Non-linear Bi-axial fatigue Testing of Wind Turbine Blades

Abstract

The wind turbine industry is required to follow quality standards and regulations for all wind turbines
components to operate safely without failing. The quality standard targets are defined by IEC 61400
(International Electricity Commissioning 61400) and are related to different aspects of wind turbines.
Regulations state that wind turbine blades must be tested, for both static and dynamic (e.g., fatigue)
loading. Wind turbine blades generally experience, through their lifetime, different loads that affect their
durability, causing damage and consequently crack. In addition, offshore wind turbine blades are more
subject to higher loads due to the turbine’s large dimensions.
Because wind turbine blades experience loads that vary in time, fatigue tests play a key role in predicting
the lifespan of wind turbine blades. Generally, fatigue tests are performed only along a single axis at a time;
these tests however might provide inaccurate results because, in reality, loads are acting simultaneously in
multiple directions. Researchers are aware of the limitation of single axis fatigue tests to predict fatigue
damage in blades. Hence there is a growing interest in improving fatigue tests from single axes to bi-axial
fatigue tests. However, running biaxial fatigue tests represents a challenge for the industry because of the
length (several months) and cost associated with these tests. Thus, the research is moving to simulate
biaxial fatigue tests using numerical methods to reduce costs and time.
Several key elements affect the choice of modelling software. The first one is represented by the
computational time, which depends on several aspects, including the complexity of the system. Secondly,
the software should give as realistic results as possible. Therefore, the choice of an appropriate tool may be
essential. Among all the software available, the MBDyn platform, an open-source Multibody Dynamics
analysis software for nonlinear simulations, allows quick results, so it is a highly suitable candidate for
biaxial fatigue simulations. In addition, some components such as actuators may be modelled, to represent
a real test. Lastly, the software allows to account for geometrical nonlinearity of flexible components such
as beams. Geometrical nonlinearity provides similar outputs to linear behaviour output if the deflections or
rotations of the components under analysis are smaller; however, this is not the case in the context of this
study, due to the large size of the wind turbine blade (more than 100m) and the significant loads that the
blade experiences. The blade under analysis is a fictional model: in other words, there is no availability of a
real component with all the properties that will be shown in next chapters. As consequence, there are not
any available tests data nor for single axis neither for biaxial fatigue tests.
The main research being proposed in this project is to investigate the nonlinearity related to wind turbine
blades, focusing on the deformation and the fatigue loads, in the context of biaxial fatigue. Particularly, the
aim is to identify the differences between nonlinear analysis results against linear results obtained, with an
interest to variation of displacements and loads experienced by the blade. Thus, this project used a
numerical simulation software, MBDYN, for the biaxial fatigue testing of wind turbine blades, combining
Matlab and MB-DYN. A Matlab code has been developed, from scratch, to interface with the MB-DYN
platform, where simulations are run for different geometries. There was a consideration of geometrical
nonlinearity of the blade to achieve more realistic results comparing the biaxial test procedure. Firstly, an
analysis on a simple cantilever beam subjected to a tip load is used to validate the software
implementation and numerical predictions against theoretical data. In the second phase, an analysis of the
blade with a tip force is investigated. In addition, a focus on gravity behaviour of the blade is performed,
with an attention to mass, tip displacement and natural frequencies. Furthermore, focus on the actuator
system used for the single axis and biaxial test is the subject of the analysis. Lastly, a numerical simulation
for fatigue test is performed, where initially single axis results will be compared, and then biaxial fatigue
will be investigated. For the first part, which included force and gravity, the results match well with
theoretical data, as well as single axis. For biaxial fatigue, some issues occurred due to damping formulation
in the nonlinear tool, and a diagonal damping matrix was introduced. For the best damping value at the
target load the results gave convergencies of the value at the root and differences at tip for the bending
moments. Displacements have good matches in general. The discrepancies arisen, as expected, as one of
the applied load increases, affecting the other component.

Date of Award	19 Mar 2024
Original language	English
Awarding Institution	University of Bristol
Supervisor	Terence Macquart (Supervisor)