- Title
- The development and testing of a new fatigue life procedure for small composite wind turbine blades incorporating new empirical fatigue life prediction and damage accumulation models for glass fibre reinforced plastics
- Creator
- Epaarachchi, Jayantha Ananda
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2002
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- The work in this thesis is concerned with developing a suitable fatigue life evaluation procedure for the blades of small wind turbines. Detailed strain gauge measurements from the blade of an operating 2.5m long composite-constructed blade were used to establish a relationship between upstream wind speed and blade response. A blade fatigue test procedure has been developed using historical wind data and the wind speed/blade response relationship; a detailed description of the methodology and procedure used is documented in this thesis. Existing empirical models to determine the fatigue life of glass-fibre reinforced composites were found to be inadequate for composite-constructed small wind turbine blades as they do not adequately address the effects of load frequency and stress ratio, and cannot be easily extended to address cumulative fatigue life calculations. A model has been developed which overcame these limitations, an is detailed in this thesis. A fatigue test rig has been designed and built to test blades 2.5m in length and has been used successfully to evaluate the fatigue life of a composite blade using the developed fatigue test procedure. The Wind Energy Group at the University of Newcastle had a 5kW prototype wind turbine at Fort Scratchley, Newcastle. Here one of the turbine's composite blades was instrumented with strain gauges with signals acquired simultaneously with those representing wind speed and direction, turbine direction and turbine generator power. Results show that the blade does not respond instantaneously to all changed in wind speed but follows the wind profile. A detailed finite element model of this blade was solved for load due to aerodynamic pressures and blade rotation. Predictions from the model were found to be in good agreement with the measurements. Comprehensive meteorological data were obtained through the Australian Bureau of Meteorology for 18 sites. Here the maximum and mean wind speeds were presented for a 30 minuted time increment with data acquired and averaged over a 10 minute time period in each increment with data acquired and averaged over a 10 minuted time period in each increment. This wind data was found to fit to a Weibull distribution. The data were also rainflow counted to isolate blade fatigue cycles. The detailed wind data acquired at 0.5Hz rate from the turbine test site at Fort Scratchley were also rainflow counted to determine the blade fatigue cycles in 10 minute sampling periods. From these data, a relationship between the average and maximum wind velocity and the number of wind cycles was determined. A method has been developed, and is detailed in this thesis, to determine all likely blade loading cycles using the Bureau's data and the Fort Scratchley data. The worst-case yearly fatigue loading spectrum was determined to have a total of 1803705 cycles representing stress levels for wind velocities between 1 m/s to 20 m/s and binned under stress ratios between 0 and 0.9 inclusive. This loading cycle was used to test the 2.5m composite. An empirical model for the fatigue life of glass-fibre reinforced plastic composites under various stress ratios and loading frequencies has been proposed. The model has been tested with fatigue test data from the literature as well as from various research laboratories. The model shows close agreement to all test data. This model was extended to predict cumulative fatigue damage and to overcome limitations of traditional cumulative fatigue rules such as Miner's rule. Predictions from this cumulative fatigue model were in good agreement with cumulative fatigue data obtained from the literature and the results of experiments performed by the author. The fatigue properties for the composite materials used in the wind turbine blade were determined from standard fatigue tests. Using a minimal about of experimental fatigue data, the proposed fatigue life of the blade for any loading spectrum. This cumulative fatigue model was found to perform better than traditional cumulative fatigue models. A mechanically operated test rig where the load on the blade was applied through a crank-level mechanism has been designed and built to test the 2.5m long wind turbine blades using the developed load spectrum. This test rig can apply blade loading at a maximum frequency of around 3Hz. Fatigue testing of the 2.5m composite blade shows that the material's properties continuously degradation with time. Predictions of blade fatigue life using material properties based on uni-axial fatigue data were found to be significantly lower than those based on flexural fatigue data. This disparity was found to be due to the difference between the stress state of the blade's critical section under full scale fatigue test conditions and the stress state under standard fatigue test conditions. Furthermore it was analytically shown that the blade's life span predictions were unaffected by the loading sequence.
- Subject
- glass fibre; reinforced plastics; wind turbine blades; blade fatigue; blade fatigue test
- Identifier
- http://hdl.handle.net/1959.13/1312474
- Identifier
- uon:22404
- Rights
- Copyright 2002 Jayantha Ananda Epaarachchi
- Language
- eng
- Hits: 1063
- Visitors: 1052
- Downloads: 0