Large-scale fatigue testing of a retrofitted 3rd shear web in a 34m wind turbine blade section

Utilizing a fatigue-rated multi-axial strong-floor based test rig, the effect of a retro-fitted fish-mouth third shear-web geometry detail located within the double-curved trailing edge sandwich panels are evaluated in an inner 15m root section from a 34m wind turbine blade manufactured by SSP Technology. From previous research, a load configuration is identified, capable of triggering the breathing/pumping deformations in the trailing edge panels within the root and transition zone, which will drive the propagation on the shear-web disbonding mechanism. Investigation and evaluation of the shear web disbonding and associated mitigation are acquired through strain gauges, wire potentiometer and digital image correlation (DIC) measurements inside the root section.


Introduction
Wind turbines are progressively used as a substitute to fossil fuel energy resources, enhancing the demand for larger and more efficient wind turbine blades [1].With an increasing size of wind turbine blades a growing need is emerging to address structural challenges due to higher utilization of the structural capacity -especially within the root, transition zone and max.chord region [2].Among one of the increasingly encountered in-field damages found in wind turbine blades, regardless of blade make and model, is transverse cracks within the transition zone and max chord region [3].This hypothesis is from an operational perspective, supported by the wind turbine owners (WTOs), who are reporting a gradually increasing amount of transverse cracks.Believed to be the root cause of these transverse cracks are, among others, the out-of-plane deformations in the large double curved trailing edge aerodynamic sandwich panels on the pressure side within the max-chord region and towards the root -referred to here as breathing [4].These deformations, often referred to as 3D longitudinal out-of-plane bending, leads to critical bending stresses in the area, where the trailing edge panels are connected/kinked into the stiff cylindrical root section.To mitigate these critical bending stresses in the trailing edge panels, a retro-fitted fish-mouth third shear-web geometry detail is installed in the inner 15m section of a 34m wind turbine blade designed and built by SSP Technology -referred to here as the root section, connecting the inner skin of the trailing edge pressure side (PS) and suction side (SS) sandwich panels through an adhesive bond line.The scope of this paper is to investigate and evaluate the overall structural integrity of the retro-fitted third shear-web along with the mitigation associated with the shear web disbonding mechanism triggered by breathing of the trailing edge panels.This breathing is primary triggered by i) the edgewise dominated loading governed by gravity which have to be carried by the large curved sandwich panels combined with ii) a torsional moment around the longitudinal axis of the wind turbine blade, generated by the flap-wise deformation, which acts as an in-plane eccentricity to the edgewise load [5].Utilizing a fatigue rated multi-axial test rig, this dual degree-of-freedom (dof) load configuration is applied to the free end of the root section through a servo-hydraulic loading arrangement.Magnitude and phase shift of the dual dof load configuration capable of triggering the maximum breathing deformations in the trailing edge panels within the root and transition zone, which will drive the propagation on the shearweb disbonding mechanism, are identified in accordance to [6].Investigation and evaluation of the shear web disbonding and associated mitigation are acquired through internally mounted strain gauges, wire potentiometer and digital image correlation (DIC) measurements.

Test specimen and installed third shear-web
The 34m wind turbine blade used in this application was used on a 1.5MW wind turbine.General blade dimensions and properties are outlined in table 1.The blade is produced using a high quality production method including pre-impregnated fibre mats without autoclave [7].To ensure adequate resin quantity towards the core and mold a high resin content (HRC) is selected for the laminates next to those regions.The resin system used is unknown to the authors, but according to SSP Technology a modification of the Ampreg 22 epoxy laminating system [8] was used.At a distance of 4m from the root (z = 4m) a retrofitted fish-mouth third shear-web is installed connecting the inner skin trailing edge PS and SS sandwich panels through a single lap adhesive bond line according to figure 1.The main body of the shear-web consists of a sandwich structure with 10mm core material covered by 4 layers of bi-axial (±45º) glass fibre on both sides (equivalent to a skin thickness of approximately 2mm).The fatigue rated clamped support of the root section is achieved using a mobile strong wall consisting of i) two strong floor mounted concrete towers and ii) steel plate attached to the concrete towers.The root section is center mounted on the steel plate.All connections are established using pretensioned threaded bars.Pretensioning levels of critical connections are monitored with "donut" load cells of the type: K-181/N550-G31 by Lorenz Messtechnik GmbH.
The complete loading arrangement -consisting of two structural actuators -is capable of applying discrete loads at the free end of the root section comprising two dofs including: edgewise load (Fy) along with a torsional moment (Mz) around the longitudinal axis of the blade.Actuator specifications are provided in figure 3b.A servo hydraulic control system is operated in load control through a Proportional Integral Derivative (PID) controller of the type MTS FlexTest using the MTS 793 software suite.The coupling between the control point, located in the center of the load carrying box girder and the corresponding force of each actuator, are defined through the MTS 793.15 Degree of Freedom Control software following the assumption of rigid body motion.Related coordinate system and notation is presented in figure 3b.The load is transferred to the free end of the root section through a steel bulkhead, which is extending 750mm into the free end of the load carrying box girder, and fixed to the inner surface of both spar caps using glue and threaded bars.To avoid critical peeling stresses in the adhesive bondline connecting the trailing edge (TE) and leading edge (LE) panels with the spar caps, the free end of the root section is fully constraint against in-plane distortion by closing the cross section with plates of over laminated plywood.The tip end region -covering approximately 2m towards the root -is over laminated using GFRP fabrics.This over-lamination is solely added to avoid structural damages generated by the localized loading induces trough the steel box girder.The zone of interestreferred to here as the gauge zone -is located around the trailing edge panels at the transition zone.According to a numerical study further described in [6], no influence on the breathing response were detected within the gauge zone.The loading arrangement and clamped support presented in figure 2a constitutes a fatigue rated structural test setup with an edgewise load capacity of 100kN and moment capacity of 100kNm around the longitudinal axis.
Investigation and evaluation of the retrofitted fish-mouth third shear-web disbonding and associated mitigation is monitored through strain gauges, wire potentiometer and DIC.Strain gauge rosettes are applied on all four corners of the shear-web to monitor the overall transfer of shear loads from the connected trailing edge panels.These strain gauge rosettes are applied at the surface towards the trailing edge of the shear-web and labelled according to figure 4a.The breathing of the large double curved trailing edge aerodynamic sandwich panels are monitored through a wire potentiometer located 200mm aft of the shear-web according to figure 2c.The out-of-plane and in-plane deformation of the PS single lap adhesive bonde-line relative to the adjacent trailing edge sandwich panel (see figure 4b) is monitored through full-field 3D DIC system of the type: ARAMIS 12M supplied by Gesellschaft für Optische Messtechnik GmbH (GOM GmbH).The camera setup and performance are presented in table 2.

Spatial resolution Resolution
In plane Out of plane 1.62mm, 17px 0.045mm, 0.48px 0.090mm, 0.96px By tracking the out-of-plane (z-direction) and in-plane (xy-direction) deformation of the three points located at the PS single lap adhesive bonde-line (named: shear-web) and adjacent pressure side trailing edge panel (named: ps panel) through DIC, the relative peak/valley deformation is derived.

Results and discussions
The root section is loaded in a fatigue test campaign comprising approximately 280.000 load cycles with a combined load configuration of two dofs including Fy and Mz.With a sinusoidal waveform, execution frequency of 0.1Hz and phase shift of 90 degrees -Fy and Mz is applied a max load of 100kN and 100kNm respectively both with R=-1 according to figure 5.This load configuration is according to [6] found to generate the highest magnitude of breathing of the large double curved trailing edge aerodynamic sandwich panels.As the retrofitted 3 rd shear web is installed to mitigate these deformations, that load configuration is established with the highest possible load magnitude to redice the number of cycles required for design validation process -even though the shear forces are different and higher than the in-service loads of the full-scale blade.A stable phase shift error of 0.7 sec between Fy and Mz were detected throughout the entire testing campaign.This phase shift error could not be lowered further with the available compensators.From the max-and min principle strain provided in figure 6, no significant and consistent variation of the magnitude is detected throughout the fatigue test campaign indicating that the peak/valley strain field throughout the third shear-web is unchanged during the fatigue test campaign.Furthermore, no rotation of the principal axis is detected during the fatigue test campaign.From the DIC measurements of the peak/valley in-plane (resulting deformation in the xy-plane) and out-of-plane (deformation in the z-direction) deformation of the PS single lap adhesive bonde-line and adjacent trailing edge panel is stable throughout the entire fatigue test campaign.From all the above measurements on the third shear web and visual inspection of both the shear-web and SS and PS bondline, it is concluded that the adhesive connection between the trailing edge sandwich panels and single lap bondline is undamaged.

Conclusion and further work
To investigate and evaluate the overall structural integrity of the retro-fitted third shear-web along with the mitigation associated with the shear web disbonding mechanism triggered by breathing of the trailing edge panels, a fatigue test campaign was conducted covering 280.000 load cycles.Monitored by strain gauges, a wire potentiometer and DIC, the structural behavior of the third shear-web was evaluated.Here it was found that the shear-web and single lap bondline connecting the shear web with the trailing

Figure 1 :Figure 2 :
Figure 1: Retro-fitted 3 rd shear-web including: a) overall position and b) photographic illustration of installation3.Experimental test setupA fatigue rated multi-axial test rig for structural assessment of the root section has been established on the strong floor[6].A 3D illustration of the test setup including the loading arrangement and clamped support is presented in figure2.a

Figure 3 :
Figure 3: Load train including: a) detailed 3D illustration of multi-axial loading arrangement and b) active coordinate system and notation

Figure 4 :
Figure 4: Labelling of internal measuremens including a) in-plane DIC measurements, b) out-ofplane DIC measurements and c) strain gauges

Figure 5 :Figure 6 :
Figure 5: Fatigue loading of root section including a) timed loading sequence, b) peak/valley edgewise load and c) peak/valley torsional load The wire potentiometer, monitoring the breathing of the PS and SS trailing edge sandwich panels, are stable throughout the entire test campaign with an amplitude of approximately 7mm. a b

Figure 7 :
Figure 7: Relative deformation of PS single lap adhesive bonde-line and adjacent trailing edge panel including a) in-plane and b) out-of-plane

Table 1 :
Overall blade dimensions

Table 2 :
Setup and performance of the 3D-DIC system