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Probability distribution of fatigue strength of rotor blades

Objective



OBJECTIVES OF THE PROJECT

The fatigue strength of rotor blades show some variation, among others due to the inherent scatter of the material properties. To take into account this uncertainty a safety factor for fatigue strength is prescribed in design codes. Likewise an additional test load factor has to be applied in full scale blade testing. To determine these factors in a rational manner the fatigue strength distribution of rotor blades is a prerequisite. As this distribution is not available at the moment the corresponding factors are determined empirically and a lot of uncertainty concerning the values used nowadays is present.

For this reason the main objectives of this project are:
- acquisition of a deeper understanding of the blade to blade variation and determination of the statistical distribution functions of the fatigue strength of rotor blades;
- determination of the correlation between the measured fatigue strength of rotor blades and coupon data.

Besides, the following additional objectives are defined:

- formulation of a more generic methodology to determine the fatigue strength distribution of rotor blades based on the above mentioned correlation; - definition of the test load factors to be applied in a full scale blade fatigue test as function of the number of tests to be performed; - transfer of specific knowledge on the application of statistical techniques to characterize the fatigue strength of blades to certification bodies.
TECHNICAL APPROACH

To achieve the project objectives the blade to blade variation in fatigue strength will be quantified by testing a large number of test specimens with a realistic structural design. This will either be commercial available small scale blades eventual modified for this project or test specimens special developed for this project. Fatigue tests will be carried out at different stress levels. Besides fatigue other failure modes like ultimate load might be incorporated in the test sequence.
The results of these tests will be correlated with coupon data for corresponding material. The differences in stress (re)distribution between the test specimen and the coupon due to the realistic design of the test specimen will be part of this correlation. Based on this correlation a procedure for the determination of the strength distribution of full scale blades will be set up. At the same time the correlation will be used for the determination of the test load factors.

EXPECTED ACHIEVEMENTS

- The availability of the fatigue strength distribution of rotor blades makes it possible to optimize the design economically more easily. - The use of safety factors deviating from the ones prescribed in codes has to be negotiated with the certification bodies. These negotiations can go much easier by using the results obtained in this project instead of using an ad hoc approach. This means that the time to introduce a new design on the market can be shortened.
- Recommended values for the test load factor for a certain design (combination of materials and structure) will be available and can be used for the formulation of a recommendation in this field.
- With the availability of the statistical distribution functions for the fatigue strength of rotor blades, the possibility arises to include fatigue more substantially in the design phase by applying structural reliability techniques. Optimization for fatigue can be included in the probabilistic design process.

Coordinator

ENERGY RESEARCH CENTRE OF THE NETHERLANDS
Address
3,Westerduinweg 3
1755 LE Petten
Netherlands

Participants (6)

CENTRE FOR RENEWABLE ENERGY SOURCES
Greece
Address
Marathonos Avenue, Km. 19
19009 Raphina (Pikeermi)
Det Norske Veritas Danmark A/S
Denmark
Address
16,Nyhavn
1051 København
GERMANISCHER LLOYD WINDENERGIE GMBH
Germany
Address
Johannisbollwerk 6-8
Hamburg
Risø National Laboratory
Denmark
Address

4000 Roskilde
Stichting CIWI Holland
Netherlands
Address
30,Utrechtseweg 30
6812 AR Arnhem
Technische Universiteit Delft
Netherlands
Address
1,Stevinweg 1
2600 GA Delft