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ECN publication
Probability distribution of fatigue strength of rotor blades (profar) : publishable final report; [EU joule III project JOR3-CT98-0266]
Braam, H.; Heijdra, J.J.; Delft, D.R.V. van; Leeuwen, H. van; Jorgensen, E.; Lekou, D.; Vionis, P.S.
Published by: Publication date:
ECN Wind Energy 1-11-2001
ECN report number: Document type:
ECN-C--01-116 ECN publication
Number of pages: Full text:
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In the period June 1997 ? July 2001 the EU funded project ?Probability Distribution of Fatigue Strength of Rotor Blades ? PROFAR? was carried out. The main objective of this project is to get a deeper understanding of the blade to blade variation of the fatigue strength of rotor blades and to determine the statistical distribution function by which this variation can be described.Furthermore the project aimed at the determination and quantification of the correlation between the measured fatigue strength of rotor blades and coupon data. To achieve the project objectives 37 small-scale blades have been tested both static and in fatigue. Next to these blades, a number of coupons of identical material as that of the blades has been tested statically and under fatigue loading. The tests served two main purposes, finding the scatter in the blade properties and relating the coupon test data to actual failure of real blades. In order to serve these purposes the two different sets of test were done. One set aimed at failure in the prismatic outboard section of the blade in the parent glass fibre material, while the other aimed at failure in the geometrically complicated root section of the blade where failure of the bonding line, collapse of the spar or other mechanisms can be observed. These latter tests were done both in edgewise and in flapwise direction. The fatigue tests have been carried out at different stress levels. Besides fatigue, other failure modes like ultimate load are incorporated in the test sequence. The tests have been performed at three laboratories, the Stevin laboratory of the Technical University of Delft, The Netherlands; the test facilities of the Centre for Renewable Energy Sources, CRES in Pikermi, Greece; and a test facility at Risø National Laboratory, Roskilde, Denmark. The results of the blade tests have been correlated with coupon data for similar material obtained from coupon tests performed within this project and with corresponding material obtained from the existing data base FACT. Based on this correlation, test load factors were determined. The results of the tests show that:
? Although a serious attempt has been made to harmonise the tests within the different laboratories, the available equipment, the in house procedures and culture makes inevitable that differences in the tests occur between the different institutions.
? Ignoring the slight "laboratory influence" scatter is most pronounced for tests focussed on the prismatic (tip) section.
? The results of the coupon tests are in good agreement with the FACT results when divided by the UTS value of the relevant material.
? For the blade tests and for the coupon tests the slope of the S-N curves is close to 10, for R = 0.1 as well as for R = -1.
? The scatter in the results of the blade tests is of the same order of magnitude as the scatter found in the coupon test results.
? It is shown that the Goodman based curves are close to the data obtained from the tests and therefore are to be used for design of wind turbine blades.
? Elementary statistical analysis of the blade test results gives way to the assumption that a partial safety factor on the tests load to account for the scatter in blade properties seems to be appropriate.

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