EN IEC 61400-5:2020

SLOVENSKI STANDARD
01-oktober-2020
Sistemi za proizvodnjo energije na veter - 5. del: Rotorski listi vetrnih turbin (IEC
61400-5:2020)
Wind energy generation systems - Part 5: Wind turbine blades (IEC 61400-5:2020)
Windenergieanlagen - Teil 5: Rotorblätter von Windenergieanlagen (IEC 61400-5:2020)
Systèmes de génération d'énergie éolienne - Partie 5: Pales d’éoliennes (IEC 61400-
5:2020)
Ta slovenski standard je istoveten z: EN IEC 61400-5:2020
ICS:
27.180 Vetrne elektrarne Wind turbine energy systems
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN STANDARD EN IEC 61400-5

NORME EUROPÉENNE
EUROPÄISCHE NORM
August 2020
ICS 27.180
English Version
Wind energy generation systems - Part 5: Wind turbine blades
(IEC 61400-5:2020)
Systèmes de génération d'énergie éolienne - Partie 5: Windenergieanlagen - Teil 5: Rotorblätter von
Pales d'éoliennes Windenergieanlagen
(IEC 61400-5:2020) (IEC 61400-5:2020)
This European Standard was approved by CENELEC on 2020-07-21. CENELEC members are bound to comply with the CEN/CENELEC
Internal Regulations which stipulate the conditions for giving this European Standard the status of a national standard without any alteration.
Up-to-date lists and bibliographical references concerning such national standards may be obtained on application to the CEN-CENELEC
Management Centre or to any CENELEC member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by translation
under the responsibility of a CENELEC member into its own language and notified to the CEN-CENELEC Management Centre has the
same status as the official versions.
CENELEC members are the national electrotechnical committees of Austria, Belgium, Bulgaria, Croatia, Cyprus, the Czech Republic,
Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, the
Netherlands, Norway, Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland,
Turkey and the United Kingdom.

European Committee for Electrotechnical Standardization
Comité Européen de Normalisation Electrotechnique
Europäisches Komitee für Elektrotechnische Normung
CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2020 CENELEC All rights of exploitation in any form and by any means reserved worldwide for CENELEC Members.
Ref. No. EN IEC 61400-5:2020 E

European foreword
The text of document 88/759/FDIS, future edition 1 of IEC 61400-5, prepared by IEC/TC 88 "Wind
energy generation systems" was submitted to the IEC-CENELEC parallel vote and approved by
CENELEC as EN IEC 61400-5:2020.
The following dates are fixed:
• latest date by which the document has to be implemented at national (dop) 2021-04-21
level by publication of an identical national standard or by endorsement
• latest date by which the national standards conflicting with the (dow) 2023-07-21
document have to be withdrawn
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CENELEC shall not be held responsible for identifying any or all such patent rights.

Endorsement notice
The text of the International Standard IEC 61400-5:2020 was approved by CENELEC as a European
Standard without any modification.

Annex ZA
(normative)
Normative references to international publications
with their corresponding European publications
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments)
applies.
NOTE 1  Where an International Publication has been modified by common modifications, indicated by (mod), the relevant
EN/HD applies.
NOTE 2  Up-to-date information on the latest versions of the European Standards listed in this annex is available here:
www.cenelec.eu.
Publication Year Title EN/HD Year
IEC 60050-415 - International Electrotechnical Vocabulary - - -
Part 415: Wind turbine generator systems
IEC 61400-1 - Wind energy generation systems - Part 1: EN IEC 61400-1 -
Design requirements
IEC 61400-2 - Wind turbines - Part 2: Small wind turbines EN 61400-2 -
IEC 61400-3-1 - Wind energy generation systems - Part 3- EN IEC 61400-3-1 -
1: Design requirements for fixed offshore
wind turbines
IEC 61400-3-2 - Wind energy generation systems - Part 3- - -
2: Design requirements for floating offshore
wind turbines
IEC 61400-23 - Wind turbines - Part 23: Full-scale EN 61400-23 -
structural testing of rotor blades
IEC 61400-24 - Wind energy generation systems - Part 24: EN IEC 61400-24 -
Lightning protection
ISO/IEC 17021-1 - Conformity assessment - Requirements for EN ISO/IEC 17021-1 -
bodies providing audit and certification of
management systems - Part 1:
Requirements
ISO 10474 - Steel and steel products - Inspection - -
documents
ISO 2394 - General principles on reliability for - -
structures
ISO 9000 - Quality management systems - EN ISO 9000 -
Fundamentals and vocabulary
ISO 9001 - Quality management systems - EN ISO 9001 -
Requirements
- - Metallic products - Types of inspection EN 10204 -
documents
ISO 16269-6 - Statistical interpretation of data - Part 6: - -
Determination of statistical tolerance
intervals
IEC 61400-5 ®
Edition 1.0 2020-06
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Wind energy generation systems –

Part 5: Wind turbine blades
Systèmes de génération d’énergie éolienne –

Partie 5: Pales d’éoliennes
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 27.180 ISBN 978-2-8322-8335-6

– 2 – IEC 61400-5:2020 © IEC 2020
CONTENTS
FOREWORD . 4
INTRODUCTION . 6
1 Scope . 7
2 Normative references . 7
3 Terms and definitions . 8
4 Notation . 10
4.1 Symbols . 10
4.2 Greek symbols . 11
4.3 Subscripts . 11
4.4 Coordinate systems . 11
5 Design environmental conditions . 12
6 Design . 13
6.1 Structural design process. 13
6.1.1 General requirements . 13
6.1.2 Building block approach for composite structural design . 13
6.1.3 General blade design process . 14
6.1.4 Design loads . 17
6.2 Blade characteristics . 18
6.2.1 Blade properties . 18
6.2.2 Functional design tolerances . 18
6.3 Aerodynamic design . 19
6.3.1 General . 19
6.3.2 Aerodynamic characteristics . 19
6.3.3 Power performance characterisation (informative) . 20
6.3.4 Airfoil noise (informative) . 20
6.4 Material requirements . 20
6.4.1 General . 20
6.4.2 Material properties for blade design . 20
6.4.3 Qualification of materials for manufacture . 24
6.5 Design for manufacturing . 25
6.5.1 General . 25
6.5.2 Requirement for manufacturing tolerances . 25
6.6 Structural design . 26
6.6.1 General design approach . 26
6.6.2 Structural analysis . 27
6.6.3 Verification requirements . 29
6.6.4 Partial safety factors for materials . 30
6.6.5 Structural design verification. 34
6.6.6 Additional failure modes . 47
7 Manufacturing requirements . 48
7.1 Manufacturing process . 48
7.2 Workshop requirements . 48
7.2.1 General . 48
7.2.2 Workshop facilities . 49
7.2.3 Material handling and storage facilities . 49
7.2.4 Tools and equipment . 50

IEC 61400-5:2020 © IEC 2020 – 3 –
7.2.5 Personnel . 51
7.3 Quality management system requirements . 52
7.4 Manufacturing process requirements. 52
7.4.1 General manufacturing requirements . 52
7.4.2 Gelcoat application to the mould . 52
7.4.3 Building up the laminate . 53
7.4.4 Adhesive bonding process . 54
7.4.5 Curing . 55
7.4.6 Demoulding . 55
7.4.7 Trimming, cutting, and grinding . 55
7.4.8 Surface finish . 56
7.4.9 Sealing . 56
7.4.10 Additional component assembly processes . 56
7.4.11 Mass and balance . 57
7.4.12 Manufacturing and assembly processes outside controlled environment . 57
7.5 Manufacture of natural fiber-reinforced rotor blades . 57
7.6 Other manufacturing processes . 58
7.7 Quality control process . 58
7.7.1 Manufacturing quality plan . 58
7.7.2 Incoming inspection . 58
7.7.3 Manufacturing and quality control records . 58
7.7.4 Non-conformity process . 59
7.7.5 In manufacture corrective action processes . 59
7.7.6 Final manufacturing inspection and conformity review . 60
7.7.7 Documentation . 60
7.8 Requirements for manufacturing evaluation . 61
8 Blade Installation, operation and maintenance . 62
8.1 General . 62
8.2 Transportation, handling and installation . 62
8.3 Maintenance . 63
8.3.1 General . 63
8.3.2 Scheduled inspections . 63

Figure 1 – Chordwise (flatwise, edgewise) coordinate system . 11
Figure 2 – Rotor (flapwise, lead-lag) coordinate system . 12
Figure 3 – The building block approach . 13
Figure 4 – Typical process for design and analytical evaluation of blade . 15
Figure 5 – Application of limit states design approach for blade verification . 16

Table 1 – Typical manufacturing effects . 33

– 4 – IEC 61400-5:2020 © IEC 2020
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
WIND ENERGY GENERATION SYSTEMS –

Part 5: Wind turbine blades
FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote
international co-operation on all questions concerning standardization in the electrical and electronic fields. To
this end and in addition to other activities, IEC publishes International Standards, Technical Specifications,
Technical Reports, Publicly Available Specifications (PAS) and Guides (hereafter referred to as “IEC
Publication(s)”). Their preparation is entrusted to technical committees; any IEC National Committee interested
in the subject dealt with may participate in this preparatory work. International, governmental and non-
governmental organizations liaising with the IEC also participate in this preparation. IEC collaborates closely
with the International Organization for Standardization (ISO) in accordance with conditions determined by
agreement between the two organizations.
2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
consensus of opinion on the relevant subjects since each technical committee has representation from all
interested IEC National Committees.
3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC
Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any
misinterpretation by any end user.
4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
transparently to the maximum extent possible in their national and regional publications. Any divergence
between any IEC Publication and the corresponding national or regional publication shall be clearly indicated in
the latter.
5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity
assessment services and, in some areas, access to IEC marks of conformity. IEC is not responsible for any
services carried out by independent certification bodies.
6) All users should ensure that they have the latest edition of this publication.
7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and
members of its technical committees and IEC National Committees for any personal injury, property damage or
other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and
expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC
Publications.
8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of
patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 61400-5 has been prepared by IEC technical committee 88: Wind
energy generation systems.
The text of this International Standard is based on the following documents:
FDIS Report on voting
88/759/FDIS 88/767/RVD
Full information on the voting for the approval of this International Standard can be found in
the report on voting indicated in the above table.
This document has been drafted in accordance with the ISO/IEC Directives, Part 2.
Future standards in this series will carry the new general title as cited above. Titles of existing
standards in this series will be updated at the time of the next edition.

IEC 61400-5:2020 © IEC 2020 – 5 –
A list of all parts of the IEC 61400 series, under the general title Wind energy generation
systems, can be found on the IEC website.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under "http://webstore.iec.ch" in the data related to
the specific document. At this date, the document will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
IMPORTANT – The 'colour inside' logo on the cover page of this publication indicates
that it contains colours which are considered to be useful for the correct
understanding of its contents. Users should therefore print this document using a
colour printer.
– 6 – IEC 61400-5:2020 © IEC 2020
INTRODUCTION
The blades of a wind turbine rotor are generally regarded as one of the most critical
components of the wind turbine system. In this International Standard, a minimum set of
requirements for the design and manufacturing of wind turbine blades are defined.
An approach to a structural design process for the blade is set forth in the general areas of
blade characteristics, aerodynamic design, material requirements and structural design.
Furthermore, in order to efficiently facilitate the transfer of a blade design to the production
environment, this document includes demands for designing for manufacturing.
The requirements for structural design of the wind turbine blade have been developed in a
manner to reward innovation, validation, quality and testing. Specifically, the designer will be
able claim lower partial safety factors based on, among other items, the diligence of the
validation of models and the correlation to testing results.
To ensure a production environment that can facilitate the manufacturing of a blade in
accordance with the design, the manufacturing requirements included in this document
provide a minimum basis for a quality management system and workshop requirements. In
addition, requirements for blade handling, operation and maintenance are described in the
close of this document.
IEC 61400-5:2020 © IEC 2020 – 7 –
WIND ENERGY GENERATION SYSTEMS –

Part 5: Wind turbine blades
1 Scope
This part of IEC 61400 specifies requirements to ensure the engineering integrity of wind
turbine blades as well as an appropriate level of operational safety throughout the design
lifetime. It includes requirements for:
• aerodynamic and structural design,
• material selection, evaluation and testing,
• manufacture (including associated quality management),
• transportation, installation, operation and maintenance of the blades.
The purpose of this document is to provide a technical reference for designers,
manufacturers, purchasers, operators, third party organizations and material suppliers, as
well as to define requirements for certification.
With respect to certification, this document provides the detailed basis for fulfilling the current
requirements of the IECRE system, as well as other IEC standards relevant to wind turbine
blades. When used for certification, the applicability of each portion of this document should
be determined based on the extent of certification, and associated certification modules per
the IECRE system.
The rotor blade is defined as all components integrated in the blade design, excluding
removable bolts in the blade root connection and support structures for installation.
This document is intended to be applied to rotor blades for all wind turbines. For rotor blades
used on small wind turbines according to IEC 61400-2, the requirements in that document are
applicable.
At the time this document was written, most blades were produced for horizontal axis wind
turbines. The blades were mostly made of fiber reinforced plastics. However, most principles
given in this document would be applicable to any rotor blade configuration, size and material.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their
content constitutes requirements of this document. For dated references, only the edition
cited applies. For undated references, the latest edition of the referenced document (including
any amendments) applies.
IEC 60050-415, International Electrotechnical Vocabulary (IEV) – Part 415: Wind turbine
generator systems
IEC 61400-1, Wind energy generation systems – Part 1: Design requirements
IEC 61400-2, Wind turbines – Part 2: Small wind turbines
IEC 61400-3-1, Wind energy generation systems – Part 3-1: Design requirements for fixed
offshore wind turbines
– 8 – IEC 61400-5:2020 © IEC 2020
IEC 61400-3-2, Wind energy generation systems – Part 3-2: Design requirements for floating
offshore wind turbines
IEC 61400-23, Wind turbines – Part 23: Full-scale structural testing of rotor blades
IEC 61400-24, Wind energy generation systems – Part 24: Lightning protection
ISO/IEC 17021-1, Conformity assessment – Requirements for bodies providing audit and
certification of management systems – Part 1: Requirements
ISO 10474, Steel and steel products – Inspection documents
ISO 2394, General principles on reliability for structures
ISO 9000, Quality management systems – Fundamentals and vocabulary
ISO 9001, Quality management systems – Requirements
EN 10204, Metallic products – Types of inspection documents
ISO 16269-6, Statistical interpretation of data – Part 6: Determination of statistical tolerance
intervals
3 Terms and definitions
For the purposes of this document, the terms and definitions given in
IEC 60050-415 and the following apply.
ISO and IEC maintain terminological databases for use in standardization at the following
addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
3.1
blade root
that part of the rotor blade that is connected to the hub/pitch-bearing of the rotor
3.2
blade subsystem
integrated set of items that accomplish a defined objective or function within the blade (e.g.,
lightning protection subsystem, aerodynamic braking subsystem, monitoring subsystem,
aerodynamic control subsystem, etc.)
3.3
buckling
instability characterized by a non-linear increase in out of plane deflection with a change in
local compressive load
3.4
characteristic value
value having a prescribed probability of not being attained (i.e. an exceedance probability of
less than or equal to a prescribed amount)

IEC 61400-5:2020 © IEC 2020 – 9 –
3.5
chord
length of a reference straight line that joins the leading and trailing edges of a blade aerofoil
cross-section at a given spanwise location
3.6
creep
time-dependant increase in strain under a sustained load
3.7
design limits
maximum or minimum values used in a design
3.8
design loads
loads the blade is designed to withstand, including appropriate partial safety factors
3.9
design properties
material and geometric properties (including design limits)
3.10
edgewise
direction that is parallel to the local chord
3.11
environmental conditions
characteristics of the environment (wind, altitude, temperature, humidity, etc.) which may
affect the wind turbine blade behaviour
3.12
flapwise
direction that is perpendicular to the surface swept by the undeformed rotor blade axis
3.13
flatwise
direction that is perpendicular to the local chord, and spanwise blade axis
3.14
inboard
towards the blade root
3.15
lead-lag
direction that is parallel to the plane of the swept surface and perpendicular to the longitudinal
axis of the undeformed rotor blade
3.16
limit state
state of a structure and the loads acting upon it, beyond which the structure no longer
satisfies the design requirement
3.17
load envelope
collection of maximum design loads in all directions and spanwise positions

– 10 – IEC 61400-5:2020 © IEC 2020
3.18
natural frequency
eigen frequency
frequency at which a structure will vibrate when perturbed and allowed to vibrate freely
3.19
partial safety factors
factors that are applied to loads and material strengths to account for uncertainties in the
representative (characteristic) values
3.20
prebend
blade curvature in the flapwise plane in the unloaded condition
3.21
spanwise
direction parallel to the longitudinal axis of a rotor blade
3.22
stiffness
ratio of change of force to the corresponding change in displacement of an elastic body
3.23
strain
ratio of the elongation (or shear displacement) of a material subjected to stress to the original
length of the material
3.24
sweep
blade curvature in the lead-lag plane in the unloaded condition
3.25
twist
spanwise variation in angle of the chord lines of blade cross-sections
3.26
critical to quality
CTQ
process or design value that is measurable and specifies critical acceptance criteria
4 Notation
4.1 Symbols
F load
F design value for the load
d
F characteristic value for the load
k
R resistance of material or structure against the corresponding limit state
R characteristic material resistance
k
PSF Partial Safety Factor
S() function for structural response to the load
T glass transition temperature
g
p (−) negative Puck inclination parameter
┴║
p (+) positive Puck inclination parameter
┴║
IEC 61400-5:2020 © IEC 2020 – 11 –
4.2 Greek symbols
γ Partial safety factor
4.3 Subscripts
m materials
m0 materials as a “base” material factor (to be included in all analyses)
m1 materials for environmental degradation (non-reversible effects)
m2 materials for temperature effects (reversible effects)
m3 materials for manufacturing effects
m4 materials for calculation accuracy and validation of method
m5 materials for load characterization
n consequence of failure
f factor for loads
4.4 Coordinate systems
Coordinate systems for loads and design reference are shown in Figure 1 and Figure 2.

Loads are along and perpendicular to the local blade chord directions.
Key
M edgewise bending moment
a
M flatwise bending moment
b
M torsion moment
c
F flatwise shear force
a
edgewise shear force
F
b
F axial force
c
1 torsion angle
2 flapwise translation
3 lead-lag translation
Figure 1 – Chordwise (flatwise, edgewise) coordinate system

– 12 – IEC 61400-5:2020 © IEC 2020

Loads are along the rotor plane reference directions.
Key
M lead-lag bending moment
x
M flapwise bending moment
y
M torsion moment
z
F flapwise shear force
x
F lead-lag shear force
y
F spanwise force
z
1 flapwise translation
2 lead-lag translation
Figure 2 – Rotor (flapwise, lead-lag) coordinate system
5 Design environmental conditions
Wind turbine blades are subjected to environmental conditions that may affect their loading,
durability and operation. To ensure the appropriate level of safety and reliability, the design
environmental conditions shall be taken into account and explicitly stated in the design
documentation. This shall include but is not limited to the environmental conditions specified
in IEC 61400-1, IEC 61400-3-1 or IEC 61400-3-2, and IEC 61400-24 (for lightning).
The environmental conditions are divided into normal and extreme categories. The normal
environmental conditions generally concern recurrent structural loading conditions, while the
extreme environmental conditions represent infrequent external design conditions. The design
load cases defined in IEC 61400-1, IEC 61400-3-1 or IEC 61400-3-2 include combinations of
these environmental conditions with wind turbine operational modes and other design
situations.
When additional environmental conditions not listed in the above references are specified by
the designer, the parameters and their values shall be stated in the design documentation.

IEC 61400-5:2020 © IEC 2020 – 13 –
It shall be taken into account that these environmental conditions may vary for different
phases of the product lifecycle (manufacturing, transport/storage, installation, operation or
dismantling).
6 Design
6.1 Structural design process
6.1.1 General requirements
The structural design process shall ensure that the required operation safety levels are met
for the entire design lifetime and loading of the blade.
The design shall be sufficiently described and specified to ensure that assumptions made
during the design process can be met and complied with during the manufacturing process.
The allowable manufacturing tolerances and acceptance criteria shall be defined by the
designer and specified in the design documentation.
Any of the requirements of this document may be altered if it can be suitably demonstrated
that the safety of the wind turbine system is not compromised.
6.1.2 Building block approach for composite structural design
The traditional detailed design (analytic and numerical calculation together with validated
material data and full blade testing) of FRP structures can be enhanced by a building-block
approach, starting with coupon-level tests, analysis and testing of more complicated
structures; and culminating in a full blade test. This relationship is shown in Figure 3, where
increasingly more complex tests are developed to evaluate more complicated loading
conditions and failure modes.
Figure 3 – The building block approach
The approach can be summarized as follows:
Coupons: A number of tests are conducted at the coupon level, where confidence in
repeatable physical properties is developed. Procurement specifications are developed for the
individual constituents, and allowable design variables developed for lamina/laminate
combinations.
– 14 – IEC 61400-5:2020 © IEC 2020
Elements and details: Critical areas from the design analysis identify elements for further
testing and analysis at the design conditions with representative specimens. This may include
such tests as the spar cap to web bond line or ply drops in the spar cap laminate.
Sub-components: Parts and sections representative of the blade design are tested to evaluate
specific loading conditions and failure modes. Examples include spars, shells and root
sections. The test components may be full or partial scale where demonstrated to be
representative.
Full blade: A full blade or significant part of a blade, representative of the blade design is
tested to evaluate specific loading conditions and failure modes. The blade may be full or
partial scale where demonstrated to be representative.
The number of tests required for each level should be tailored for each design activity, with
the blade designer responsible for the development of a reasonable number of tests at each
stage.
Tests on the element and detail as well as sub-component level will increase the confidence
in the structural design.
For design values (strength, stiffness, etc.) developed from test at any building block level
(material sample, sub-component, etc.), the validity of such design values shall be described
and tolerances to be met in the final design.
and limited by acceptance criteria
6.1.3 General blade design process
A typical process, provided for informative guidance only, for the design and analytical
evaluation of a blade is illustrated in Figure 4. In addition to the steps shown, the design
process can include the development of critical inputs, such as establishing aerodynamic
characteristics of airfoils, and characterization of materials properties.
The iteration loops shown are only indicative and may not represent all specific design
processes. For example, if an aerodynamic design evaluation is not found satisfactory, the
designer may re-consider the airfoils used (as shown in the figure), or iterate at another step
of the aerodynamic design process.
___________
Note on acceptance criteria (example only): for a laminate coupon sample tested for fatigue strength, the
acceptance criteria may amongst other include definition of raw materials (reference to material specifications),
fiber volume fraction, fiber alignment angles, manufacturing and curing process, etc.

IEC 61400-5:2020 © IEC 2020 – 15 –

Figure 4 – Typical process for design and analytical evaluation of blade
As noted in Figure 4, the blade structural integrity is to be evaluated for avoidance of specific
failure modes. Evaluations can be based on analysis or tests or a combination of analysis and
tests (see building block approach, Subclause 6.1.2). This is in conformity with IEC standards
(e.g., IEC 61400-1, IEC 61400-3-1, IEC 61400-3-2) which require the use of the limit states
design approach.
In the most general sense, the limit states design approach involves the characterization of
structural responses resulting from loads (e.g., stress, strain or deflection) and resistance to
those responses (e.g., strength, stiffness). Partial safety factors (PSFs), γ, are applied to
account for uncertainties in the calculated response and resistances so that the probability of
exceeding limit states is acceptably low.
Characteristic loads are those predicted to occur with a specified probability. The design
values for loads are determined by multiplying by loads partial safety factors, γ .
f
Resistance is normally a function of material properties. Characteristic resistance is
calculated from test results, where the default is 95 % exceedance with 95 % confidence level
according to ISO 16269-6. It should be stated if statistical tolerance limit factors for known or
unknown population standard deviation are used.
The resistance of the structural materials as embodied in the full blade structure may be
different than as measured at the coupon level. In some cases, this may be due to predictable
effects of scale, geometry, and load-introduction. Other effects could include variations in
material properties (e.g., composition, mechanical properties, orientation). The material partial
safety factor, γ , is intended to cover combined uncertainties in the relationship between
m
coupon-based resistance and the resistance in the as-built blade. Subclause 6.6.4 gives
detailed definition of how γ is defined.
m
According to IEC 61400-1, partial safety factors for consequences of failure, γ , shall also be
n
included. In principle, γ can be applied either as an increase in the response, or a decrease
n
in resistance as shown in Figure 5.

– 16 – IEC 61400-5:2020 © IEC 2020
In all verifications, the design value of response shall not exceed the design value of
resistance. Figure 5 shows these two values being separated by a safety margin. Verification
requires safety margin values greater than or equal to zero.
For some limit states, the relationship between material properties and resistance against
failure in the limit state is not linear (e.g., in a fracture mechanics and buckling analyses). For
such cases, the PSFs shall be applied in such way that they have a linear relation with the
load carrying capability as in the following equation:
R
k
SF()γ ×≤
fk
γγ
mn
where S is a function for the structural response to the load and R is the characteristic
k
material resistance.
This general formulation is shown in Figure 5, where partial safety factors are introduced in a
relationship between the structural response due to loads and the resistance for the
corresponding limit state.
Figure 5 – Application of limit states design approach for blade verification
Material resistance may be expressed either in stress or strain.
As discussed in 6.6, γ is derived by consideration of numerous contributions to the overall
m
resistance uncertainty. At the time this document was written, typical industry practice was
consistent with the illustration in Figure 3, with material resistance developed by coupon-level
testing to determine material properties which are then translated to blade-level using a value
of γ that covers a wide number of uncertainties. Numerous strategies are possible by which
m
uncertainties in the transfer of material-level properties to the resistance in the as-built blade
can be reduced.
For example, by using the building block approach, a combination of failure modes and limit
states are taken into account with a lower uncertainty with increased size and complexity of
the building block tested. Obtained strength and stability values will define failure modes for
material samples and sub-components, and shall be compared with analytical methods for
validation.
IEC 61400-5:2020 © IEC 2020 – 17 –
6.1.4 Design loads
6.1.4.1 Design envelope
The design loads for a blade can be specified for a single wind turbine design, or it can be
given as a wider load envelope intended to cover a range of wind turbine designs.
The defined loads/load envelope may be based on the design load cases specified in
IEC 61400-1, including non-operational situations (e.g., transportation, handling, installation,
maintenance, loading of attachment points). For offshore turbines, the requirements in
IEC 61400-3-1 and IEC 61400-3-2 should be considered.
6.1.4.2 Load interaction
The structural characteristic of the blade interacts with its aerodynamic loading and the
turbine controller. For a safe operation of the blade in rotational and stand still conditions, it
shall be assured that any instability (e.g., flutter) or resonance as a result of the interaction of
aerodynamic loading, blade structural design, turbine control (rotational speed, pitch angle,
etc.) and support structure has been considered in the load calculation.
The structural design of the blade shall reflect the defined operational, transportation and
handling load envelope, and the structural verification shall prove that the blade can withstand
the specified ultimate and fatigue design loads.
6.1.4.3 Load envelopes
Due to the asymmetry of rotor blade structures, the assumed loads in the analysis may not
reflect the most critical loading direction for any given failure mode. There are two approaches
to account for the critical loading direction.
• Basic approach: Usage of four matched sets of loads (moments and forces) corresponding
to the extremes of flatwise and edgewise loads. The angle of the resulting load vector
shall be considered. For certain verifications, this may be insufficient to cover the most
critical load directions for combined loading. This shall be considered in the design.
• Advanced approach: Consideration of all potentially critical loadi
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