On certification aspects of wind turbine blade load carrying structure

On certification aspects of wind turbine blade load carrying structure Tomasz Sieradzan, Bente Vestergaard Lead author: Tomasz.sieradzan@dnvgl.com Direct: +45 3945 4879 DNV GL Tuborg Parkvej 8, 2 nd floor 2900 Hellerup Denmark Summary With continuously increasing size of the turbines, the requirements for the blade structure are getting ever more demanding. The energy from wind is to be captured from the entire blade length and transferred to the rotor hub. As a consequence, functionality of the blades must not only consider aerodynamic efficiency but ensure stable load transfer from the outer parts with high linear velocity to high bending moment carrying root section. The requirements put ahead of blade designers often necessitate unique, project specific solutions. Variation in structural functionality along the blade s length needs to be properly addressed and account not only for design requirements, but also for manufacturing capabilities and process robustness. Over the last decade the complexity in the design of the blade internal load carrying structure has significantly evolved. Substantial improvement, to great extent caused by controlled manufacturing processes, is also visible in the quality of the finished parts. Availability of high quality materials and innovative solutions for blade construction is no longer an in-house advantage of the blade manufacturer alone. Along with the material and manufacturing development, the certification procedures and approaches need to be revised and address the relevant issues, which may have not appeared before. Understanding of various material interactions requires not only standard specified factors, but involves close cooperation with the customers. Evaluation of the design needs to be supported with detailed manufacturing considerations on quality and feasibility, involving, if necessary, manufacturing inspections. Higher structure utilization is often accompanied by reduction in partial safety factors applicable to manufacturing techniques. Design Evaluation, Manufacturing Evaluation and Type Testing certification modules are becoming inseparable, as only their interactions give the true picture of the structure performance and robustness. 1/10

Introduction Wind turbine blades are considered one the most complex components of a wind turbine due to material composition, demanding manufacturing techniques and high material utilization of the structure. Blades are first components of a wind turbine exposed to wind loads and their ability of transferring energy from wind into rotational movement of the rotor determines turbine s efficiency. With continuously increasing size of the turbines, the requirements for the blade structure are getting ever more challenging. The energy from wind is to be captured from over 80 meters of blade s length and transferred to the rotor hub. As a consequence, functionality of the blades must not only consider aerodynamic efficiency but ensure stable load transfer from the outer parts with high linear velocity to high bending moment carrying root section. Evolution of the wind turbine blades has accelerated over the last decade, leading to optimized designs, often incorporating unique, patented solutions. The limitations are being pushed as the manufacturing capabilities allow more repeatable and controlled processes. Along with the material and manufacturing development, the certification procedures and approaches need to be revised in order to address relevant issues, which may have not appeared before. Understanding of various material interactions requires not only standard specified factors, but, involves close cooperation with the customers. Commonly used certification systems may not always be up-to-date with the latest techniques and processes, as they allow some flexibility and trust certification bodies in their evaluation. Certification of wind turbine blades is evolving and dedicated blade standards are being developed as a response to market demands. Commonly used certification scheme of International Electrotechnical Commission (IEC) distinguishes the following assessment modules: 1. Design Evaluation 2. Manufacturing Evaluation 3. Type Testing Each module can be completed independently and an appropriate Statement of Compliance can be issued. This generic scheme is applicable not only for blades, but also for other components. What makes the certification of wind turbine blades unique is the material composition, various possible manufacturing techniques and extensive full scale testing program. The main load carrying structure is a comprehensive representation of the blade complexity accounting for different structural solutions, high material utilization, bonded joints and geometrical transitions. Verification activities have to reflect complexity and functionality of each and every subcomponent as well as their mutual interactions. DNV-GL interpretation of the IEC design standards is complemented in [4] and [5]. Both the DNV-DS-J102 and the GL Guideline show similar principles, starting with material identification, evaluating global response and getting deeper into details potentially critical to the design. Materials The variety of materials used in the blade design makes the qualification of materials and their properties an important step in the certification process. Available and commonly used fabrics include regular and high modulus glass fibre, and carbon fibres. Currently available choices of resin system cause the average laminate properties to vary. Therefore to validate the strength and elastic properties of laminates, coupon testing is advised. The desired material properties to be tested are indicated in [4] and [5] together with the applicable international testing standards. In sandwich construction, commercially available core materials are usually sufficiently defined by the supplier, 2/10

and provided mechanical properties can be used in the design. To have a full picture of the material composition, qualification of adhesives cannot be neglected as loss of their structural functionality may result in a catastrophic failure of the entire blade. Equally important in the material characterization is their quality assurance accounting for repeatability of mechanical properties, qualification of raw materials suppliers and in-house quality control procedures. Coupon testing and simple loading scenarios used in material characterization give solely an indication of the possible structural response. The real interactions between different materials, geometrical effects and combined loading may only be seen in more complex structural tests, as indicated in [4], see Figure 1. Figure 1 The testing block approach, [4] To account for uncertainty, selection of corresponding Partial Material Safety Factors (PMSF) has to be properly applied. The effort put into detailed characterization of structure composition is compensated by reduction of the PMSF, allowing higher utilization of the materials in well-defined areas of the blade. The benefit is even more pronounced on the state of the art designs, using hybrid material compositions and tailored techniques to eliminate defects e.g. prefabricated sub components, fabrics with varying areal weight. The applicability of the standard PMSF may turn out to be more conservative than in conventional designs. This leaves space for technical argumentation and discussion. The more demanding the design, the higher the need for the dialog necessary to understand each other s concerns and expectations. Material characterization provides basis for the design assessment. The risk associated with not fulfilling the design assumptions is very high. It is clearly highlighted to the manufacturer in the very early stage of the certification process. With the design of offshore multi-megawatt wind turbines, the blade length reached over 80m whereas the longest blade in 2010 was just above 60m. The limits of scaling are being crossed. Blades become more slender and the root connection significantly increases in size. Consequently, structural challenges so far mitigated by the structure itself arise and can no longer be overlooked. Rapid development of the material science and the numerical methods, which enable calculations of a full blade by a standard PC, pushes the designers forward, and provokes them with application of the latest achievements in material science analysis i.e. inter fibre failure, lamina level fatigue. Nonetheless, the designs remain similar, and typical blade load carrying structure follows one of the 3/10

concepts presented in Figure 2, [11]. The design drivers for load carrying elements are straightforward and follow I-beam approximation. The caps need to provide bending stiffness to the structure, whereas webs transfer shear, compression and keep the distance between the caps constant. (a) (b) (c) Figure 2 Cross sections of different structural blade designs. a) Single shear web, b) Double shear webs, c) Box girder Spar caps The design of spar caps is driven by the longitudinal stiffness along the blade. The usual layup consists of UD fibres, consolidated in a thick laminate. For glass fibre designs, in many cases the global flap-wise response of the blade can be estimated using only caps properties and section thickness and a close match is found for thin sections. The thick glass cap design has the longest record in the industry and its response is considered to be well defined. Even though the definition of the local behaviour is well known, the challenges originating from significant increase in the blade length cannot be disregarded. During certification process, the potential risks are identified and the design robustness is checked. Although functionality of the caps is well known, materials are subjected to highest strains, and therefore very sensitive to manufacturing issues, among others: - Fibre misalignment within the fabric inter-fibre failure - Fabric misalignment: Off-axis position - UD fibres not aligned with the blade axis Wrinkles - Infusion problems due to caps thickness 4/10

- Distribution of the material in a chord wise direction - thickness jumps - Distribution of the material in longitudinal direction: Overlaps Cutting patterns and cutting procedures - Fabric willingness to follow mould s curvature - especially for wide and dense UD fabrics and cured or pre-cured subassemblies Despite potential problems, caps designed in glass fibre have significant advantages: - Raw material availability and cost - Robust manufacturing techniques - Easy handling and repairs - Well-known design compatible with resin systems and other blade material composition Accounting for both benefits and drawbacks, many of the manufacturers successfully use glass fibre for long off-shore blades, among others: - Siemens Wind Power, B75-75m Glass fibre epoxy, [6] - LM Wind Power, LM73.5-73.5m Glass fibre polyester, [8] Due to rather low specific stiffness for E- and H- glass fibres, the weight of the blades is high, and the corresponding loads on the rotor nacelle assembly are significant. To decrease the weight, caps can be designed and manufactured in carbon fibre composites. With the specific stiffness of carbon being more than twice higher than for glass, the possibilities in weight reduction are clearly visible. So-called carbon blades gain more importance in the off-shore wind turbines, with blade length not applicable to onshore market. The main development has recently been seen in : - Vestas V164 turbine, with 80m carbon blade, [7] - Samsung 7MW, with 83.5m carbon blade from SSP Technology A/S, [9] Manufacturing and handling of carbon fibre is a true challenge enforcing much more strict precautions to be taken than when dealing with the glass fibre. Moreover, many additional design considerations not present in glass fibre designs have to be accounted for: - Zero tolerance for fibre waviness or wrinkles - Resin penetration during vacuum infusion - Hybrid material composition Compatible resin system Curing accounting for different material thermal expansion coefficients Joints between carbon and glass induce jumps in a caps neutral axis introducing local bending and reducing the load carrying capabilities - Thinner caps prone to buckling and transversal bending Core material often utilized in the caps construction Sensitivity to geometrical distortions - Strain limits in compression significantly lower than in tension - High Influence of the ply drops - Lightning protection in a conductive structure In carbon design a big focus is put on ensuring carbon fibre straightness. Any misalignments, or waviness may results is significant reduction of already low ultimate compression strain. The quality plays the most important role in carbon structure. To fulfil the strict requirements, load carrying carbon can be sub-assembled off the mould to allow better process control and accessibility for profound quality inspections. Despite the high precautions, an additional nondestructive testing (NDT) of the cured carbon laminates is conducted. 5/10

Many of the issues are directly related to quality of composite construction, with higher restrictions than indicated for the glass fibre. However, buckling and local bending of the caps may necessitate modifications to the caps design, by increasing the caps thickness with low density materials, e.g. balsa core. This approach enabled caps to work as sandwich structure, with longitudinal stiffness provided by carbon fibre face sheets and the transversal resistance to local bending and buckling carried by the separated face layers. Introduction of the core into the caps requires detailed considerations on the core bonding, and geometrical properties which may result in stress concentrations. The benefits form the carbon application are more visible on the system level, where by reducing the blade mass, larger rotors can be fitted on existing turbine designs. Therefore the interest in this technology is limited, as the necessary precautions require very high quality manufacturing processes and the associated risks are more serious than in case of the glass fibre construction. In spite of all factors, many manufacturers and designers e.g. Vestas, Gamesa and Blade Dynamics find the carbon technology most suited for their purposes. Throughout the certification process, the DNV-GL verifies the pure design consideration as well as the manufacturing procedures which are found critical for the design. Material characterization, coupon testing and identification of associated risks provide an input to the certification process which also takes into account the manufacturing feasibility of the design. Processes that are found crucial for the functionality are being verified also throughout the manufacturing inspection which among general procedures focuses on issues indicated by the design review. The webs Webs are commonly made in sandwich, with glass fibre face sheets and various core materials. Utilized fabrics are often ±45º in order to resist shear forces, with additional transversal unidirectional plies to withstand compression. Webs functionality is to stabilize the load carrying structure, keep constant distance between the caps and resist Brazier s forces [10]. The geometry of the webs is fairly simple, and as they are manufactured separately, the quality is not considered an issue. The main certification considerations in web design come down to: - Sandwich core geometry - Corner design for the transition to the caps or to the web foot - Design of the bonding area, web feet The webs themselves are considered robust. However, the connection between webs and caps is to be verified in details, as the consequences of the loss of its functionality are catastrophic for the entire blade. Moreover, web start geometry at the root and finishing of the free edge has to be evaluated due to complex load state and increased risk of stress concentrations. In the box girder design, webs can be connected to the caps with or without adhesive. The adhesive free corner design is visible in girders which are winded or split in the middle of the webs, [13]. Due to very high curvature of the area and complicated material composition, including web sandwich core edge and laminate thickness change, the positioning and handling of materials is considered a critical issue. Positioning mismatches may result in resin reach areas, with considerable decrease of the load carrying capacity, possibly forming an elastic hinge. Consequently, buckling capacity of the webs may not be able to withstand the design loads. For girders other than winded (with a joint running along the length) see [ 13], [ 14], the adhesive connections have to account for loading state, flange design and the adhesive patterns, addressed 6/10

in the next section. In case if the girder is split in half, the challenges from both adhesive free corners and the adhesive joints have to be considered. Adhesive Joints Adhesive joints are present in almost every wind turbine blade. In the load carrying structure, they are located between webs and caps, and ensure proper load transfer within blade structure. The criticality of the bonded connection cannot be underestimated, as the consequences in adhesion lost are often catastrophic. Load carrying girder shows considerably good performance within the adhesive connection durability. However, as the girder has yet another adhesive connection to the shell, the amount of bonded joints increases and so does the complexity. The uncertainties in the bonded connections are high due to limited control accessibility and short pot life, which limits the time for quality checks. In order to properly estimate the strength and durability of the adhesive joint the glue line is assumed to have fixed constraints on the geometry. The thickness is assumed to be within a certain limits, and the bond is assumed to be continuous. In reality, the thickness of the glue joints needs to be checked with a dry-fit, and the recorded values are evaluated for compliance with design assumptions. What is more, the width and the edge geometry are to be ensured, although that it is no longer possible with the dry-fit. For designs with a structural shell technology, the minimum width of the web to cap adhesive joint is determined accounting for manufacturing imperfections via PMSF for bonded joints. Nonetheless, the bond lines are often checked with NDTs for continuity. Shall the check fail, the areas have to be reinjected with adhesive using standard repair procedures. The length of the connection is close to the length of the blade, which means that the adhesive has to be applied to the long surface of the webs feet at once, with hardly any automation. The process is prone to errors of a human nature, which are the most difficult to detect and control. Even a slight offset from the bond line may result in considerable reduction of the strength or fatigue durability of the joint. By introducing stress concentration areas, which are hard to detect even with a NDT, the blade structure may suffer a significant loss of its functionality. While performing design evaluation, a focus is put on the adhesive joints in terms of their limit states and manufacturing robustness. Wind turbine blade manufacturers do not aim to decrease conservatism in the design as they are aware of high manufacturing variation of the process. Root The design of the blade root may appear simplified, but it is highly affected by the nature of loading in the area. Circular section provides same resistance in any of the loading directions, making the root capable of transferring any angular load fluctuations. High density, multi-directional glass fabrics are used, in order to accommodate required bolted connection e.g. T-bolts, inserts, or prefabricated sub-components [9]. The achieved strain level is low compared to the remaining part of the blade; however challenges occur mainly in other areas: - High density fabrics and thick laminates are very hard to infuse. - Bonding of prefabricated components to the root structure necessitates proper handling and storage. Contamination of the bond surface may result in loss of structural functionality. - High curvature of the root area may induce wrinkles in the laminate. - High number of layers makes it is difficult to localize the defect and perform the repair. 7/10

To ensure good quality of the blade root, especially close to the root bolted connection elements, prefabricated sub components are commonly used. Root spears, or root sections fabricated off the blade mould [9] show much better quality and reduce the risks associated with the infusion process. Pre-fabricated elements Full repeatability of the final product is the key to optimized designs. Automation in the wind turbine blade manufacturing processes is very low. Nonetheless, the development continues towards sub-assemblies that can be fabricated off the main mould, quality approved and put together with limited uncertainties in the manufacturing process. The more simple element, the better quality can be expected. Among the prefabricated components, one can notice: - Root elements [9], [12] - Load carrying caps [9] - Webs or web sections - Box girder as a sub-component of a blade Utilization of sub-assemblies comes directly from lean manufacturing approach, being adapted to serve also the wind turbine blade industry. This allows new sub-suppliers to provide high quality composite elements using unique manufacturing techniques not applicable for blade manufacturing until now. From the certification perspective, new materials and high quality sub-components require proper characterization, which is very similar to what is expected at material level. Advantages are seen in reduction of PMSF due to repeatability and control of the processes. Testing Development of sub-components for the wind turbine blade necessitate their capacities to be tested, and corresponding results to be used as an input for the design. Depending on characteristic, small scale tests are conducted for e.g. root connection, adhesive joints and design specific solutions. Testing is advised if the analytical and numerical calculations may not be able to reproduce the complex material composition or loading. Moreover, if the structure or its elements cannot be validated by full scale blade test, small or medium scale testing is required. The last of the evaluation IEC modules is Type Testing. For a wind turbine blade to be certified, a full scale blade testing campaign is required. Having identified and mitigated the risks coming from the design, it is the ultimate laboratory test of the blade structure and all interactions between the utilized materials and solutions. The full scale blade test campaign requirements are indicated in [3] and [ 4]. The test sequence consists of pre-fatigue static test, fatigue test in two main directions, and the post fatigue static test. Static tests prove blade resistance to the design loads by accounting for uncertainties with a test load factor. A blade is loaded along main axis, both directions each, resulting in 4 loading directions for a static test. Fatigue test is conducted as a subsequent stage of the testing campaign. Its ultimate purpose is to reproduce lifetime operational cycles of a blade, and prove blade operational capabilities. Due to complexity in the fatigue load levels, an approximation has to be made to meet feasibility of the real test configurations. Consequently, the fatigue formulation of the design loads accounts a test load factor to compensate for possible. Fatigue testing is to be performed in flapwise and edgewise directions simulating wind and gravity loads. 8/10

As indicated before, wind turbine blades are very sensitive to the manufacturing quality and applied processes. Manufacturing defects which are not critical for ultimate blade capacity often reveal themselves throughout the fatigue testing. Therefore, fatigue tests are not only a check of the blade design, but give also a true picture of manufacturing capabilities and robustness of the design. The wind turbine blade structural integrity may not be compromised at any point of the operation. The post fatigue static test validates blade capacity of resisting the extreme static loads in the last day of designed lifetime. Conclusion Certification of load carrying structure in wind turbine blades takes into account design and manufacturing considerations as well as the interface between conceptual and real life challenges. Design evaluation needs to be supported with detailed manufacturing study on quality and feasibility, involving, if necessary, dedicated manufacturing inspections. Higher structure utilization is often accompanied by reduction in partial safety factors applicable to manufacturing techniques. Design Evaluation, Manufacturing Evaluation and Type Testing certification modules are becoming inseparable, as only their interactions give the true picture of the structure performance and robustness. DNV GL as a world leading certification body is contributing to International Electrotechnical Commission standards with guidelines [ 5], and recognized blade design standard [4]. Experience from numerous projects among various manufacturers enables assessment of the associated risks. Profound insight into the design details and manufacturing aspects is selected as the key combination for the assessment. Throughout the certification process, step-by-step assessment mitigates the risk in an early stage and diminishes possible consequences. The 3-party assessment provides a fresh view on the structure, and ensures that no critical item is overlooked. DNV GL strives to meet the need for constant dialogue with the customers throughout certification process. By hand-in-hand cooperation towards sustainable future, the Company drives implementing new methodologies for wind turbine blade design, testing and manufacturing. Completed Component Certificate adds value to the product by assuring its consistency and quality. It proves that the blade is designed and manufactured with the sound engineering knowledge, and capable to fulfil design requirements over the entire lifetime. 9/10

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