dnv-os-j102 (draft october 2004) design and manufacturing of wind turbine blades (offshore)

8/16/2019 DNV-OS-J102 (Draft October 2004) Design and Manufacturing of Wind Turbine Blades (Offshore)

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OFFSHORE STANDARD

DNV-OS-J102

DESIGN AND MANUFACTURING OF

WIND TURBINE BLADES

DRAFT OCTOBER 2004


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DNV Offshore Standard: 'Design and Manufacturing or Wind Turbine Blades’, Draft, 041025

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SECTION 1 INTRODUCTION...............................5

A. Objectives............................................................5 A 100 Objectives ............................................. 5

B. Application..........................................................5

B 100 Application............................................5 C. Normative references .......................................... 6 C 100 IEC Type certification standards...........6 C 200 DNV guidelines and standards..............6 C 300 Other references....................................7

SECTION 2 PROCEDURES FOR MATERIALQUALIFICATION..........................................................8

A. General................................................................8 A 100 General..................................................8 A 200 Quality system requirements.................8

B. FRP materials...................................................... 8 B 100 FRP materials........................................8

C. Sandwich core materials......................................9 C 100 Sandwich core materials ....................... 9 D. Adhesives.......................................................... ..9

D 100 Adhesives..............................................9 E. Laminated wood..................................................9

E 100 Laminated Wood ....................................... 9 F. Metallic materials................................................9

F 100 Metallic materials for bushings etc............9

SECTION 3 DESIGN CALCULATIONPROCEDURES 10

A. General..............................................................10 A 100 Objective.............................................10

A 200 Quality system requirements...............10 A 300 Processes.............................................10 A 400 Failure Modes and Failure Mechanisms 12 A 500 Design criteria.....................................14 A 600 Load and load factors..........................15 A 700 Consequence of failure factor ............. 15 A 800 Material resistance and resistance Factors 15 A 900 Geometrical Parameters........... ...........16

B. Verification of input for design loads and towerclearance analysis ....................................................... 17

B 100 Verification of input for load and tower

clearance analysis...................................................17 C. Analytical models for linear analysis of bladestrains..........................................................................17

C 100 General................................................17 D. Finite Element Analysis .................................... 17

D 100 Modelling of Structures – General......17 D 200 Software Requirements.......................19 D 300 Execution of Analysis.........................19 D 400 Evaluation of Results .......................... 20 D 500 Validation and Verification.................20

E. Buckling Analysis ............................................. 20 E 100 Concepts and definitions..........................20 E 200 General ............................................... .....21

E 300 Calculation of buckling............................21 E 400 Buckling analysis of isolated components22

E 500 Buckling analysis of more complexelements or entire structures .................................. 23

F. Tower clearance ................................................ 23 G. Fibre failure.......................................................24

G 100 General................................................24

G 200 Fibre failure at the ply level ................24 G 300 Fibre failure check using a modifiedTsai-Wu criterion...................................................25 G 400 Special considerations for fibre failureunder inplane compressive loads ........................... 27 G 500 Fracture mechanics approach..............27

H. Matrix failure .................................................... 27 H 100 General................................................27 H 200 Matrix failure based on simple stresscriterion 28 H 300 Matrix failure based on Puck's criterion 29 H 400 Obtaining orientation of the failure

surface 30 H 500 Matrix cracking caused only by shear.31 H 600 Matrix yielding ...................................31

I. Delamination and bond failure..........................31 I 100 General ................................................ ....31 I 200 Onset of delamination..............................31 I 300 Delamination growth ............................... 31

J. Sandwich failure................................................31 J 100 General ................................................ ....31 J 200 Failure of Sandwich Faces.......................32 J 300 Failure of the Sandwich Core .................. 32 J 400 Failure of the Sandwich Skin-Core Interface 32

J 500 Buckling of sandwich structures..............32 J 600 Yielding ................................................... 33 J 700 Ultimate Failure of orthotropic homogenousmaterials.................................................................33

K. Fatigue Limit State............................................35 K 100 General................................................35 K 200 Cyclic Fatigue ..................................... 35 K 300 Fibre failure in fatigue ........................36 K 400 Change of elastic properties................36 K 500 Initiation of fatigue damage ................ 36 K 600 Growth of fatigue damage...................39

L. Other Failure modes..........................................39 L 100 Wear ...................................................... ..39

L 200 Chemical decomposition / Corrosion.......39 L 300 Lightning protection ................................ 39 L 400 Impact..................................................... .40 L 500 Creep........................................................40 L 600 Extreme temperatures in the blade...........40 L 700 Draining...................................................41

SECTION 4 QUALIFICATION OFMANUFACTURING PROCEDURES........................ 42

A. General..............................................................42 A 100 Manufacturing procedure manuals......42

SECTION 5 TEST PROCEDURES......................44

A. General..............................................................44 A 100 General................................................44


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B. Selection of test blade and specification of testing 44

B 100 Selection of test blade ......................... 44 B 200 Parts of the blade to be tested..............45 B 300 Determination of natural frequencies anddamping 45 B 400 Static testing........................................45 B 500 Fatigue testing.....................................47 B 600 Final static testing ............................... 49

C. Quality management of testing ......................... 49 C 100 General................................................49

D. Reporting...........................................................50 D 100 General................................................50

SECTION 6 DOCUMENTATION FORDETALIED SPECIFIC BLADE DESIGN..................51


A 200 Design Documents..............................51 A 300 Technical Documents..........................51 A 400 Work Instructions and Drawings ........51 A 500 Test Documents .................................. 51 A 600 Installation and Service Documents....52

SECTION 7 MANUFACTURING OF BLADES.53



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SECTION 1

INTRODUCTION

A. Objectives

A 100

Objectives

101 This standard provides principles, technical requirements and guidance for the entire process for design andmanufacturing of wind turbine blades. The objective of the standard is to serve as:

— technical reference document between clients, contractors, suppliers, consultants and third parties — detailed interpretation of IEC WT 01 for type certification of wind turbine blades — supplementary interpretation of ISO 9000 for certification of quality management systems for design and manufacturing of

wind turbine blades — supplementary standard for Manufacturer Product Quality Assessment (MPQA) services — supplementary interpretation of ISO 17025 certification of blade test laboratories — detailed standard for third party blade manufacturing inspection

Guidance note:The type certification of blades according to IEC WT 01 involves design evaluation (verification of material qualification, design calculation and work

instructions), manufacturing evaluation (inspection of the manufacturing of one blade) and testing of blades.

The IEC WT 01 design evaluation will both cover the generic elements and specific elements of the design documentation. Verification of the generic elements

will only be carried out in details if these elements are critical for the specific design.The IEC WT 01 blade testing evaluation is normally carried out as a review of test report from an accredited laboratory and a report for manufacturing of the test

blade.The IEC WT 01 manufacturing evaluation is normally carried out once. The evaluation is carried out when the manufacturer consider tools (moulds) and quality

procedures ready for serial production. Often the quality procedures are in development during the manufacturing of the prototype test blade(s). In such cases the

manufacturing evaluation is carried out at a later stage and the documentation for manufacturing of the test blades is reviewed in the light of the final tools.

If design, materials or manufacturing procedures are changed necessary elements of design verification, testing and manufacturing evaluation must be repeated before the IEC type certificate can be reissued.

DNV may offer additional services to the IEC WT 01 type certification to assist the manufacturer in developing and maintaining the quality system, to enable afaster type approval process for new blade designs and to inspect blade manufacturing as a third party. These services can be arranged as MPQA, ISO 9000

certification, ISO 17025 certification of blade test laboratories and/or project related blade third party manufacturing inspection. The basis for these additional

services is provided with the present standard as it is divided in self contained sections.

---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---

102 This standard does not cover details of the determination of design loads for wind turbine blades. The safety level isspecified according to loads and partial safety factors specified in IEC 61400-1.

B. ApplicationB 100

Application

101

Manufacturing and design of wind turbine blades are normally carried out both on generic and specific levels at themanufacturers. The generic level involves qualification of design procedures, materials, manufacturing and test methods. Thespecific level involves design, manufacturing and test of individual blade types.

102

The wind turbine blade manufacturer should preferably establish and control general design procedures, qualification ofmaterials, manufacturing procedures and applicable test procedures through for example revision-controlled manuals. Themanuals should be self-contained and preferably provide all relevant justification of procedures and qualification.

103

This standard is thus divided into a generic part concerning documentation of the qualification process and a specific partfor blade design and manufacturing.

104 The certification of wind turbine blades can be carried out in three steps as illustrated in Figure 1:

— The first step covers verification of the blade manufacturer procedure manuals for control of design calculations, materialsqualification, manufacturing and blade testing. The verification is carried out on a generic level.

— The second step involves approval of the specific design which is carried out according to the procedure manuals verified inthe first step and documented in form of design drawings, work instructions, design reports and test reports.

— The third step consists of inspection of manufacturing of individual blades according to the design drawings and workinstructions verified in the second step. The procedure manual for manufacturing verified in the first step is used as guidancefor the inspection.


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105

Type certification is limited to a specific design. The type certification may not cover a verification of the complete

material qualification and all the generic procedures. Only elements of the material qualification and the generic procedures thatare critical for this design will be verified.

Guidance note:

The wind turbine blade designer and manufacturer must decide how to communicate design and manufacturing to the certifying body. In cases with manydifferent blade design based on the same principles it may help the communication not to mix generic documents and specific documents for the individualdesigns in the documentation packages issued and in the verification statements to be requested from the certifying body.


Figure 1. Wind turbine design and development processes with reference to the sections of this standard

C. Normative referencesThe latest revision of the following documents applies:

C 100 IEC Type certification standards

IEC WT 01, IEC System for Conformity Teststing and Certification of Wind Turbines, Rules and ProceduresIEC 61400-1, Wind Turbines Generator Systems, Safety Requirements

IEC 61400-23, Full scale testing of wind turbine bladesIEC 61400-24, Lightning protection for wind turbines

C 200 DNV guidelines and standards

DNV Guidelines for Certification of Wind Turbine Power Plants

DNV/Risø Guidelines for Design of Wind TurbinesDNV OS-501-C501, Composite components


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C 300 Other references

ISO 9001:2000, Quality management systems, requirementsISO 17025, General requirements for the competence of calibration and testing laboratoriesEnergistyrelsen: Recommandation for Design Documentation and Test of Wind Turbine Blades.

Guidance note:The number of latest edition of DNV documents may be found in the publications list at the DNV website www.dnv.com

The latest edition of DNV OS-C501 may be read in the viewing area for offshore standards on http://exchange.dnv.com/

The latest edtion of the publications issued from Energistyrelsen may be found at www.dawt.dk ---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---


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SECTION 2

PROCEDURES FOR MATERIAL QUALIFICATION

A. General

A 100 General

101 The material qualification should at least include:

— Requirements to manufacturing processes (e.g. curing control for resins and adhesives) — Requirements to traceability for material (e.g. name and trademark of manufacturer, material grade, batch number) — Requirements for storage (e.g. control of temperature, humidity and shelf life) — Characteristic material parameters for all relevant limit states including minimum and maximum service temperatures and

(e.g.: strength, toughness, density, cold deformability, ageing characteristics, resistance to rot and sun light) — Purchase specifications for the individual materials. The specifications shall at least cover strength properties, testing

methods, batch size, frequency of testing, certification, marking (labels/colour codes) — Qualification scheme for new suppliers of the individual materials. The scheme shall identify test methods that can be used

to document that a new material is compatible with the existing approved materials and the specified characteristic material parameters. — Qualification records for the approved suppliers.

102

The characteristic material resistance is defined in this standard as either the low 2.5% or the low 5% quantile in thedistribution of the arbitrary strength. This is equivalent to the 97.5 % or 95% tolerance respectively. Partial safety factors shall beused in accordance with the definition of characteristic strength, see Section 3, A 100.

103

The material strength shall refer to 95% confidence. The confidence requirement is important for test series that onlyinclude a few tests.

104

The material stiffness shall refer to 95% confidence and mean value.

105 The qualification records shall cover all interfaces to neighbouring materials.

A 200 Quality system requirements

201 The documentation for the material qualification shall be open to all individuals involved in design, purchase andmanufacturing. The documentation should preferably be organised as a manual.

B. FRP materialsB 100

FRP materials

101

The material qualification for FRP materials should be carried out according to DNV-OS-C501.

102 A qualification of a fibre ply type shall consider all applicable resins in the justification of the characteristic static andfatigue strength.

Guidance note:

One resin is typically selected to be used with a ply type early in the qualification phase. Later other resins may need to be qualified as substitutes for the first

resin. All relevant parts of the qualification shall be repeated for such substitutes.


103 The qualification of a resin shall consider all relevant fibre plies and all relevant adhesives.

104 The tensile and compressive strength of FRP laminates shall take into account the ply drops, fibre misalignment,manufacturing procedure and workmanship and be corrected for fibre content.

105

Through thickness tearing shall be taken into account in the qualification of FRP laminates. The minimum tearingstrength shall not be lower than 9 MPa according to ASTM C 297.


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C. Sandwich core materialsC 100

Sandwich core materials

101 The material qualification for sandwich core materials should be carried out according to DNV-OS-C501.

102 The qualification of a sandwich core material shall consider the interface to the alternative laminates.

Guidance note:

The qualification of a sandwich core material shall involve the interface to the skins as this interface may be critical for the strength in some failure modes. The

qualification records shall clearly specify the laminate skins that have been part of the qualification


D. AdhesivesD 100 Adhesives

101 The material qualification for adhesives should be carried out according to DNV-OS-C501.

102 Qualification of adhesive shall be carried out according the specified maximum bond thicknesses.

103

Qualification of adhesives shall both consider ductility, shear strength and peeling strength. The acceptance criteria shall be verified in full scale blade tests.

104

Qualification of adhesives shall consider thermal effects and ageing.

105 Adhesive joints shall be protected against exposure to water.

E. Laminated woodE 100 Laminated Wood

101 The qualification of wood should be based on the same principles as in DNV-OS-C501.

102

Data for strength of tensile and compressive wood shall be corrected for the density. The purchase specification shallspecify acceptable densities

103

The procedures for purchase and storage of wood shall control the moisture content at the manufacturing. Thecharacteristic strength of wood shall be based on test data that are corrected to represent the worst possible humidity. Thevariation in moisture content for the wood for a blade should be lower than 4%.

Guidance note:

The wood can normally be enclosed such that the moisture content will not increase when the blade is in service. Creep and ageing of the wood over the servicelife can normally be neglected for a controlled moisture content below 10%.


F. Metallic materialsF 100 Metallic materials for bushings etc.

101 Metallic materials do normally not need qualification if the design is carried out according to a recognised standard i.e.DNV Guidelines for certification of wind turbine power plants.

102 Bushings should be qualified in a test with both adjoining laminate, adhesives and the bushing arranged in a testingmachine where all the load is transferred from the bushing to the laminate in the same way as it is for the blade.

103

Bushings may be tested by inducing tension in one bushing and compression in other bushings. This kind of tests do nothave the same stress distribution as in the blade. Strength values can only be used for quality control and not for qualification ordesign.


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SECTION 3

DESIGN CALCULATION PROCEDURES

A. General

A 100 Objective

101 This section provides the general framework for analytical verification of wind turbine blade strength.

102 The analytical verification is carried out a check of the response of the structure from exceeding specific limit statesdefined as design criteria.

103 The verification is based on the principles of DNV-OS-C501. Reference is made to this standard for composite laminatesand sandwich structures.

A 200 Quality system requirements

201 The design calculation manual at the blade designer should specify the details of the processes covered in this sectionwhen the emphasis is on the specific materials, structural lay-out and processes relevant for the actual designs.

202 The qualification scheme shall be specified for engineers performing or supervising finite element analysis.

A 300 Processes

301 The processes in the analytical verification of blade strength are illustrated in Fig. 3.1 and are commented below.

302 Selection of partial safety factors and definition of characteristic values are not marked in Figure. 3.1. The selection isspecified later in this section.

303

The structural input for aero-elastic model shall be verified. In the initial design phase this is accomplished through finiteelement calculations or analytical models for blade stiffness and mass. In the final design phase the verification is followed up

through verification with measured data. Further details of the verification are given in Section B.

304

The aero-elastic calculation shall be carried out on the basis of IEC 61400-1 and DNV Risø Guidelines for design of wind

turbines. The aero-elastic calculation results in combined time-series of all relevant deformation modes of the blade. Theaero-elastic calculation is not covered further in this standard.

Guidance note: The worst loads in a circular root section may not occur in the flap or edgewise directions. Sensors for blade bending in the aero-elastic model are defined in acoordinate system where one axis typically is located in the rotor plane at the root and one axis is located in the direction of the aerodynamic profile along the

blade. The load histories for the individual sensors may not cover the most critical loads as these may occur in other directions than those of the sensors. Post

processing of the sensor histories with relevant transformation matrices for arbitrary positions on the root may be need to find the most critical positions and

loads.


305 A full scale static and dynamic blade test shall be carried out for all new blades designs as specified in IEC WT01 andIEC-61400-23. The test loads shall be specified by the designer based on the aero-elastic calculation and the materialcharacteristics. The test loads cannot be taken directly as the design loads as this would require many load directions and a

fatigue test running several years. Instead the test loads are taken as a transformation of design loads where reference is made tothe most critical limit states. The transformation is carried out such that analytical utilisation ratios for are not reduced. Relevantload factors are applied to variability in loads and strength. The specification of test loads are detailed in and the procedures fortesting wind turbine blades are detailed in Section 5.

306

A strain analysis shall be carried out for the worst ultimate loads. The strain analysis can be carried out as a linear analysissupplemented with a simplified local evaluation of the resistance against buckling. If the local buckling analysis indicates thatthe effect of buckling is moderate and is limited to a few elements that do not interact, then the final strain distribution can betaken as the linear response with a correction for buckling based on simplified analysis.

307

The strain analysis shall be carried out as a global buckling analysis if there is a significant influence of buckling on straindistribution when the blade is subjected to ultimate loads or if there is an interaction between buckling of neighbouring structural

elements in the blade. The global buckling analysis is normally carried out as finite element analysis

308

Buckling of the blade under normal operating loads is not acceptable. Accordingly, the strains for the fatigue analysis can be based on a linear analysis.


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Figure 3.1 Processes in analytical verification of wind turbine blade strength


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A 400 Failure Modes and Failure Mechanisms

401

Fracture of the blade may be caused by any combination of axial tension, compression, torsion or bending moments.

402

The global analysis of the wind turbine blades shall provide the worst combination of the above loads for local analysis.

The local analysis shall establish load effects (stresses or strains) on the ply level. Effects of buckling shall be taken into accountin the local analysis.

403 Every possible failure mode and failure mechanisms shall be identified for the blade.

404 General analysis methods for composite laminates and sandwich structures are given in DNV-OS-C501 Section 9. One ofthe suggested analysis methods shall be used for wind turbine blades.

405 The most common failure mechanisms related to fracture of composite laminates and sandwich structures aresummarised in Table A1.

Table A1: Typical failure mechanisms that can lead to fracture

Failure

Mechanisms

Comments Section in this standard

Fibre breaking

Fibre buckling

Ply failure with main stress in the fibre direction. Is assumed to causefracture. Fibre failure can be caused by exceeding the tensile orcompressive strength of the individual fibres. The fibres may start to buckle at micro level or on a higher level. The buckling may reducethe compression strength drastically. Imperfections further increasethe effect of buckling. Shall always be checked.

3G

MatrixCracking

Is assumed to cause fracture in UD laminates, in 0/90 laminates loadedin in-plane shear and in jointsMay reduce stiffness and compressive fibre strength.May initiate delamination.

Failure in the matrix shall consider both the matrix and the interface tothe fibre. The seizing of the fibre controls may have a significant

impact on the strength of the interfaceOtherwise a failure mode that does not influence fracture.

3H200-600

Matrix yielding Shall be checked, unless structure can tolerate large deformations ofthe material investigated.

3H700

Delamination Is assumed to cause fracture if a structure is exposed to through

thickness stresses.May be acceptable for in-plane loads.May reduce buckling strengthIf limited delamination is accepted it shall be verified that thisdelamination do not grow to an unacceptable size during the servicelife

3I

Sandwich corefracture

Shall always be checked. 3J700

Sandwich coreyield

Shall be checked, unless structure can tolerate large deformations ofthe material investigated.

3J600

Fatigue Repeated loading lead to accumulation of fatigue damage 3K

Guidance note:

Interlaminar tensile failure and interlaminar shear failure are in this standard and in DNV-OS-C501 described as matrix cracking and/or delamination.

Yielding is typically a failure mode for polymeric foams, but not a failure mode for most fibre reinforced laminates. If yielding can happen two options may be

used:

• The design does not allow yielding.• A fully nonlinear analysis may be done considering the effects of yielding.


406 For each failure mechanism its critically with respect to fracture shall be evaluated. Criticality may be different at

different locations in the blade.


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407 If a failure mechanism is found to be critical design calculations or testing shall show that this failure mechanism will nothappen within the lifetime of the blade.

408

The failure mode for fatigue is normally fatigue damage accumulation. See section 3.I.

409

In addition to the above mentioned failure modes that are initiated from the global ultimate and fatigue loads the

following other failure modes shall be considered in design of a wind turbine blade: impact from hail, lighting, wear (from sand particles in the air), corrosion, creep in areas under constant high (pre-)load (i.e. at T-bolts bolts) and temperature.

410

Special consideration shall be given to interfaces between laminates and steel or laminates and laminates, seeDNV-OS-C501 Section 7.

Guidance note:

The mechanisms of failure of composites can be discussed at different material levels. Failure can be considered to happen in the matrix, in the interface between

the matrix and the fibre or in the fibre. On a larger scale, it can happen to the individual ply (or core). Eventually, one can consider the whole thickness of thestructure as one quantity, i.e. the laminate or the sandwich structure.

Design criteria are often investigated on the most detailed level, i.e. first ply failure are used for as a criteria for laminate strength. For some structure there is a

high redundancy and there may be significant conservatism in this approach.


411

In some cases, a critical sequence of mechanisms of failure may be required for a failure mode to occur. That sequenceshould be specified (considering the “domino effect”), if relevant.

Guidance note:

Different sequences may lead to the same failure mode. In this case, the structure shall only be considered as failed, if the whole sequence of mechanisms of

failure modes has happened.


412 The type of failure (brittle or ductile failure) is determined for each failure mechanism in DNV-OS-C501. This is not donehere, because simplified material resistance factors are used.

413

Laminates and sandwich structures typically show a sequence of failure mechanisms. These sequences should be

considered. If one failure mechanism cannot be well described it may be sufficient to design the component in a way that the preceding failure mechanism will not occur.

414 Typical sequences for laminates are:

— matrix cracking => delamination => fibre failure — debonding and matrix cracking => fibre buckling => fibre failure — delamination => crack propagation due to fatigue => global buckling — An unusual but possible sequence is: — Wedge shaped matrix cracks => component failure in compression

415 Typical sequences for sandwich structures is:

under fatigue loading, crack initiates in the core due to core shearing => crack then propagates in core material =>

face-core delamination starts when shear crack reaches interface => face-core delamination propagates along theinterface until final catastrophic failure.

416

Typical failure mechanisms of sandwich structures are shown in the Figure below.


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(a) face/core yielding/fracture;(b) core shear;(c) buckling - face wrinkling;(d) delamination;(e) general buckling;(f) buckling - shear crimping;(g) buckling - face dimpling;(h) core indentation - core yield.

Figure 1: The Failure Mechanisms in a Sandwich Beam.

Guidance note:

The types and directions of loading shown in the figure 1 are indicative, and are characteristic of loading associated with the elementary failure mechanisms ofsandwich structures. However, in real structures, a failure mechanism can occur under various loading conditions.


A 500 Design criteria

501 A design criterion shall be assigned to each relevant mechanism of failure.

502 The general design criterion in the case of a single load for the Load and Resistance Factor Design format is takenaccording to IEC 61400-1:

mn

k k f k

R F S

γ γ

γ

.

)( ≤

where,

γf Partial load effect factorFk Characteristic loadSk Local stress or strain based design load effectR k Characteristic resistance

γm Partial resistance factorγn, Consequence of failure factor

503

The last term in 502 is the design resistance or the design strength

504 The selection of the partial safety factors shall be determined according to Section A700.

505 The characteristic value of the local stress or strain based on the characteristic load shall be determined. Nonlinear effectsin the analysis should be considered to obtain the proper value and distribution of the local stress or strain.


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Guidance note:

The safety factor format in IEC 61400-1 differs from the safety factor format in DNV-OS-C501. The consequence of failure factor is not part of the safety factor

format of DNV-OS-C501. Instead the following factors are used: Load model factor, Resistance model factor, system effect factor and a partial factor for fatigueanalysis.

The load model factor in DNV-OS-C501 can be taken as 1.0 for wind turbine blades as the loads are carefully verified through measurements and the response of

the blade is verified through full scale blade testing. Geometrical tolerances shall be taken into account in a conservative way.

The resistance model factor in DNV-OS-C501 can be taken as 1.0 if the following accuracies are taken into account:- Non-linearity of the strain response is taken into account in the calculated strain distribution before fibre failure is evaluated.- Matrix failure models are calibrated to tests for the actual stress distributions in the given laminate- If the delamination model takes into account the crack length, See DNV-OS-C501, sec. 6E.- Failure models for brittle fracture of core materials under multidirectional loading are calibrated to tests for the actual material under the same type of

loading.

The system effect factor in DNV-OS-C501 can be taken as 1.0.

The design criteria in DNV-OS-C501 are only valid for linear stress and strain response to design loads.

The fatigue factor in DNV-OS-C501 is not relevant as IEC-61400 and the present standard specify the safety in fatigue at strain level as for the ultimate strength.The present standard does not specify the safety in fatigue on fatigue life level as DNV-OS-C501. The safety level in the present standard is of the same level as

the safety level in DNV-OS-C501 for the slope of typical SN curves for materials for wind turbine blades (8 < m < 25 for curves on log-log scale).

The safety factor format in DNV-OS-C501 is based on the hypothesis that effects of low cycle fatigue, size effects of test specimens, long term effects ofhumidity, temperature, ageing and degradation all can be taken into account in a concise way through testing. The present standard defines a penalty factor for

material strength that can be used when detailed testing for these effects has not been completed.

It is also assumed in DNV-OS-C501 that the statistical variation in material strength properties in the qualification of materials can be used as a basis for the

determination of partial safety factors for resistance. The partial safety factor for resistance in the present standard is based on a minimum partial safety factormultiplied with a penalty factors for increased variations in strength. The minimum safety factor is based on the lowest realistic variation in strength for present

composite materials and manufacturing methods for wind turbine blades. The penalty factors for increased variation in strength is based on experience with the

specified optional materials and manufacturing methods.


A 600

Load and load factors

601 The specification of characteristic values and partial safety factors for loads should follow IEC 61400-1 or DNV-OS-J01.

602 Load effects shall be determined for all phases of the blade, e.g., transport, installation, operation, repair.

A 700

Consequence of failure factor

701 The consequence of failure factor shall be taken a according to IEC 61400-1

Table C1: Consequence of failure factors

Factor UltimateStrength

FatigueStrength

Consequence of failure factor γn 1.0 1.15

A 800

Material resistance and resistance Factors

801

The specification of characteristic values shall refer to the material qualification The characteristic material strength asdescribed in DNV-OS-501 Section 4 should be used for all calculations. Strength values shall based on the low 2.5% quantile inthe distribution of the arbitrary strength with 95% confidence. If only data of the low 5.0% quantile in the distribution of thearbitrary strength with 95% confidence are available an additional materials factor shall be used, as stated in Table C2.

802

If the strength of the material is temperature dependent, dependent on the surrounding environment or degrades over theservice life within the range of operational conditions, the analysis should consider the range of strength using the same principles as given in DNV-OS-C501. If this influence is moderate the reductions in strength is taken into consideration by

applying a strength reduction factor when determining the partial safety factor for strength, see Table C2

803

The material resistance factor γm is given as a combination of partial factors:

γm = γm1 γm2 γm3 γm4 γm5 γm6


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where γm1 to γm6 are partial safety factors and strength reduction factors described in Table C2.

Table C2: Partial material resistance factors

Factor UltimateStrength

FatigueStrength

Basic partial resistance factor 95 % fractile strength data γm1 1.3 1.2Correction factor if strength data refer to 97.5% fractileinstead of 95 % fractile

γm2 0.95 0.95

Strength reduction factor for repeated loading/low cyclefatigue

γm3 1.1 -

Strength reduction factor for of size effects, temperature,ageing and degradation for UV radiation and humidity:- Epoxy resin- Polyester resin- Wood with fibre reinforced epoxy shielding

γm4

1.11.21.1

Strength reduction factor for effects of optional materials andmanufacturing methods:

Fibre reinforced thermo setting resin- Prepreg or resin infusion unidirectional plies – areas with

continuous plies and no ply drops and manufacturingmethods which do not allow wrinkles through laminates

- Prepreg or resin infusion with mainly unidirectional pliesincluding ply drops

- Random orientated mats and/or hand layup Laminated wood, number of veneers through the thickness-- -- More than 11 veneers- 6 veneers- 4 veneers- 3 veneers or less

Interpolation between these values shall be done for other

number of veneersSandwich core- Plastic Polymeric foam, designed for yielding (PVC)- Brittle polymeric foam designed for fracture (PMI)- Balsa wood Bonded joints- Adhesive

γm5

1.0

1.1

1.2

1.01.11.21.3

1.01.11.3

1.3

Strength reduction factor for post curing:- Post curing controlled with DSC or equivalent- Post curing without control of cured laminate- Exothermic curing only

γm6 1.01.051.1

Guidance note:The factors in table C2 have been selected according to experience with coefficient of variation COV for optional materials and manufacturing methods. The

COV is defined as the standard deviation divided by the mean value. Further details of influence on COV on partial safety factors can be found in Appendix E of

DNV-OS-C501


804

The design of components in ferrous materials should follow DNV: Guidelines for certification of wind turbine power plants and is not considered in further details in the present standard.

A 900

Geometrical Parameters

901

Nominal dimensions shall be used for all calculations related to FRP laminates or polymers except for thicknesses takeninto account in buckling calculations. thicknesses in buckling calculation shall be taken as conservative values

Guidance note: There may be large variations in fibre content/thickness in wet lay-up.


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B. Verification of input for design loads and tower clearance analysisB 100

Verification of input for load and tower clearance analysis

101

The verification of the input for the load and clearance analysis shall confirm the distribution of aerodynamic profiles andthe orientation of the local coordinate systems in the computer model and cover a comparison of measured and calculated valuesfor:

— Mass — Mass center — First and second flapwise natural frequency and damping — First edgewise frequency and damping — Flapwise blade deflections in load conditions that are reprentative for the load at minimum tower clearance. — Torsional stiffness

Guidance note: Torsional stiffness verification can be omitted if it is justified that flutter instability is not critical for the blade design.


102 The verification of the input for the load analysis shall evaluate critical effects of combined loads such that design loadsare calculated for all critical details of the blade.

Guidance note: The worst loads in a circular root section may not occur in the flap or edgewise directions. ”Sensors” additional to the flap and edgewise root bending

moment ”sensors” in the aero-elastic model may be specified to provide necessary details of the design loads for other directions.


C. Analytical models for linear analysis of blade strainsC 100

General

101 In many blade designs the global bending loads are carried by a beam structure in the blade. The beam is composed ofsymmetric balanced laminates. The Aerodynamic profile of the blade is based on curved sandwich panels attached to the beam.

For this type of structure the strains from global bending can be found with simple beam theory.102 Beams in blades are not prismatic. Simple Bernoulli-Euler beam theory can be extended by correction factors coveringthe effect of flanges that are not parallel.

D. Finite Element AnalysisD 100 Modelling of Structures – General

101 Only recognised FE programs should be used. Other programs shall be verified by comparison with analytical solutionsof relevant problems, recognised FE codes and/or experimental testing

102

Element types shall be chosen on the basis of the physics of the problem

103

The choice of the mesh should be based on a systematic iterative process, which includes mesh refinements in areas withlarge stress/strain gradients.

104 Problems of moderate or large complexity shall be analysed in a stepwise way, starting with a simplified model.

105

Model behaviour shall be checked against behaviour of the structure. The following modelling aspects shall be treatedcarefully:

— loads, — boundary conditions, — important and unimportant actions, — static, quasi-static or dynamic problem, — damping, — possibility of buckling, — isotropic or anisotropic material, — temperature or strain rate dependent material properties, — plastic flow, — nonlinearities (due to geometrical and material properties),


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— membrane effects.

Guidance note:

Bending of bolted root joints may be controlled by the stiffness of the hub and pitch bearing. The stiffness of the interface shall refer to the technical specification

for the blade.


106

Stresses and strains may be evaluated in nodal points or Gauss points. Gauss point evaluation is generally most accurate,in particular for layered composites, in which the distribution of stresses is discontinuous, and should therefore be applied when possible.

Guidance note:

The analyst shall beware that Gauss point results are calculated in local (element or ply based) coordinates and must be transformed (which is automatically

performed in most FE codes) in order to represent global results. Thus, Gauss point evaluation is more time-consuming than nodal point calculations.


107 Support conditions shall be treated with care. Apparently minor changes in support can substantially affect results. In FEmodels, supports are typically idealised as completely rigid, or as ideally hinged, whereas actual supports often lie somewhere in between. In-plane restraints shall also be carefully treated.

108

Joints shall be modelled carefully. Joints may have less stiffness than inherited in a simple model, which may lead toincorrect predictions of global model stiffness. Individual modelling of joints is usually not appropriate unless the joint itself isthe object of the study. See also requirements for the analysis of joints in DNV-OS-501.

109 Element shapes shall be kept compact and regular to perform optimally. Different element types have differentsensitivities to shape distortion. Element compatibility shall be kept satisfactory to avoid locally poor results, such as artificialdiscontinuities. Mesh should be graded rather than piecewise uniform, thereby avoiding great discrepancy in size betweenadjacent elements.

110 Models shall be checked (ideally independently) before results are computed.

111

The following points shall be satisfied in order to avoid ill-conditioning, locking and instability:

— a stiff element shall not be supported by a flexible element, but rigid-body constraints shall be imposed on the stiff element, — for plane strain and solid problems, the analyst shall not let the Poisson’s ratio approach 0.5, unless a special formulation is

used,

— 3-D elements, Mindlin plate or shell elements shall not be allowed to be extremely thin, — the analyst shall not use reduced integration rule without being aware of possible mechanism (e.g. hourglass modes).

Guidance note:

Some of these difficulties can be detected by error tests in the coding, such as a test for the condition number of the structure stiffness matrix or a test for diagonal

decay during equation solving. Such tests are usually a posteriori rather than a priori.


112

Need for mesh refinement is usually indicated by visual inspection of stress discontinuities in the stress bands. Analogousnumerical indices are also coded.

113

For local analysis, a local mesh refinement shall be used. In such an analysis, the original mesh is stiffer than the refinedmesh. When the portion of the mesh that contains the refined mesh is analysed separately, a correction shall be made so the boundary displacements to be imposed on the local mesh are consistent with the mesh refinement.

114 For nonlinear problems, the following special considerations shall be taken into account:

— the analyst shall make several trial runs in order to discover and remove any mistake, — solution strategy shall be guided by what is learned from the previous attempts, — the analyst shall start with a simple model, possibly the linear form of the problem, and then add the nonlinearities one by

one,

115

Computed results shall be checked for self-consistency and compared with, for example, approximate analytical results,experimental data, text-book and handbook cases, preceding numerical analysis of similar problems and results predicted for thesame problem by another program. If disagreements appear, then the reason for the discrepancy shall be sought, and the amount

of disagreement adequately clarified.

116 The analyst shall beware the following aspects:

— for vibrations, buckling or nonlinear analysis, symmetric geometry and loads shall be used with care since in such problemssymmetric response is not guaranteed. Unless symmetry is known to prevail, it shall not be imposed by choice of boundaryconditions,


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DET NORSKE VERITAS

— for crack analysis, a quarter point element can be too large or too small, thereby possibly making results from meshrefinement worse,

— the wrong choice of elements may display a dependence on Poison’s ratio in problems that shall be independent of Poisson’sratio,

— if plane elements are warped, so that the nodes of the elements are not co-planar, results may be erratic and very sensitive tochanges in mesh,

— imperfections of load, geometry, supports and mesh may be far more important in a buckling problem than in problemsinvolving only linear response.

117 In the context of finite element analysis (FEA) of laminate structures (one of) the following element types should beapplied:

— layered shell elements with orthotropic material properties for each layer (for in-plane 2-D analysis, — solid elements with orthotropic material properties (for 3-D and through thickness 2-D analysis.

The decision to use 2-D or 3-D analysis methods should be made depending on the level of significance of through thickness

stresses and gradients of inplane stresses through the thickness.

Guidance note:

There are two options for the solid elements: The modelling may be performed with (at least) two solid elements through the thickness of each ply. Alternatively,

one may apply layered solid elements where the thickness of a single element includes two or more plies.---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---

D 200

Software Requirements

201

Selection of finite element software package shall be based on the followings:

— software availability, — availability of qualified personnel having experience with the software and type of analysis to be carried out, — necessary model size, — analysis options required, — validated software for intended analysis.

202 Useful options for the analysis of composite structures include:

— layered solid elements with orthotropic and anisotropic material behaviour, — layered shell elements, — solid elements with correct material models or appropriate interface elements allowing for debond (for analysis of bonded

and laminated joints), — interface elements allowing for large aspect ratio (for analysis of thin layer bonds), — the possibility to select different co-ordinate systems in a clear and unambiguous way.

203 Depending on the area of application, additional analysis options should be available, such as:

— appropriate solver with stable and reliable analysis procedures, — options characterising large displacements and large strains (for geometrically nonlinear analysis), — material models describing the behaviour of, e.g., laminates beyond first failure (for materially nonlinear analysis), — robust incremental procedures (for nonlinear analysis in general),

— tools for frequency domain analysis and/or options such as time integration procedures (for dynamic analyses), — appropriate post-processing functionality, — database options, — sub-structuring or sub-modelling.

D 300

Execution of Analysis

301

Extreme care shall be taken when working with different relevant co-ordinate systems, i.e. global, ply based, laminate based, element based and stiffener based systems.

302

The approach shall be documented.


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D 400 Evaluation of Results

401 Analysis results shall be presented in a clear and concise way using appropriate post-processing options. The use ofgraphics is highly recommended, i.e. contour plots, (amplified) displacement plots, time histories, stress and strain distributionsetc.

402

The results shall be documented in a way to help the designer in assessing the adequacy of the structure, identifyingweaknesses and ways of correcting them and, where desired, optimising the structure.

D 500

Validation and Verification

501

FE programs shall be validated against analytical solutions, test results, or shall be benchmarked against a number offinite element programs.

502

Analysis designer shall check whether the envisaged combination of options has been validated by suppliers. If this is notthe case, he shall perform the necessary validation analysis himself.

503

FEA results shall be verified by comparing against relevant analytical results, experimental data and/or results from previous similar analysis.

504

Analysis and model assumptions shall be verified.

505 Results shall be checked against the objectives of the analysis.

506 Verification whether the many different relevant co-ordinate systems have been applied correctly shall be considered.

507 The calculated strains shall be verified by measured strains from the blade test.

E. Buckling AnalysisE 100 Concepts and definitions

101 Elastic buckling phenomena are commonly considered in two main categories:

— Bifurcation buckling: Increasing the applied loading induces at first deformations that are entirely (or predominantly) axialor in-plane deformations. At a critical value of applied load (elastic critical load) a new mode of deformation involving bending is initiated. This may develop in an unstable, uncontrolled fashion without further increase of load (unstable

post-buckling behaviour, brittle type of failure), or grow to large values with little or no increase of load (neutral post-buckling behaviour, plastic type of failure) or develop gradually in a stable manner as the load is increased further

(stable post-buckling behaviour, ductile type of failure). — Limit point buckling: As the applied load is increased the structure becomes less stiff until the relationship between load and

deflection reaches a maximum (elastic critical load) at which the deformations increase in an uncontrolled way (brittle typeof failure).

102

Determination of the elastic critical load of a structure or member that experiences bifurcation buckling corresponds tothe solution of an eigenvalue problem in which the elastic buckling load is an eigenvalue and the corresponding mode of

buckling deformation is described by the corresponding eigenvector.

103 Elastic buckling may occur at different levels:

— Global level for the structure. This involves deformation of the structure as a whole. — Global level for a structural member. This is confined mainly to one structural member or element but involves the whole of

that member or element. — Local level for a structural member. Only a part of a structural member or element is involved (e.g. local buckling of the

flange of an I-beam or of a plate zone between stiffeners in a stiffened plate).

104 Resistance of a structural member to elastic buckling is normally expressed as a critical value of load (applied force, orstress resultant induced in a member) or as a critical value of a nominal average stress (e.g. axial or shear force divided by area ofcross-section). However, such resistance may also be expressed as a critical value of mean strain induced at a cross-section in amember.

105

Initial geometrical imperfections (out-of-straightness, out-of-roundness, or eccentricity of applied loading) that lead to asituation where compressive forces in a structural part are not coincident with the neutral axis of that part may influencesignificantly the buckling behaviour. An idealised structure without such imperfections is referred to as “geometrically perfect”.


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106 Bifurcation buckling is essentially a feature of geometrically perfect structures. Geometrical imperfections generallydestroy the bifurcation and lead to a situation where bending deformations begin to grow as the load is increased from zero. Anelastic critical load may still be associated with the structure, and may provide a good indication of the load level at which thedeformations become large. However, some structures with unstable post-buckling behaviour are highly sensitive to geometricimperfections. In the presence of imperfections, such structures may experience limit point buckling at loads that aresignificantly lower than the elastic critical load of the geometrically perfect structure.

107

Elastic buckling deformation of a geometrically perfect or imperfect structure may trigger other failure mechanisms suchas fibre failure (compressive or tensile) or matrix cracking.

108

The presence of failure mechanisms such as matrix cracking or delamination may influence significantly the buckling behaviour of structures and structural members.

E 200

General

201

Load carrying components shall not buckle when subjected to the design load. For all other components, elastic bucklingunder the design load is acceptable. Buckling shall not occur under the characteristic value of the maximum operational load.

202

Simple analytical formulas for buckling of flat and curved orthotropic panels can normally be used to identify if buckling

will have an influence on the strain distribution in the blade or if the strain distribution at extreme loads shall be analysed203 Buckling of the wind turbine blade shall be considered as a possible failure mechanism. Global and local buckling shall be checked.

204 The interaction of buckling with other failure mechanisms, such as delamination and matrix cracking, shall be consideredcarefully.

205 Buckling may in extreme cases lead to violation of displacement requirements.

E 300 Calculation of buckling

301

The methods described in DNV-OS-C501 Section 9 for buckling analysis shall be followed.

302

The need for special buckling analysis shall be assessed carefully in every case. In particular the following aspects shall be considered in making this assessment:

— Presence of axial compressive stresses — Presence of shear stresses

303

All parts of the blade, like shell, rips, joints and fittings should be evaluated for buckling. Buckling calculations shall becarried out at least for panels and the webs in the blade.

304

Buckling shall be evaluated as described in Section DNV-OS-C501 Section F taking due account of geometricimperfections.

305

Assumptions regarding geometrical imperfections shall wherever possible be based on

— knowledge of production methods and corresponding production tolerances — knowledge of how imperfections of given shape and magnitude influence the structural behaviour, and — experience from previous measurements and tests

If an adequate knowledge base does not exist, a programme of measurement and/or testing shall be agreed to demonstrate thatthe design assumptions are justified.

306 If analytical formulae are used for estimating critical buckling loads, due account shall be taken of the anisotropic properties of the wind turbine blade.

307 Two alternative approaches may be used in analysing buckling problems:

— Analysis of isolated components of standard type, such as tubular sections, beams, plates and shells of simple shape. — Analysis of an entire structure (or of an entire, complex structural component)

308

Buckling analysis may be carried out by analytical or numerical methods. Such analyses may be applied to eithergeometrically perfect or imperfect structures. Analytical methods are mainly confined to geometrically perfect structures, exceptfor some simple structural members with imperfections of simple shape.


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309 When performing a buckling analysis the boundary conditions shall be evaluated carefully.

310

If in any buckling analysis the applied load is higher than the load that would introduce partial damage in the structure,e.g. matrix cracking or delaminations, the buckling calculations shall take this partial damage into account.

311

For structures or structural elements that are expected to exhibit bifurcation buckling, an analysis to determine the elastic

critical load (or critical stress or strain) shall normally be carried out before more complex non-linear analysis is performed. The purpose of this is to establish:

— whether it may be acceptable to perform only geometrically linear analysis, — whether the structure is clearly under-dimensioned against buckling (if the applied load clearly exceeds, or is close to, the

elastic critical load);

— in the case of finite element analysis, the required fineness of mesh for a more complex buckling analysis

312

Analytical formulae for elastic buckling shall be checked carefully if they contain empirical safety factors or not and ifthey are based on structures with or without imperfections. Without this knowledge analytical formulas should not be used.

313

Except in the case of analysis for elastic critical loads (or elastic critical stresses or strains), the analysis model shallincorporate the least favourable geometrical imperfections that are possible within the specified production tolerances.Alternatively, a series of analyses may be performed incorporating a representative range of geometrical imperfections that mayarise in the intended production process; these may then be combined with statistical information about the imperfections thatarise in practical production, in terms of their distributions of shape and amplitude.

314

If imperfections are not directly included in the buckling analysis as described in, their effects shall be evaluated by othermeans such as supplementary analysis or testing. However, if it is demonstrated that the eccentricities or local bending momentsinduced by the least favourable geometrical imperfections are less than 10% of the corresponding quantities resulting from other

features inherent in the structure or its loading, such as out-of-plane loads, the geometrical imperfections may be neglected.

315 In assessing buckling-induced failure, the design criteria shall normally be applied at the level of local stress or strainstate, considered at all points in the structure. The criteria related to fibre failure, matrix cracking, delamination, yield andultimate failure shall be applied as appropriate. For sandwich structures the special criteria shall be applied in addition.

Additionally the displacement criterion shall be applied both globally and locally to ensure that there are no excessive buckling

displacements.316 To obtain the resistance quantities required for the checks, geometrically non-linear analysis shall be performed.Reduction of mechanical properties due to local failure such as matrix cracking or delamination shall be taken into account.

317 The calculated resistance is to be considered from a probabilistic point of view.

318

Variability is introduced by:

— Uncertainties in the stiffness parameters that are used in the buckling calculations — Uncertainties in geometric parameters — Uncertainties in size of imperfections and how imperfections are considered

319 The uncertainty introduced by geometrical imperfections shall be evaluated when determining the characteristic strength.

E 400

Buckling analysis of isolated components

401 When a member or component that is a part of a larger structure is analysed separately a global analysis of the structureshall be first applied to establish

— the effective loading applied to the member/component by the adjoining structural parts; — the boundary conditions for the structural member, in terms of translational and rotational stiffness components in all

relevant directions.

402 For simple members or components standard formulae or tables may be used to estimate elastic critical loads ( P e), criticalstresses (σ e) or critical strains (εe), and the corresponding elastic buckling mode shapes. Alternatively these quantities may becalculated using analytical or numerical methods. It shall always be checked that the buckling mode shape is consistent with the boundary conditions.

403

An assessment shall be made of the shape and size of initial, geometrical imperfections that may influence the buckling behaviour of the member. Normally the most critical imperfection shape for a given buckling mode has a similar form to the


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buckling mode itself. However, any geometrical feature (including eccentricity of loading) that results in compressive forces thatare not coincident with the neutral axis of the member may require consideration. The assumed form and amplitude of the

imperfection shall be decided on the basis of the production process used with due consideration of the relevant productiontolerances. Refer to DNV-OS-C501 Section 6H .

Guidance note:In some cases a geometrically non-linear analysis may be avoided as follows. The elastic critical load (without imperfections) P e is calculated. In addition anultimate failure load P f is estimated at which the entire cross-section would fail by compressive fibre failure, in the absence of bending stresses at the section in

question. If P e > P f the further assessment may be based on geometrically linear analysis provided geometrical imperfections are included and the partial loadeffect modelling factor is increased by multiplying it by the factor

e f P 4 P 1

1

−

In cases where it is possible to establish the bending responses (stresses, strains or displacements) associated with an in-plane loading separately from the

in-plane (axial) responses, a first estimate of the influence of geometrical non-linearity combined with the imperfection may be obtained by multiplying therelevant bending response parameter obtained from a geometrically linear analysis by a factor

eee 1

1or

1

1

P P 1

1

ε ε σ σ −−−,

and combining the modified bending responses with the (unmodified) in-plane responses.

The above procedures may be non-conservative for some cases where the post-buckling behaviour is unstable or where delamination and/or matrix failure are

critical. Examples include cylindrical shells and cylindrical panels under axial loading and panels with delamination defects. Such cases shall be subject to

special analysis and/or tests.


E 500 Buckling analysis of more complex elements or entire structures

501

Buckling analysis of more complex elements or entire structures shall be carried out with the aid of verified finite elementsoftware or equivalent.

502

Initially an eigenvalue buckling analysis shall be performed assuming initial (non-degraded) elastic properties for thelaminates. This shall be repeated with alternative, finer meshes, until the lowest eigenvalues and corresponding eigenmodes are

not significantly affected by further refinement. The main purposes of this analysis are to clarify the relevant buckling modeshapes and to establish the required mesh density for subsequent analysis.

503

Careful attention shall be paid to correct modelling of boundary conditions.

504 If the applied load exceeds, or is close to, the calculated elastic critical load, the design should be modified to improve the buckling strength before proceeding further.

505 A step-by-step analysis shall be carried out. Geometrical non-linearity shall be included in the model. The failure criteriashall be checked at each step. If failure such as matrix cracking or delamination is predicted, any analysis for higher loads shall be performed with properties reduced as described in DNV-OS-C501 Section 4 I .

506

Alternatively to the requirement in 505 a geometrically non-linear analysis may be performed using entirely degraded properties throughout the structure. This will normally provide conservative estimates of stresses and deformations. Providedreinforcing fibres are present in sufficient directions, so that the largest range of un-reinforced directions does not exceed 60º,

such an estimate will not normally be excessively conservative.

507

The influence of geometric imperfections should be assessed, on the basis of the production method and productiontolerances.

F. Tower clearance101 The deflection of the blade shall be kept to a certain maximum to avoid contact with the tower or other components.

102

The partial safety factors to be applied in design criteria for tower clearance are specified in IEC 61400-1

103 The partial safety factor for material in deflection criteria (mainly tip to tower distance) shall be multiplied by 1.1 whenmean values for stiffness is used in the analysis. It is accepted that this factor is reduced to 1.0 if:

— The deflection analysis is carefully calibrated with the full scale static testing of the blade — A quality control instruction covers retesting of the flapwise stiffness for manufactured blades on a spot check basis


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G. Fibre failure

G 100 General

101

Fibre failure is defined here as the failure of a ply by fracture of fibres. The fibre strength or strain to failure is based ontest results from plies or laminates. Ply failures are measured as rupture of the ply in fibre direction.

102

The maximum strain criterion should be used to check fibre failures.

103 Other design criteria may be used if it can be shown that they are equal or conservative compared to the maximum straincriterion given here.

104 Fibre failure should be checked at the ply level, not at the laminate level.

105 If laminates have a layup with fibre orientation seen through the entire thickness that are more than 45o apart, matrixcracking or deformation due to in plane ply shear stresses may cause rupture of the laminate. In this case matrix cracking due to

ply shear should also be checked to avoid fracture, unless it can be shown that matrix cracks or deformations can be tolerated bythe laminate under the relevant loading conditions.

Guidance note:A pipe made of +55 laminate with a liner can tolerate matrix cracks and shear deformations, as long as the pipe sees only internal pressure. If the pipe must carry

axial loads or bending moments in addition to the pressure, fibres would want to reorient themselves to a different angle, a complicated condition. This is only

avoided as long as the shear properties of the pipe are intact.A pipe made of a 0/90 laminate can tolerate matrix cracks and shear deformations under internal pressure and axial loads. This pipe would have problems with

axial torsion, since the stresses due to torsion have to be carried by the matrix.


106

Regardless of the analysis method used, these laminates should always be analysed with non-degraded in-plane shearmoduli G12.

107 If laminates have a layup with fibre orientation seen through the entire thickness that are more than 70o apart, matrix

cracking or deformation due to in plane ply shear stresses or stresses transverse to the fibres may cause rupture of the laminate. Inthis case matrix cracking due to all possible stress components should also be checked to avoid fracture (see also J100 to J300),unless it can be shown that matrix cracks or deformations can be tolerated in by the laminate under the relevant loadingconditions.

Guidance note:

This condition is typical for UD laminates where all fibres run parallel in one direction throughout the thickness of the laminate. Great care should be taken whenusing such laminates due to their low properties in all other directions than the fibre direction.


108

Regardless of the analysis method used, these laminates should always be analysed with non-degraded matrix dominated

elastic constants, i.e., E2 , G12 , ν12 .

G 200

Fibre failure at the ply level201

For single loads, the maximum strain design criterion is given as:

fiber k nk

∧

< ε ε

where:

εnk Value of the local response of the structure (strain) in the fibre direction n vor the design loadε ̂ k

fiber Design value of the axial strain to fibre failure

202 For each fibre direction of the laminate the partial analysis factor shall be given by

γΑ = Elin/Enonlin,


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where Elin and Enonlin are laminate moduli (stiffnesses) related to loading in the fibre direction of consideration. Elin is thelaminate stiffness based on initial (non-degraded) ply properties, while Enonlin is the reduced laminate stiffness obtainedfrom degraded ply properties due to matrix cracking. A further explanation is provided by figure 1 and the Guidance note below

Guidance note:

The introduction of partial analysis factors, γA, above may be thought of as a reduction of the effective strain to failure fromε ̂ toε ̂ corr (mean values). Figure 1 shows a typical laminate stress-strain curve for a laminate containing 0, 45 and 90 layers when loaded in the 0 direction.

A partial analysis factor shall be calculated for each fibre direction of the laminate, which in this example corresponds to obtaining laminate stress-strain

relations for loading in the 0, 45 and 90 degrees directions for the laminate in figure 1.


Matrix cracking

in 45 layers

Elin

ε

Enonlin

ε ̂εcorr

Fibre failure

in 0 layer.

Laminate

failure

Matrix cracking

in 90 layers

σ

σ ̂

Figure 2. Typical stress-strain relation for a laminate containing 0, 45 and 90 layers

203 The maximum strain criterion shall be checked in all n directions parallel to the fibres, and for tensile and compressivestrains.

204

ε ̂ k fiber is the time dependent characteristic strength of the ply in fibre direction. One value for one fibre and weave type

shall be used.

G 300

Fibre failure check using a modified Tsai-Wu criterion

301

In many cases the maximum fibre strain criterion is not available in commercial software packages. As an alternative the

Tsai-Wu criterion may be used with modified input parameters as described here.

302

The Tsai-Wu criterion is described in 3-D as:

( )( )( ) 1

222

332211

322331132112

2

2323

2

1313

2

1212

2

333

2

222

2

111


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ct

F

11

11

1∧∧=σ σ

,

ct

F

22

22

1∧∧=σ σ

,

ct

F

33

33

1∧∧=σ σ

212

12

1∧

=σ

F ,2

13

13

1∧

=σ

F ,2

23

23

1∧

=σ

F

ct

F

11

1

11∧∧ −=σ σ

,

ct

F

22

2

11∧∧ −=σ σ

,

ct

F

33

3

11∧∧ −=σ σ

2211

12*

12

F F

H H = ,

3311

13*

13

F F

H H = ,

3322

23*

23

F F

H H =

Where:n The co-ordinate system is the ply co-ordinate system, where n refers to the directions 1, 2, 3, 12, 13 and 23

σn Value of the local load effect of the structure (stress) in the direction n for the design load

nt

∧

σ Design tensile strength in the direction n

nc

∧

σ Design compressive strength in the direction n

nk

∧

σ Design shear strength in the direction nk

303 The interaction parameters*

12 H ,*

13 H ,*

23 H should be determined experimentally for each material.

304 Since Tsai-Wu criterion is here only used to check for fracture of the laminate and small matrix cracks are acceptable,strength properties should be taken as described below. Characteristic strengths as described in Section 4 B 400 should always beused.

t 1

∧

σ tensile ply strength in fibre direction.

c1

∧

σ compressive ply strength in fibre direction.

t t

E

E 1

1

22

∧∧

= σ σ modified inplane tensile ply strength transverse to the fibres.

cc

E

E 1

1

22

∧∧

= σ σ modified inplane compressive ply strength transverse to the fibres.

t 3

∧

σ tensile through thickness ply strength in fibre direction.

c3

∧

σ compressive through thickness ply strength in fibre direction

12

∧

σ inplane shear strength.

13

∧

σ through thickness shear strength.


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DET NORSKE VERITAS

23

∧

σ through thickness shear strength.

305

If tensile and compressive fibre strength differ by more than 60% it should be checked that the individual design criteria,i.e. fibre failure in 200 and matrix cracking in J200 or J300, do not give lower allowable stresses than this criterion.

G 400 Special considerations for fibre failure under inplane compressive loads

401

The orientation of matrix cracks shall be checked if the compressive strength of a laminate is important (Section J400).

402 If matrix cracks with an orientation of 30o-60o relative to the plane of the laminate may be present, the compressive strainto fibre failure used in the design criteria of this section shall be obtained from measurements on laminates with the presence ofmatrix cracks with an orientation between 30o and 60o. Alternatively, the compressive strain to failure may be reduced by 50%,or a component test shall be carried out.

G 500 Fracture mechanics approach

501 The fibre design criteria described above can always be used. However, in the presence of stress concentrations that reach

infinity a fracture mechanics approach may be applied.

502

Stress concentration can be caused by the following factors:

— cut-outs, — discontinuous linear and smooth geometry (including rough edges), —