidelines by Wedel-Heinen, Tadich et al. (2006). The most important observations related to measurements of stiffness and ultimate strength of coupons were: Strength and fibre volume ratios go hand in hand. Test results should be corrected for the fibre volume ratio before characteristic values for ultimate strength are established. Coupon geometry for stiffness testing blade materials is not a critical issue as long as the monitoring of strains is based on strain gauges mounted on both sides of the coupons to correct for bending in the test rig. 118

5 Qualification of materials and blades for wind turbines The layout of the coupon in tensile testing and compression is not very critical as long as buckling of coupons for compressive testing is avoided e.g. through sufficiently stiff designs for anti-buckling. It is recommended to determine the full range of tests (including fatigue tests) at the start of the material qualification programme, and keep the number of different test specimens and test setups as low as possible to allow for meaningful comparison of test results between the different test series. The effects of moisture and temperature on the material should be investigated as part of the qualification of composite materials. These effects may lead to a significant reduction in strength in particular in compression and at loading transverse to the fibres where the resin and the resin to fibre interface strength is important. 2.2 Fatigue design. The fatigue loads for wind turbines are calculated from typically 10 minute simulations of the response at 10 different wind speeds in the operational interval. Turbines are normally in operation from 4 to 25 m/s. Loads in frequent transient events such as start and stop are also included. The rain flow counting procedure is applied to arrange the continuous histories for the individual load components in discrete load cycles. Each load cycle is represented by a mean value and a load width. The number of cycles in the 10 minute simulations is scaled according to the number of hours at each wind speed interval for a specified wind speed distribution over the turbine s 20 year life. The load cycles are summarised for the complete 20 year life of the turbine. The result is presented in the form of a Markov matrix containing the number of load cycles for each specified load interval in terms of width and mean load. For further details of rainflow counting see DNV/Risø (2002). The representative fatigue strength for blade material is typically established through constant amplitude coupon testing. The fatigue life of composite materials depends both on the mean stress and the stress width. Normally test series for fatigue strength are organised as testing at different ratios between minimum and maximum stress in the load cycles on the coupons. This ratio is referred to as the R-value. Stress width (S) to number of cycles to failure (N) diagrams can be established for each R-value by testing at different stress widths. The fatigue behaviour is thereby characterised by the mean S-N curve, and the scatter. Typically, the S-N data imply a linear relationship when log S is plotted versus log N as shown at Figure

6 Wedel-Heinen and Tadich S max S-N curve S amp S width S mean S min 1 cycle log N Fig. 3. Constant amplitude load cycles and S-N diagram for a particular R value. Characteristic S-N curves for design are taken according to 95% survival probability and 95% confidence. See also DNV (2006). On the turbine blade in operation the typical R-values depend on the location, see also Figures 1 and 2. For the windward side of the blade the stresses are predominantly in tension. The leeward side is predominantly in compression. The leading and trailing edges have less predominant mean loads than the sides. To map the characteristic fatigue life as a function of both R-value and stress width constant life diagrams can be drawn. The constant life diagrams are used for interpolation to R-values that were not part of coupon testing. Figures 4 and 5 illustrate constant life diagrams developed for unidirectional (UD) laminates in the OPTIMAT project. Figure 4 refers to coupon testing where the load is applied in the direction of the fibres and Figure 5 refers to load applied transverse to the fibres. CLD for UD OB_ R σa [MPa] E E E E E E E E E σ m [MPa] Fig. 4. Constant Life Diagrams for OPTIMAT UD material at longitudinal loading (Wedel-Heinen et al. (2006)) 120

7 Qualification of materials and blades for wind turbines CLD for UD OB_ R σa [MPa] E E E E E E E E E σ m [MPa] Fig. 5. Constant Life Diagrams for OPTIMAT UD material at transverse loading (Wedel-Heinen et al. (2006)) Miners Rule assumes that all stress cycles contribute to the same damage growth, and that fatigue failure occurs when the accumulated damage exceeds a certain value. With Miners Rule constant life diagrams can be used as maps for partial damage for a stress cycle. With the Markov matrix as a map of the load cycles over the design life and with the constant life diagram as a map for the partial damage for a stress cycle the accumulated fatigue damage over the design life is calculated by simple summation of damage over the design life according to Miners rule. Reference is made to DNV/Risø (2002). From the above description it is observed that the fatigue design methodology commonly used in the industry is similar to the methodology that was originally established for steel structures. It includes three fundamental elements: Rain flow counting Calculation of partial damage with constant life diagrams developed on basis of constant amplitude testing of coupons Cover ultimate fatigue strength as two separate issues and calculate fatigue failure according to the fatigue damage accumulation with Miners Rule Several assumptions are inherent in this methodology. These assumptions will lead to a modelling uncertainty in the calculation of both ultimate and fatigue strength. Some of the assumptions were challenged by detailed testing in the recent OPTIMAT research project. Constant amplitude testing was substituted by variable amplitude testing. 121

8 Wedel-Heinen and Tadich The sensitivity of ultimate strength to damage accumulation in fatigue was investigated. The relevance of testing at varying R-values was investigated. Exact values for the modelling uncertainty are difficult to establish based on the OPTIMAT research alone. Improved models for damage accumulation could lead to reduced partial safety factors and an improved basis for optimisation of design. For further details see Janssen, Wingerde et al. (2006) and Wedel-Heinen, Tadich et al. (2006). It was concluded that it is very easy to introduce a considerable amount of modelling uncertainty for fatigue life with the linearization of the SN diagrams and the Constant Life Diagrams. It is important to test coupons at R-values and stress widths that are critical for the calculated fatigue life of the blade structure. In addition to the issues studied in the OPTIMAT research project, the above described procedure assumes that coupon strength is representative for the strength of laminates in composite structures. DNV experience with blade failures in the field suggests that manufacturing defects and imperfections are also very important for strength, this is an issue that we will revisit in the following sections. 3. IEC BLADE TESTING The IEC document covers the details of full scale blade testing. It is noted that IEC is today formally issued as a technical specification and is in the process of being translated into a standard. The fundamental purpose of full scale blade testing is to demonstrate, to a reasonable level of certainty, that a blade type when manufactured according to defined specifications possesses the strength and service life as expected in the design. Full scale blade tests are carried out on typically one prototype blade. IEC require that the testing is carried out as both static and dynamic (fatigue) testing. 3.1 Static testing for ultimate loads. The ultimate loads for wind turbine blade can in general not be attributed to only one event and they are different from the external loads due to the dynamic nature of blade response. The ultimate loads along the blade axis are taken as an envelope for internal loads in the response during the life. The static testing is carried out by fixing the blade root and pulling the blade with slings attached to the blade in cradles. Due to the non-symmetric layout of a blade the testing is carried out in four directions. The test load is taken as the characteristic ultimate load multiplied with the partial safety factor for load according to IEC and test factors to account for the facts that the test blade may not represent the lowest strength for all blade, and that the test conditions (including temperature and humidity) may be less severe than that actually experienced by the installed blade on an operational turbine. Test loads are also in 122

9 Qualification of materials and blades for wind turbines principle adjusted according to their consequence of failure, however the load factors for ultimate strength in IEC are calibrated according to a consequence of failure factor with a value of one. For further details see IEC (2001). 3.2 Dynamic testing for fatigue loads. Fatigue loads for dynamic testing cannot be developed directly according to the internal loads during the service life as this would require tests lasting for decades. Furthermore; it is for practical reasons much easier to test in series of constant amplitudes. The fatigue damage accumulation rules based on rainflow counting, Constant Life Diagrams and Miners rule are used to transform the design loads to test loads. As for static testing also dynamic testing is carried out with factors for variation in blade strength, a correction for the environment in the laboratory and a penalty in form of a factor for consequence of failure. For further details see IEC (2001). The testing shall be as representative as possible for the design loads. This means that the number of cycles for the test loads shall not be taken too small and that the blade preferable shall be preloaded to have mean loads that are of the same magnitude as for blades in operation. Normally the number of cycles is taken between one and 10 millions in both the flapwise and edgewise directions. In practice the dynamic testing is carried out by attaching masses and exciting the first eigenmode of the blade with a rotating exciter or by hydraulic actuators attached along the blade. IEC does today not specify the order of the individual static and fatigue tests on the blade. As in IEC ultimate and fatigue strength are considered as two separate issues. This means that damage due to combined damage growth from ultimate and fatigue loads may not be covered by the type test. This contribution to modelling uncertainty may be critical for composite structures. The authors recommend that static testing is carried out both prior and after fatigue test of blade to conservatively consider the combined damage accumulation due to ultimate and fatigue loads. It is important to emphasise that fatigue testing is carried out in order to mitigate the possibility of gross modelling uncertainties leading to insufficient blade strength. However, both design and testing are based on rainflow counting, constant life diagrams from constant amplitude testing of coupons and Miners rule. For this reason type testing cannot cover all contributions to modelling uncertainty in the fatigue strength analysis. 4. CHALLENGES FOR FURTHER DEVELOPMENT OF THE IEC STANDARDS As already mentioned IEC (2005) and IEC (2001) are to a large extent based on the common design methodology for steel structures. The following two issues are noted: 123

10 Wedel-Heinen and Tadich Temperature effects are not part of detailed load or resistance calculations for load carrying steel components of wind turbines. Thermal induced strains and stresses during cure are important for wind turbine components. At present these are controlled by active heating and cooling systems during cure. The format for structural safety is based on rational assessment of variability in load and basic material strength (i.e., strength that can be measured on coupon level). Variability of defects and imperfections are assumed to be part of the basic material strength. This implies that they shall be categorised and controlled with standardised procedures. Differences in structural behaviour at elevated temperatures and differences in size and complexity of defects complicates the use of the above mentioned methodology for composite structures. We will illustrate this by three examples. 4.1 Example 1 Composite materials at elevated temperatures. The purchase of materials and the manufacturing of wind turbine blades introduce important constraints on design. One constraint is that the required glass transition temperature for thermosetting resins and adhesives shall be as low as possible. A low glass transition temperature will both open up for a broader range of alternative suppliers and products that can be qualified for the blade and allow for short curing cycles and reduced requirements to high curing temperatures. Short curing cycles and low temperatures are necessary to speed up the product flow through the work shop. In the OPTIMAT project coupon testing of epoxy GFRP laminates was carried out at both room temperature and at +60 C which is slightly above the extreme ambient temperature 50 C for the IEC standard wind turbine classes. Mengis, Brøndsted and Eriksen (2005) report that the measured tensile and compressive strength at 60 C were reduced by 6 and 44% respectively compared to their corresponding room temperature values. The middle point glass transition temperature for the coupons manufactured for the testing in OPTIMAT were in the range from 65 to 78 C and the epoxy system was a typical infusion system commonly used for wind turbine blade manufacture. The lay up of wind turbine blades includes both thin and think laminates and these laminates are cured in the same process. The temperature history in the laminate during the exothermic curing process will depend on the thickness of the laminate. This is particularly the case for un-heated moulds. Control of glass transition temperatures for cured blades is normally an important quality control measure in the blade work shops. From the OPTIMAT observations it can be concluded that load and resistance at elevated temperatures can be very critical for the ultimate strength of wind turbine blades. IEC does not specify how to calculate the design loads that are associated with high temperatures and does not directly specify which temperature interval the 124

11 Qualification of materials and blades for wind turbines characteristic strength refers to. On the contrary the standard specifies that the air density is kg/m 3 for all the load scenarios in the type classes. From an analysis point of view the effect of the temperature on the air density is straight forward. The effect of a smaller air density on the loads can also be calculated by simple sensitivity studies of aeroelastic analysis. However, a rational approach to issues such as the way the ambient temperature is combined with heating from solar radiation and the correlation of high temperatures with extreme wind speeds and other less frequent load scenarios should be part of the text in the standard in order to form rational wind turbine class definitions. 4.2 Example 2 Thermally induced strains. An increasing amount of CFRP has been introduced in commercial wind turbine blades over the last 5 years. Uni Directional CFRP laminates tested in the laboratory have typically a tensile strength in the order of 1500 MPa and a compressive strength of 1000 MPa. A complicated part of design of CFRP reinforced blades is the transfer of stresses from the CFRP to the steel bushings in the root joint. A simple approach is to introduce a traditional GRP transition between the CFRP and the steel bushings. Further optimisation of blades may require that CFRP and steel is bonded. A critical issue for the bonding of CFRP with other materials is that the thermal induced stresses that will arise from unequal coefficient of thermal expansion. A simple example of bonded CFRP steel joint is illustrated in Figure 6. Fig. 6. Simple example of a bonded steel joint between CFRP laminate and steel. A simple assessment of the thermally induced strains after the curing is carried out in Table 1. It is assumed that the cross sectional area of the steel is much larger than for the CFRP. Furthermore, it is assumed that the bonding of the steel fitting to the CFRP laminate is cured at a temperature of 120 C and that the joint is cooled to - 30 C after curing. Such cooling could occur as soon as the blade leaves the work shop and is 125

12 Wedel-Heinen and Tadich stored outside. The approximate stress width during a typical daily temperature cycle of 30 K is also listed for reference in Table 1. Table 1. Characteristic values for material properties and typical thermally induced strains and stresses in a bonded CFRP steel joint CFRP Steel Coefficient of thermal expansion K K -1 Young s Modulus 133 GPa 210 GPa Strain at curing of adhesive joint 120 C 0 Strain at 30 C Stress at 30 C MPa ~ 0 MPa Stress width a daily temperature cycle of 30 K 40 MPa ~ 0 MPa With partial safety factors for design and knock down factors for steel to CFRP interfaces the maximum allowable compressive stress is in the range of 500 MPa for strength at a joint. From the values of the stresses in the example in Table 1 it is clear that thermal induced stress can be of the same magnitude as stress introduced by the aeroelastic response for the turbine. The prevention of large stress concentrations at the ends of steel and CFRP components requires careful design in form of taperings etc. It is noted that simple upscaling will reduce the aeroelastically introduced strains in the interface but not the thermally introduced strains. For complicated composite joints of dissimilar materials further specification is needed in the design standards to better define the thermally induced response for design and the combination of thermal and aeroelastic response in ultimate and fatigue load combinations. Dissimilar materials also suffer from corrosion which must be considered during the design by isolating the two materials or by protecting the joint from moisture. 4.3 Example 3 Defects and Imperfections. The traditional safety format in structural design codes for steel structures is focussed on the variability in loads and materials resistance. The variability in strength refers to coupons testing. Figure 7 is taken from DNV/Risø (2002) and illustrates the background behind the format. The characteristic values for loads and resistance L C and R C refers to certain survival probabilities in the frequency distributions. Allowable design is specified as the allowable margin between L C and R C. The margin depends on the consequence of failure and the shape of the tails of the frequency distributions of loads and resistance. The allowable margin is in IEC specified in form of partial safety factors covering loads, resistance and consequence of failure. 126

13 Qualification of materials and blades for wind turbines Fig.7. Frequency distributions for loads L and resistance R (from DNV/Risø (2002)) For composite materials variability in strength on coupon level and variability in load is not sufficient to cover the structural reliability as for steel structures. The size and complexity of defects and imperfections in composite structures is typically not represented in the coupon testing. Defects and imperfections need to be considered as stochastic parameters in addition to load and coupon strength. Figure 8 is taken from Berry (2006) and shows a small GRP wind turbine blade with significant surface cracks after being in operation for a limited period. A cut transverse to cracks reveals a large wrinkle in the laminate. It is clear that the defect controls the local strength. Fig. 8. Surface crack and laminate wrinkle in GRP wind turbine blade (from Berry 2006) The variability of defects and imperfections is specific for each material and each manufacturing method. This implies that blade manufacturers must have individual measures for allowable defects and imperfections. The fact that suitable NDT methods for composite laminates are still in their mature development complicates the 127

14 Wedel-Heinen and Tadich implementation of rational quality control procedures. A well known example of defects to be considered in the design of wind turbines is wrinkles in laminates as discussed by Brøndsted, et al. (2005). The first step in incorporating defects and imperfections in the design and test standards could be to require that test blades include representative worst case defects at critical locations instead of including a blade to blade variation factor in the test loads. Such defects should also cover the general repairs that are carried out in the work shop to remove manufacturing flaws, as well as typical field repairs. Another step could be to specify requirements to fracture mechanical verification of defect and damage growth. The present safety factor format in IEC is not relevant for this purpose. 5. CONCLUSION The existing procedures for qualification of materials and testing of wind turbine blades are specified in the IEC series of standards. The procedures are based on the established design methodology for steel structures. This methodology is characterised by four issues. Assessment of material ultimate strength through coupon testing Rainflow counting of the stochastic load histories Constant Life Diagrams as maps of partial damage for a number of load cycles. Miners Rule for damage growth and fatigue failure. Type testing is carried out to avoid gross errors leading to a significant modelling uncertainty in design. As the above four issues are also used for deriving of the test loads in fatigue not all modelling uncertainty can be assessed in type testing. The existing procedures in the IEC (2005) and IEC (2001) may not be sufficient for optimal design of advanced wind turbine blades in composite materials in the future. This is illustrated by three examples. The examples conclude that initial steps for improving the standards could be: The temperature and preferably also other ambient conditions should be specified in more details for the design load scenarios in the IEC standard. The allowable defects and imperfections should have more attention in qualification of materials, design and testing of wind turbine blades. A rational approach should be part of IEC The IEC standard should require that test blades include worst case manufacturing defects and imperfections. 128

15 Qualification of materials and blades for wind turbines Type testing of a blade should be carried out as first static testing then fatigue testing and finally static testing again to conservatively address combined damage growth for ultimate and fatigue loads. The authors propose that improved design requirements are developed in research in joint industry programmes such as UPWIND, as previous experience with joint industry efforts for standardisation in the wind turbine industry has been quite promising. ACKNOWLEDGEMENT The present paper has been prepared based on the input and discussions in working group of the EU supported UPWIND Project Work Package 1A1, Integrated Design and Standards. REFERENCES Berry, D: (2006) Wind Turbine Blades, Blade Manufacturing and Materials Development, Presentation at Sandia Blade Work Shop, April Brøndsted, P., Lilholt, H. and Lystrup, A. (2005). Composite Materials for Wind Power Turbine Blades, Annual Review of Materials Research Vo DNV/Risø (2002) Guidelines for Design of Wind Turbines (Risø National Laboratory) DNV-OS-J102, Draft, (2006) Design and manufacturing or Wind Turbine Blades (DNV Copenhagen) IEC (2005), Wind Turbine Part 1, Design Requirements, edition 3 IEC (2001), Wind Turbine Generator Systems Part: 13, Measurement of Mechanical Loads IEC (2001), Technical Specification, Wind Turbine Generator Systems Part: 23, Full-Scale Structural Testing of Rotor Blades IEC WT 01 (2001), IEC system for Conformity Testing and Certification of Wind Turbines, Rules and Procedures Jansen, L.G, Wingerde, A.M. et al. (2006), Reliable Optimal Use of Materials for Wind Turbine Rotor Blades (Optimat report OB_PC_017, WMC & ECN the Netherlands) Mengis, M., Brøndsted, P. and Eriksen, K.P. (2005) Effects of extreme conditions on properties of the reference material (Optimat report OB_TG3_R015, Risø Denmark) Wedel-Heinen, Tadich et al. (2006), Implementation of OPTIMAT in Technical Standards (Optimat report OB_TG6_R002, DNV Copenhagen) 129