Where IEC 61400-23 sits

IEC 61400-5 sets the requirements for blade design, manufacturing quality and documentation, and it requires that the design be verified by full-scale testing. IEC 61400-23 sets out how that testing is done: what is measured, how test loads relate to design loads, how the fatigue test represents the design life, and what the report has to contain. A certification body evaluates the design against 61400-5 and the test results against 61400-23 as part of issuing a blade or turbine type certificate.

The standard is therefore a procedure for validating a design, carried out once, on one or a small number of blades, at the start of a blade type's life. Everything that follows from that sentence is the point of this article.

What a full-scale test consists of

A test programme under IEC 61400-23 typically includes:

  • Property measurements. Mass, centre of gravity, natural frequencies and stiffness distribution, checked against the design values. A blade that is heavier or softer than designed is the first sign that it was not built as drawn.
  • Static tests. The blade is clamped at the root and loaded in each principal direction, flapwise and edgewise, to a test load derived from the design's extreme loads with test load factors applied. Strain is measured at many points and compared with the design's predictions.
  • Fatigue tests. The blade is cycled in flapwise and edgewise bending, for a number of cycles chosen so that the damage represents the design life, usually many millions of cycles over months. The load is higher and the cycle count lower than in service, which is what makes the test possible in a laboratory.
  • Post-fatigue static tests. Static tests repeated after fatigue, to show that the fatigued blade still carries the extreme loads.
  • Inspection and reporting. The blade is inspected during and after the programme, and the report records the loads, the measurements, any damage and any deviation from the design.

Passing means the blade survived the static loads, completed the fatigue cycles, and survived the post-fatigue static loads without failure or unacceptable damage, and that the measured behaviour matched the design closely enough.

What a passed test proves

A passed test proves that the design, as embodied in the tested blade, carries the design loads and survives a represented design life in the tested directions. That is a strong statement about the structural concept: the spar cap dimensions, the bond line design, the root joint, the ply schedule at the transitions. It is the reason blade designs that pass rarely fail from a design error.

It also gives real strain data against which the design's calculations can be checked, which is what makes the design evaluation meaningful.

What a passed test does not prove

It does not prove production quality. The tested blade is one blade, often an early one, built with more attention than a production blade will receive. A laminate wrinkle in a production spar cap, dry fibres from a poor infusion, or a starved bond line on a night shift are manufacturing defects that the tested blade did not have and the test could not see. Most serial blade failures are of this kind, in designs that passed their test.

It does not prove the blade you own was built as tested. Production processes drift, plants change, materials are substituted, and design changes are made after testing. Whether a later change was re-tested, or justified by analysis and similarity to the tested blade, is documented somewhere, and it is worth asking where.

It does not test every direction or every combination. The test loads the blade in principal directions. Service loads combine them, include torsion, and include load cases the test cannot reproduce. The design evaluation covers these by analysis; the test does not.

It does not represent the site. The test loads come from the design load cases for the turbine's design class. A site with turbulence, wind shear or wake conditions beyond that class, or a control strategy the design did not assume, loads the blade differently.

It does not cover ageing, environment or repairs. The tested blade was new, dry and unrepaired. Ultraviolet, moisture, erosion, lightning and repair laminates are outside the test.

The right way to state it: the test proves the design; the manufacturing quality system, the inspection regime and the site assessment prove everything else, and they are different documents.

Certification by similarity

Blade variants, such as a tip extension, a root change or a material substitution, are often certified by similarity to a tested blade rather than by a new full-scale test. That is legitimate where the analysis shows the change does not alter the structural behaviour in the critical regions, and it saves months and a prototype. It is also a place where the chain of evidence can thin. A buyer or insurer looking at a variant should ask which blade was physically tested, what changed between it and the blade on the turbine, and what evidence supports the claim that the change did not matter.

What an owner, insurer or buyer can ask for

In a technical due diligence or after a failure, the test documentation is one of the more useful things to request:

  • the full-scale test report, with the identity and serial number of the tested blade
  • the property measurements, compared with the design values and with the as-built values of the production blades
  • the list of design changes since the test, and for each whether it was re-tested or justified by similarity
  • the manufacturing quality documentation that connects the tested blade's process to the production blades' process
  • any non-conformance reports from the test blade's manufacture

A manufacturer that can produce these promptly has a controlled process. The gaps in what is produced are themselves evidence.

Frequently asked questions

Does a blade that passed IEC 61400-23 testing have a guaranteed design life?

No. The test shows the design carries the design loads for a represented design life. Whether a specific production blade reaches that life depends on how it was built, where it operates and how it is maintained, none of which the test covers.

How many blades are tested?

Typically one, sometimes two, for a blade type. The test is a validation of the design, not a sample inspection of production.

Can a design change be made after testing without a new test?

Yes, where the change is justified by analysis and similarity to the tested blade. The documentation of that justification is what an owner or insurer should ask for.

Does the test report tell me whether my blades have manufacturing defects?

No. It tells you the tested blade did not fail. Manufacturing defects in production blades are found by production quality control, inspection, and, after a failure, by investigation.

How Apex Wind can help

We are an independent blade engineering consultancy in Denmark. Our founder has worked on blade design, full-scale testing and certification, including certification by similarity, and Apex Wind is independent of every turbine and blade manufacturer. Our blade design and structural assessment supports development, testing and certification, and reviews the evidence behind a variant. In a transaction, our blade technical due diligence reads the test and certification record for what it proves about the blades being bought. If you are working out what a test report tells you, contact us.