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Fatigue Life of GFRP Tidal Turbine Blades
C. R. Kennedy, C. M. Ó Brádaigh, S. B. Leen
Mechanical and Biomedical Engineering, National University of Ireland, Galway
c.kennedy8@nuigalway.ie
Abstract
A fatigue life prediction methodology for ocean
energy structures is presented. Starting with a tidal
velocity model, the maximum strains in a tidal turbine
blade are predicted. This is then compared to fatigue
test results via a fatigue model, to give a blade life
prediction. The effect of R ratio and matrix material is
investigated and both are found to have significant
effects on the predicted fatigue life.
Introduction
Glass-fibre reinforced polymers (GFRP) are
candidate low cost materials for use in ocean energy
structures. Quasi-isotropic (QI) laminates are useful
where (i) the loads are not very well understood, or (ii)
the loads are complex and multi-directional in nature.
The fatigue of QI laminates is investigated as part of
research investigating the fatigue behaviour of GFRP
laminates while immersed in seawater.
Methodology
The flowchart shown in Figure 1 depicts the
methodology proposed here to estimate the fatigue life
of a tidal turbine blade.
Fig 1. Flowchart of fatigue life methodology
The tidal velocity is approximated by a combination
of two sinusoids one that accounts for the twice daily
tides and a second that models the 14 day spring-neap
cycle[1].
A hydrodynamic model (stream tube momentum)
then converts that tidal velocity into loads on the tidal
turbine blade and recommends chord lengths and angles
of twist for the blade. A finite element model (e.g. Fig.
172
2) of a 5m blade is constructed in the ABAQUS
software program using these design recommendations.
Fig. 2 Deflected shape of blade at maximum load
Following practice in the wind turbine blade
industry[2] a box section spar with thick laminates is
used as the main structure of the blade with lighter
laminates used for the nose and tail fairings. Loads
corresponding to the maximum expected tidal velocity
are applied to the FE model of the blade and
representative strains are extracted.
A series of fatigue tests has established a Strain/Life
curve for both Vinyl Ester/E-Glass and Epoxy/E-glass
quasi-isotropic laminates. This information is used in
the fatigue model to estimate the life of the turbine
blade under the assumed load regime. The 7 day
repeating pattern in the tides is relatively short and
allows explicit modelling of each rotation of the tidal
turbine as a fatigue cycle. Tower "shadow" is assumed
to cause unloading of the blade on each cycle. Both the
case where it becomes completely unloaded (R = 0.1)
and where it is 50% unloaded (R = 0.5) are considered.
Results
The combination of relatively flat -N curve for the
material and the cyclic load spectrum modelled here
leads to a high degree of sensitivity of predicted fatigue
life to predicted strain level. Just 13% (epoxy) or 16%
(vinyl ester) increase in strain levels will decrease life
from 20 to 5 years. A detrimental effect of decreasing
R-ratio is established from the fatigue test programme
and associated combined structural-hydrodynamic, tidal
turbine analyses. Furthermore, the fatigue „strength‟ of
the vinyl ester matrix laminates is shown to be 25%
lower than that of the epoxy laminates.
Future work
Testing to establish the effect of long term water
immersion on the fatigue life of these materials is
ongoing. The methodology presented here will be used
to assess the impact these effects will have on the
fatigue life of tidal turbine blades.
References
[1] F. O Rourke, F. Boyle, and A. Reynolds, “Tidal energy
update 2009,” Applied Energy, vol. 87, no. 2, pp. 398-
409, Feb. 2010.
[2] C. W. Kensche, “Fatigue of composites for wind
turbines,” International Journal of Fatigue, vol. 28, no.
10, pp. 1363-1374, Oct. 2006.