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Contract No. DE-AC36-08GO28308

Development and 

Commissioning of a Small/MidSize Wind Turbine Test Facility

Preprint

D. Valyou, T. Arsenault, K. Janoyan, 

and P. Marzocca

Clarkson University

N. Post

National Renewable Energy Laboratory

C. Grappasonni, M. Arras, and G. Coppotelli

University of Rome Sapienza

D. Cárdenas and H. Elizalde

Tecnológico de Monterrey Campus Ciudad de Méx

O. Probst

Tecnológico de Monterrey

To be presented at AIAA SciTech 2015

Kissimmee, Florida

January 5‒9, 2014

Conference Paper

NREL/CP-5000-63051

January 2015
NOTICE

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Development and Commissioning of a Small/Mid-Size 

Wind Turbine Test Facility

Daniel Valyou1, Tyler Arsenault2, Kerop Janoyan3, and Pier Marzocca4,5

1,2,3,4 Clarkson University, Potsdam, NY, 13699, USA 5 Royal Melbourne Institute of Technology University, Bundoora VIC 3083 Australia.

Nathan Post6

National Renewable Energy Laboratory, Golden, CO, 80401, USA

Chiara Grappasonni7

, Melissa Arras8, and Giuliano Coppotelli9

University of Rome Sapienza, 00184 Rome, Italy

Diego Cárdenas10 and Hugo Elizalde11 

Tecnológico de Monterrey Campus Ciudad de México, D.F. CP14380, México

and

Oliver Probst12 

Tecnológico de Monterrey, Campus Monterrey, CP64849, México

This paper describes the development and commissioning tests of the new Clarkson 

University/Center for Evaluation of Clean Energy Technology Blade Test Facility. The 

facility is a result of the collaboration between the New York State Energy Research and 

Development Authority and Intertek, and is supported by national and international 

partners. This paper discusses important aspects associated with blade testing and includes

results associated with modal, static, and fatigue testing performed on the Sandia National 

Laboratories’ Blade Systems Design Studies blade. An overview of the test capabilities of the 

Blade Test Facility are also provided.

I. Introduction

Wind turbine blades are designed to capture wind effectively to deliver maximum energy production; however, 

manufacturing and logistic constraints require wind turbine blades to be made as light and affordably as possible. As 

a result, these highly stressed structures are subjected to variable and stochastic dynamic environmental loads during 

their operational life, which can lead to catastrophic failure. For example, rotor blades comprise roughly 7% of total 

wind turbine components failures

1

, as shown in Figure 1. During operation, large and frequent fluctuations in wind 

intensity and direction can cause severe strain on wind turbine blades. Wind turbine blade failures impact safety, 

cause turbine downtime, and can pose a risk to individuals located within the vicinity of wind turbines. Safety is of 

particular concern when considering small and mid-size wind turbines, as they are often located in residential, 

commercial, and community areas in close proximity to the public. Current standards (Table 1) do not require 

extensive physical testing of small wind turbine blades; however, static and fatigue structural testing is required for 

_____________________________

1 Blade Facility Manager, MAE Dept., 8 Clarkson Ave, Potsdam, NY, 13699-5725, AIAA Member.

2 PhD Student, 8 Clarkson Ave, Potsdam, NY, 13699, AIAA Member.

3 Associate Professor, CEE Dept., 8 Clarkson Ave, Potsdam, NY, 13699, AIAA Senior Member.

4 Professor, MAE Dept., 8 Clarkson Ave, Potsdam, NY, 13699-5725, AIAA Associate Fellow.

5 Professor and Deputy Head of School (Aerospace and Aviation), PO Box 71 Bundoora VIC 30833 Australia, AIAA Associate Fellow.

6 Engineer (Blade Testing), National Wind Technology Center, 15013 Denver West Parkway, Golden, CO, 80401. 

7 Post-Doc Associate, AIAA Member.

8 PhD Candidate, Dipartimento di Ingegneria Meccanica e Aerospaziale, Via Eudossiana, 18, AIAA Student Member.

9 Assistant Professor, Dipartimento di Ingegneria Meccanica e Aerospaziale, Via Eudossiana, 18, AIAA Associate Fellow.

10 Assistant Professor, Mechatronics Engineering Department, Calle del Puente 222 Col. Ejidos de Huipulco Tlalpan.

11 Professor, Mechatronics Engineering Department, Calle del Puente 222 Col. Ejidos de Huipulco Tlalpan. 

12 Professor, Physics Department, Av. Eugenio Garza Sada 2015 Sur, Col. Tecnológico.
2

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Figure 1. Share of major wind turbine component 

failures.

1

the blades of wind turbines with more than a 200-m2 swept 

area, which is typically a blade length of >8 meter (m) for 

horizontal-axis wind turbines. As deployment of small and 

community wind turbines expands, it is important to ensure 

that testing methods and facilities are available to evaluate 

blade designs for performance, reliability, and safety.

Wind manufacturers, operators, investors, insurance 

companies, and wind farm developers will benefit from thirdparty testing, inspection, and evaluation of small and mid-size 

wind turbine blades. Third party review of design, 

manufacturing and testing serve to extend the life cycle of 

wind turbine blades and minimize the risks from design flaws 

and manufacturing defects; however, there are few blade test 

facilities worldwide and their capabilities for testing wind 

turbine blades focus on current standards and primarily serve 

the market for large wind turbines. As a result, this scarcity 

makes it difficult and costly for small and mid-size blade 

manufacturers to test their products. 

With this current reality in mind, and with the goal of extending the life cycle of small wind turbines and minimizing 

the potential for failures, a grant from the New York State Energy Research and Development Authority 

(NYSERDA) established the Center for Evaluation of Clean Energy Technology (CECET) and created the 

opportunity to develop the small-to-mid-size wind turbine Blade Test Facility (BTF), installed and operated by 

Clarkson University. The facility will help manufacturers test their blades to American Wind Energy Association 

(AWEA) and international design standards, in particular to the International Electrotechnical Commission (IEC)

61400–2, Wind turbines – Part 2: Design requirements for small wind turbines, and IEC 61400-23 “Wind turbines -

Part 23: Full-scale structural testing of rotor blades,” among others (Table 1).

Table 1. Partial list of small wind turbine testing standards

North America:

Standard Title

AWEA 9.1 Small Wind Turbine Safety and Performance

CAN/CSA C61400-2 Wind Turbines - Part 2: Design Requirements for Small Wind Turbines

International:

Standard Title

EN/IEC 61400-2 Wind Turbines - Part 2: Design Requirements for Small Wind Turbines

MCS 006 Product Certification Scheme Requirements: Micro and Small Wind Turbines

RUK 2014 (formerly British Wind 

Energy Association 2008) RenewableUK Small Wind Turbine Standard

The BTF leverages ongoing collaborative efforts with national laboratories such as the U.S. Department of Energy’s

(DOE) National Renewable Energy Laboratory (NREL), the Massachusetts Clean Energy Center’s Wind 

Technology Testing Center, international partners such as the University of Rome Sapienza, the Instituto 

Tecnológico y de Estudios Superiores de Monterrey, and private industry partners. The BTF performs full-scale 

structural testing of wind turbine blades up to 15 m in length, such as static and dynamic load testing, blade 

experimental structural analysis including mass, stiffness, and modal testing, structural materials characterization, 

and structural design and analysis support. The BTF also supports industry participation and continuing education 

through distance learning and workforce development programs.
3

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II. BTF Description

The BTF test cell consists of an 8-m x 14-m strong floor with two 6-m-wide x 5-m-high reaction walls. The reaction 

walls are designed to sustain a moment of 1.02 Mega-Newton meters (MN-m), and the floor can sustain 445 

kilonewtons (kN) of tensile load. Both the strong floor and reaction walls are equipped with anchorages or tiedowns positioned 1.2 m apart in a uniform grid. The reaction walls’ support has two test stands (to run simultaneous 

static and fatigue tests) to perform testing of full-scale wind turbine blades up to 15 m, scaled turbines blades above 

15 m, and sub-scale blade components up to 15 m in length. Blade tests are performed with purpose-built structural 

test and data acquisition equipment, sourced from MTS Systems Corporation, and modal test equipment from LMS 

Testing Solutions and PCB Piezotronics, global leaders in mechanical testing and measurement equipment. 

Additional test equipment for the test cell includes a range of hydraulic actuators, hydraulic power supply, servo 

controllers, two 250-kip (1.1 MN) modular test frames, a 220-kip (979 kN) Instron testing frame, and a 110-kip (489 

kN) MTS test frame. For certification, blade testing is performed in accordance with IEC standards, and certification 

services are provided by Intertek.

The facility performs structural testing of turbine blades and substructures, including static, forced displacement 

and resonant fatigue, bi-axial fatigue, and static and fatigue torsion testing. It is modular in design and incorporates a 

range of servo-hydraulic actuators and test fixtures, load cells, and a variety of high-resolution data acquisition

systems. Multiple actuators can be deployed to approximate a distributed load to test articles. In addition, the BTF 

control system incorporates a number of safety features, including load, displacement, error, and data acquisition 

limits in software, as well as mechanical load abort and load-limiting systems. The control system also incorporates 

facility power-quality monitoring and UPS protection, which safely parks the test article in the event of a power 

quality event. These safety features protect test articles from excessive loads, displacements, and issues with grid 

power quality. A dedicated MTS data acquisition system allows for simultaneous recording of all measurement 

channels such as strains, loads, and deflections during the test. Furthermore, the BTF utilizes instruments required 

for precision measurement and nondestructive testing along with visual inspections to monitor the condition of the 

wind turbine blades. Figure 2 shows the facility’s layout.

A. Test Control

The MTS FlexTest 60 controller is an eight-channel, two-station real-time controller with eight valve drivers, eight

universal signal conditioners, eight channels of A/D input, eight channels of D/A output, a dual-serial interface, a 

16-channel digital input/output, two hydraulic service manifold controllers, and a hydraulic power unit control. Test 

control is accomplished with AeroPro test control software, which allows for closed-loop independent control of 

each actuator, including running multiple simultaneous tests. The system is capable of supporting a wide variety of 

hydraulic actuators, both dynamic and quasi-static. In addition, the facility currently incorporates six static actuators 

ranging in stroke length from 0.8 m to 1.6 m and capable of loads up to 67 kN, and two dynamic actuators with a 

full-range stroke of 0.6 m and a load capacity of 25 kN. 

Figure 2. Layout of the Clarkson/CECET Blade Test Facility. Side view (left) and end view 

(right) showing the commission blade on the floor and a wood beam used to verify control of 

the hydraulics mounted on one test stand.
4

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B. Data Acquisition System

The BTF data acquisition hardware consists of the MTS FlexTest 

60 controller, the MTS FlexDac data acquisition (DAQ) system, 

and the LMS SCADAS DAQ. The FlexTest controller captures the 

load, actuator displacement, and draw wire displacement 

transducers through eight 494.26 digital universal conditioners. 

These conditioners have 19-bit A/D, full-range signal conditioners, 

a 122.88-kilohertz (kHz) sample rate, and digital filtering with 

computer-controlled range, zero, excitation, filtering, and shunt 

calibration. The FlexDac data acquisition collects the strain data 

for the blade and consists of 128 channels of 24-bit simultaneous 

data acquisition, with a synchronization timer input between the 

FlexDac and the controller, with each input capable of quarter, half, 

full bridge, analog voltage, and Transducer Electronic Data Sheet 

(TEDS) input with bridge completion, software selectable hardware

filtering, signal conditioning, and an external shunt resistor. The 

FlexDac data acquisition and FlexTest data acquisition are synchronized for a unified time signature. The LMS 

SCADAS system measures and analyzes the accelerometer data for modal analysis and is a 48-channel, 24-bit, 

simultaneous data acquisition system that accepts voltage, ICP, AC/DC bridge, and active sensors with a frequency 

range and filtering optimized for vibration measurement. Each system is connected to its own independent UPS to 

ensure clean and constant power delivery. A summary of the BTF instrumentation is provided in Table 2.

The system sensors include a range of load cells for the hydraulic actuators, draw wire extensometers up to 5 m in 

length for quasi-static displacement measurements, resistive and piezoelectric strain gauges, accelerometers, 

inclinometers, linear variable differential transformers and temposonic displacement transducers to accommodate a 

wide array of test setups. The test control and instrumentation block diagram is shown in Figure 3.

1. Strain Gauge Instrumentation

The strain gauge type typically used by the CECET BTF is the uni-axial gauge Vishay L2A-06-125LT-350 and 90°

rosettes Vishay L2A-06-250LW-350. These gauges were selected to have a 350-Ohm gauge resistance, allowing for 

higher excitation voltage, and a self-temperature-compensation appropriate for composite materials. Each channel is 

wired to the DAQ in a three-wire quarter bridge configuration to compensate for wire length resistance. Half or full 

bridge gauge configurations are also accommodated by the DAQ system, but increase the cost of the gauges.

BTF strain gauge channel names are in the form:

GXX-YYYYY-AA-ZZZ

where XX is the gauge number, YYYYY is the span location of the gauge in mm, and AA is a letter code indicating 

location as follows:

HP –High-pressure spar cap of the blade

LP – Low-pressure spar cap of the blade

LE – Leading edge (nose) of the blade

TE – Trailing edge of the blade

ZZZ is the angle of the gauge relative to the span axis of the blade in degrees (typically 000, 045, 090, or 135).

Table 2. BTF Instrumentation
5

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The instrumentation plan for Sandia National Laboratories’ Blade Systems Design Studies (BSDS) blade used in 

commissioning the BTF is shown in Figure 4.

2. Other Test Instrumentation

When testing a blade at the BTF, displacement is measured using Micro Epsilon analog draw wire displacement 

transducers with analog voltage output. The calibration scale factor is verified for each transducer against an 

externally calibrated steel rule prior to running a test. For the BSDS blade, each draw wire displacement transducer

was located so the draw wire runs parallel to the loading cable. Actuator displacements are measured with tempsonic 

(static test) or Linear Variable Differential Transformer (LVDT) transducers. Actuator displacement is used as the 

primary displacement limit measurement. Applied force(s) are measured using load cells, which also provide 

control feedback. A shunt calibration check and verification with a calibrated load is performed prior to introducing 

the load cell into the load chain. Ambient temperature and humidity is measured in the laboratory at the test stand 

using a relative humidity/temperature transmitter. This device is calibrated by an external lab on an annual basis and 

is connected to an analog input of the FlexDac.

C. Test Hardware

1. Wall Mount and Adapter Plate

The adapter plate is a two-piece modular design consisting of the wall mount and blade adapter. The wall mount is a 

high-strength structural steel fixture that is capable of supporting any typical blade up to 15 m in length (shown in 

Figure 6). It is designed to support a bending moment of 2 MN-m. The blade adapter is specific to each test article, 

with the wall mount’s 1.219-m bolt circle of 36 1.25-inch-12 threaded studs for its outer ring and a bolt circle 

compatible with the test article for the inner ring, shown in Figure 5. Hardware components are designed to sustain 

the ultimate failure load for a given test with a minimum safety factor of 2.5, as analyzed using a finite-element 

analysis.

Figure 3. Instrumentation and control block diagram (static test highlighted).
6

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Figure 4. Blade Systems Design Studies (BSDS) blade strain gauge instrumentation

Figure 5. Blade wall mount and BSDS blade adapter
7

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2. Saddle Design

The blade saddles (see Figure 6) are designed to be

modular, with a wood insert cut to the local airfoil profile 

to distribute the load over the surface of the blade, which 

in turn is surrounded by an aluminum frame. The frame 

connects to the load chain through a loading arm that

pivots about the blade’s flapwise neutral axis, 

minimizing any off-axis moment introduction from the 

load chain. The load chain is connected to the arm 

through a slider that is adjusted to direct the force 

application through the blade’s chordwise neutral axis, 

minimizing the introduction of a pitching moment and 

preventing twist caused by load introduction. Each 

saddle form is machined wood fitted with a 6-mm-thick 

soft rubber isolator between the form and the blade. 

Reliefs are provided at the leading and trailing edges to prevent unwanted stress concentrations.

3. Load Introduction Hardware

All load chains consist of 19-mm diameter, 250-kN tensile strength Dyneema synthetic wire rope. Dyneema is used 

because it is 8 times lighter than steel, has less recoil energy than steel, recoils in a linear fashion versus steel’s 

snaking, and has no sharp ends when it breaks. It is also much easier to work with than steel, and can be worked 

with by using traditional marlinespike seamanship techniques. Connections are made with sliding eye splices and 

rope shackles to eyebolts attached to the load cells. The actuators are connected by means of threaded studs, eye 

nuts, and Supernuts that allow the connection to be torqued. The sheaves are 305-mm in diameter, include 75-mm

bore bushed pulleys, and the turning block and floor plate locations are determined by test geometry.

III. BTF Coordinate System

Three coordinate systems describe the blade and blade test

setup: the blade coordinate system {Xb, Yb, Zb}, the test 

stand coordinate system {Xt, Yt, Zt}, and the global 

coordinate system {Xg, Yg, Zg}. Most test information in 

this paper is given in global coordinates indicated by {Xg, 

Yg, Zg}. Specific locations on the blade including 

instrumentation and saddles will also make use of blade 

coordinates {Xb, Yb, Zb} with the blade root as the origin. 

Although many manufacturers refer to a rotor radius 

dimension measured from the center of the turbine hub, the 

BTF only references the blade span location measured 

from the root plane of the blade.

Blade coordinate axes are defined with the origin at the 

center of the root as shown in Figure 7. The Xb-Yb plane 

is the plane of the root flange with the origin on the pitch 

axis. The Yb axis is in the LE – TE direction at the blade’s 

zero angle of attack. The Zb axis is from root to tip and is 

also the blade’s pitch axis.

The origin of the test stand coordinate system is at the 

center point of the test stand diameter, flush with the face 

of the test stand as shown in Figure 8. At a minimum, there 

is a pitch angle rotation (about Zb) and a translation 

because of the adapter plate and spacer thickness to convert 

from the blade coordinate system to the test stand 

coordinate system.

Figure 6. Blade saddle design.

Figure 7. Blade coordinate axes.

Figure 8. Test stand coordinate system.
8

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The global coordinate system (Figure 9) is rotated 

about the Yt axis and translated. The Zg axis is the 

primary axis parallel to the test laboratory blade length 

direction. Yg is horizontal, parallel to the laboratory 

floor, and Xg is vertical. The origin is located at a 

point level with the strong floor in the corner created 

by the two strong walls. The global coordinate system 

is rotated from the test stand coordinate system by the 

test stand angle about the Yt/Yg axes and translated in 

all three dimensions.

Blade information is typically provided in blade 

coordinates, which is then converted to global 

coordinates for calculations. The raw measurement 

data including deflections and force angles is 

measured in global coordinates. The resulting 

deflection is presented in global coordinates and 

compared to the theoretical values in that coordinate 

system.

IV. The BSDS Commissioning Blade

The blade used for the commissioning of the 

BTF facility is the Sandia BSDS blade 

manufactured by TPI Composites. TPI designed 

and produced production molds and assembly 

fixtures for the BSDS prototype blades, and 

produced one blade in December 2012, which 

was shipped to Clarkson University in January

2013. The BSDS blade was designed to be a 

simpler-to-fabricate, less expensive, and lighter 

alternative to earlier 9-m blade designs. The 

blade is 8.325 m long from the blade root, 

weighs 130 kg, and has a center of gravity that 

is 2.83 m from the root. These specifications 

were given by the manufacturer upon delivery, 

and were verified in-house prior to 

instrumenting and mounting the blade. The 

blade was mounted in the facility for static 

testing, as shown in Figure 10, and for fatigue 

testing, as shown in Figure 11.

Specifications for the BSDS geometry are 

provided in Table 3 and Table 4. The blade 

features a flatback airfoil for half of the blade,

developing into a typical airfoil trailing edge.

Flatback airfoils differ from truncated airfoils, 

offering the structural benefits of thicker sections without large aerodynamic losses. See Ref. 2 and Ref. 3 for 

information on blade design, composite specifications, and material properties. See Refs. 4‒8 for information on the 

aerodynamics of flatback airfoils.

The blade root attachment consisted of 24 5/8-inch-18 studs extending 240 mm into the root of the blade from the 

root face, with the final 150 mm of the stud embedment tapered to avoid stress risers2

. The root flange consisted of 

24 13-mm-thick trapezoidal aluminum plates, which the mounting studs thread through, and are bonded to the 

composite blade root and each other, forming a 24-sided flange ring. 

Figure 9. Global coordinate system description.

Figure 10. BSDS blade mounted for static testing.
9

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Table 3. Blade geometry details2

Table 4. Rotor blade data2

Figure 11. BSDS blade mounted for fatigue testing.
10

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V. Commissioning Test

Three commissioning tests were performed with the BSDS reference blade as part of qualifying the facility for 

operation. Upon receipt of the blade, a series of modal tests were performed and the blade’s modal parameters were

characterized. In June 2013, the commissioning static test was performed, prior to the facility’s public 

commissioning on 8 August 2013. Finally, from mid-August to mid-September, a 30-day fatigue test was performed 

to complete the facility’s commissioning.

A. Modal Test

As shown in Figure 12, the blade was mounted on the 

wall and the test conducted by exciting the structure 

in both the Xb and Yb directions at the intersection of 

the trailing edge and blade root, and the response 

accelerations were measured by 48 accelerometers 

mounted in a combination of Xb and Yb

orientations.

The natural frequencies and damping ratios 

corresponding to this test configuration, estimated 

using the LMS Polymax software, are summarized in 

Table 5, and the corresponding mode shapes are 

depicted in Figure 14. Figure 13 shows a good 

correlation between the identified mode shapes. As 

Table 6 reveals, the wall-mounted blade has lower 

natural frequencies than published results for the freefree configuration9

. Moreover, the free-free

configuration identified additional modes.

Table 5. Modal Data for the Mounted Blade 

Figure 13. Auto-Modal Assurance Criterion for the 

mounted blade.

Figure 12. Experimental setup of the wall-mounted 

blade.
11

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Figure 14. Normal modes estimated from the impact test for the wall-mounted blade.

Table 6. Modal data estimated in previous analyses, free-free blade9

Mode # f n [Hz] ζ n (%) Mode Type

1 5.45 0.45 IF

2 13.5 0.43 IIF

3 16.5 0.97 IL

4 25.4 0.43 IIIF

5 38.6 0.6 IVF

6 40.1 0.94 IIL

7 52.5 1.2 T

8 65.1 0.82 T

9 72.1 0.72 IIIL
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B. Static Test

Commissioning of the BTF static test capabilities took place on 27 June 2013. The test was performed to 75% of the 

BSDS blade design loads in the positive flapwise direction, with three load introduction points. The design load, test 

load approximation, and load introduction geometry are similar to those given in Ref. 10. The BTF laboratory 

design enables testing to be conducted in the vertical downward direction with independent actuators for each 

saddle. This is the typical approach used by many laboratories for full-scale testing of larger multi-megawatt-scale 

wind turbine blades today. By comparison, the NREL test in 2007 used a whiffletree to distribute the load from a 

single overhead crane to the three saddles and the load was applied against gravity in the upwards direction10. The 

whiffletree approach was more common historically and is often used for static tests on smaller blades that exhibit 

limited deflection. Test loading for the commissioning test of the BTF is shown in Table 7.

Table 7. Commissioning test loading data

Saddle 

Number 

(Control 

Channel 

Number)

Saddle 

Span 

Location 

(m)

Saddle 

Target 

Compression 

(kN)

Predicted 

Saddle 

Mass 

(kg)

Predicted 

Load 

Chain 

Mass 

(kg)

Live 

Force 

(kN)

1 3 10 50 15 9.79

2 4.8 4 40 5 3.96

3 6.6 3.75 30 5 3.65

1. Test Simulation Results

The design loading, mass matrix, and stiffness matrix for the BSDS blade were interpolated to 1001 points and 

analyzed using a finite-beam-element model in Microsoft Excel. This model is used to estimate the actual load 

applied at each point, the fraction to target loading, and the deflection of the blade under 100% load. The results are 

used to ensure the fraction to target load is maintained as close as possible to one over the largest possible blade 

span, and to predict the optimal turning block locations to obtain perpendicular load introduction at 100% load. The 

results of the loading simulation are shown in Figure 15, and the blade’s predicted deflection under load is shown in 

Figure 16.

Figure 15. Blade loading simulation results.
13

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Figure 16. Blade deformation simulation results.

The test results match closely to published data. The spar cap strains at 100% load were compared with those for the 

test described in Ref. 10 (Figure 17). 

Figure 17. Blade spar cap strains at 100%load (54.36 kN-m) compared to past test of BSDS blade 

at NREL at a similar load level.
14

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Both blades showed similar strain distributions at a similar load level. Because the BTF commission test was 

conducted in the direction of gravity and the cables were setup to pull perpendicular to the saddles in the deformed 

position while the NREL test was configured to load against gravity and loaded the saddles approximately 

perpendicular to the unreformed blade, the load states in the two tests are not identical10. However, the final 

bending moment distribution is similar enough between the two tests that comparing the strain distribution along the 

blade is reasonable. Each BTF data point in Figure 17 represents the mean of 30 seconds of data at the specified 

load level, sampled at 64 hertz (Hz). The error bars represent one standard deviation from the mean. NREL data 

points represent the values recorded at a single point in time without any averaging. Throughout the test at the BTF, 

the blade displacement remained linear and showed low hysteresis (Figure 18). Spar cap strains (Figure 19) were 

also linear. Three replicates were performed for the static test, and the results demonstrated tight uncertainty bounds 

for the 95% confidence interval of the mean, as a result of the low noise and excellent signal conditioning of the 

DAQ system. 

Figure 18. Blade displacement versus moment. 

Figure 19. Spar cap strain versus moment.
15

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

C. Fatigue Test

The fatigue commissioning test was conducted between 13 August 2013 and 14 September 2013. The experiments 

performed included a flapwise fatigue blade test to 8.85x105 cycles and a post-fatigue, strain-to-moment calibration 

test. The test was a force-displacement, single actuator test with load introduction at the 4.8-m blade station. The 

dynamic actuator was bolted directly to the blade saddle frame through a monoball joint. The fatigue maximum live 

loading applied to the blade was 6000 N, with r=0.1, dictating a minimum live load of 600 N. The target test loading 

was selected through consultation with NREL to correspond to test loads performed by the lab in the past.

Periodically during the fatigue test, and after completion, a static test load was applied with the fatigue actuator to 

calibrate the response of the strain gauges and determine a strain-to-local-moment relationship. This calibration was 

used as a second metric for observing changes in local stiffness over time, and enabled the use of the strain gauges 

as moment gauges.

A similar 1 million-cycle force-displacement flap fatigue test was conducted on a Sandia BSDS blade at NREL in 

2008. The actuator saddle location was at the 4.8-m blade station and a load reversal ratio of R=0.1 was used with a 

maximum target load of 6300 N. The NREL test was performed in displacement control with the displacement 

amplitude and mean adjusted every few days throughout the test to maintain the target loads.

Overall, the BTF fatigue test displayed a barely detectable 0.35% reduction in stiffness over the course of the test, as 

shown in Figure 20. The dynamic stiffness is 

defined as the ratio of the load range to the 

displacement range for a given cycle. Each data 

point in the BTF data represents the average 

normalized stiffness over 10,000 cycles and the 

error bars are 1 standard deviation for those cycles. 

Figure 20 also shows historical comparison data to

a flap fatigue test performed at NREL in2008. The 

NREL data was down-sampled for plotting without 

any averaging. The down-sampled normalized 

data from the 2008 BSDS test at NREL nearly all 

lies within one standard deviation of the mean of 

the data collected at Clarkson, as shown in Figure 

20.

Figure 21 shows the normalized “strain stiffness”

measured at each gauge during the BTF fatigue 

test, defined as the change in blade strain with

change in fatigue load for each cycle. To calculate the error bars, a histogram was taken of the data. Sampled at 64 

Hz, each bin represents 5.6x105 samples. The error bars represent one standard deviation from the mean for each bin. 

The location of each gauge can be determined by cross-referencing the legend entries with the key in Section II.B.1. 

The one notable gauge was gauge G25, at a 6.244-m span on the HP side, whose stiffness deviated wildly from the 

other gauges. It was the only functioning gauge outboard of the load introduction point at 4.8 m. The corresponding 

LP gauge failed, but was not repaired because the blade outboard of 4.8 m was not tested. The loading of G25 was 

purely inertial, and increased with time as blade stiffness decreased. All other gauges accurately reflected the global 

stiffness of the blade.

At the conclusion of testing, a post-test static load was applied to the blade with the fatigue actuator. The relation 

between strain and the local bending moment is shown in Figure 22. This figure shows the raw data, curve fit of the 

data (solid line), and the 95% confidence interval of the curve fit (dashed lines). As can be seen by the wider spread 

of the 95% confidence interval compared to Figure 19, this test gives a less precise measure of the blade’s strain-tomoment relation than the full static test. This reduction in precision is because the moment distribution was lower, 

the blade was only loaded at a single point, and it was loaded more rapidly in a linear rather than a null-paced 

stepped fashion; however, the results correspond to the strain measurements taken in the static test prior to fatigue 

loading, indicating no significant loss in stiffness. All the strain-to-local-moment relations are linear, which also 

corresponds to the results of the static testing.

Figure 20. Comparison of the dynamic stiffness during 

fatigue testing of BSDS blades at the BTF and NREL.
16

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Figure 21. BTF BSDS spar cap stiffness.

Figure 22. BTF BSDS spar cap strain versus moment following the fatigue test.
17

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

VI. Conclusion and Continuing Work

This work discussed the Clarkson University/CECET Blade Test Facility, including its design, control, and 

instrumentation systems. The results of the facility’s commissioning blade test utilizing a Sandia BSDS reference 

blade were presented. Commissioning test data were compared to data for structural testing experiments on BSDS 

blades conducted at NREL in 2007 and 2008. What follows are the lessons learned from this work. The modal, 

static, and fatigue test results of the BSDS blade agreed well with historical test data. The modal analysis showed 

that the mounted blade has lower natural frequencies than the free boundary condition tested in previous analyses. 

This suggests that matching the mounted blade configuration in modal testing is essential to predicting the first 

natural frequency, which is critical for planning the fatigue test driving frequency. The static blade testing results 

closely matched historical data, and the fatigue test trends agreed with historical data. The results of individual 

measurements indicated low signal-to-noise ratio and tight error bounds/confidence intervals for the data collected 

during these tests.

Following the commissioning testing, the CECET BTF is now operational and several blade test projects are in 

progress and preparation. Development of the BTF’s bi-axial fatigue test system is underway and expected to be 

complete by the end of spring 2015. Plans are currently in progress to expand the BTF’s scope as well as conduct

the first structural test of an aircraft structure (fully composite monocoque unmanned aerial vehicle wing). In 

addition, research is currently underway to develop new test instrumentation that will greatly increase strain data 

acquisition density, enabling a more comprehensive strain characterization of structures such as wind turbine blades 

and aircraft wings.

Acknowledgments

The authors would like to thank the generous financial support of the New York State Energy Research and 

Development Authority (NYSERDA) under grant agreement # 18460, and the commitment made by the Center for 

Evaluation of Clean Energy Technologies and Intertek, along with the support and fruitful discussions with Stephen 

Nolet at TPI Composites, Inc. We would also like to acknowledge MTS, LMS, and PCB for training, technical 

consultancy, and support.

The work performed in 2007 and 2008 generating the Sandia BSDS blade test data used for comparison purposes 

in this paper was supported by the U.S. Department of Energy under Contract No. DE-AC36-99GO10337 with the 

National Renewable Energy Laboratory. Funding for this work was provided by the DOE Office of Energy 

Efficiency.

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