Impedance-based structural health monitoring

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CHAPTER 2 Impedance-Based Structural Health Monitoring of CX-100 Wind Turbine Blade Section


To evaluate IBSHM on WTBs, an actual section of a WTB was obtained from SNL’s Wind Energy Technology Department. A series of experiments utilizing piezoceramic (PZT) patches attached to the blade section as self-sensing actuators to detect both indirect and direct forms of damage were performed to test the impedance method. The purpose of these experiments was to validate that the method could detect damage on the blade and to determine the limits of sensing. A large portion of this chapter was presented at the 2007 SPIE Smart Structures and Materials and Nondestructive Evaluation and Health Monitoring conference (Pitchford et al. 2007).The WTB section tested is from a CX-100 (Carbon eXperimental-100 kW) blade.The CX-100 is a 9 meter long subscale experimental blade that was manufactured by TPI Composites in Rhode Island and was developed by SNL as part of research to incorporate carbon fiber in WTBs. This research included building subscale blades that incorporate carbon fiber which would be tested to static failure, modal tested, fatigue tested, and field tested on actual 100 kW WTs. The results of these tests were then compared to modeling(Paquette et al. 2007a; Paquette et al. 2006). The section used for this testing comes from a blade that was static tested to failure at NREL’s NWTC. The CX-100 has carbon fiber in the spar cap region of the blade infused with the blade skin, which is made of more typical glass fiber and balsa. This can be seen in Figure 2.1 and an overall picture of a similar blade, TX-100, can be seen in Figure 2.2.The blade section from a CX-100 used for impedance testing is shown in Figure 2.3. In the figure the low pressure (LP) side of the blade is facing upwards and the high pressure (HP) side of the blade is facing downwards. Also in the figure, the crosssectional geometry of the blade, as well as the materials used can be seen. The blade is made up of HP and LP skins and an internal spar which runs the majority of the length of the blade. A protective gelcoat covers the outside of the blade. The leading edge region of the skins is made up of layers of fiberglass, while the trailing edge regions as well as the spar are made of balsa covered by fiberglass. Carbon fiber is incorporated into the spar cap regions of the skins. The section used for testing is from just over two-thirds of the way down the length of the blade and is around 40 cm long.Typical damage locations and modes were suggested by SNL based on experience, blade models, and physical testing of the blade. Damage mechanisms are shown in Figure 2.4 on a cross section of the CX-100. Location 1 is on the LP side of the blade, which is where the carbon fiber spar cap meets the balsa skin. The damage mode here is separation of the two sections. Location 2 is adhesive between the spar and the spar cap on the HP side of the blade. The damage mode is cracking of the adhesive.Location 3 is the adhesive between the spar and the spar cap on the LP side of the blade.The damage mode here is pitting of the adhesive.The damage locations suggested by SNL dictated where the indirect and direct forms of damage were simulated. For the indirect testing, damage was simulated in these areas by changing the local mass and stiffness by attaching magnets and clamps to the blade. These types of test were performed because they do not directly damage the blade and are repeatable. This allowed the method to be validated on the blade while getting an idea of proper placement and frequency ranges of sensors before beginning tests that actually physically damage the blade. For the direct damage testing, actual physical damage was simulated in the locations.

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List of Figures
CHAPTER 1 Introduction
1.1 Wind power
1.2 Wind turbine blades 
1.3 Related work 
1.4 Impedance-based structural health monitoring
1.5 Thesis objectives and overview
CHAPTER 2 Impedance-Based Structural Health Monitoring of CX-100 Wind Turbine Blade Section 
2.1 Introduction
2.2 Experimental setup
2.2.1 Procedure
2.2.2 Indirect damage testing
2.2.3 Actual damage testing
2.3 Indirect Damage Results
2.3.1 Added mass results
2.3.2 Added stiffness results
2.3.3 Added stiffness, higher frequency results
2.3.4 Moving mass results
2.3.5 Indirect damage conclusions
2.4 Actual Damage Results
2.4.1 Location 1 results
2.4.2 Location 3 results
2.4.3 Actual damage conclusions
2.5 Conclusions
CHAPTER 3 High-Frequency Response Function Structural Health Monitoring of CX-100 Wind Turbine Blade Section 
3.1 Introduction
3.2 Experimental setup and procedure
3.3 Indirect damage results 
3.3.1 Added mass and stiffness results
3.3.2 Moving mass results
3.3.3 Indirect damage conclusions
3.4 Actual damage results
3.4.1 Location 1 results
3.4.2 Location 3 results
3.4.3 Actual damage conclusions
3.5 Conclusions
CHAPTER 4 Impedance-Based Structural Health Monitoring of TX-100 Wind Turbine Blade Fatigue Test.
4.1 Introduction
4.2 Preliminary tests on CX-100 wind turbine blade section 
4.2.1 Baselines
4.2.2 MFC actual damage test
4.2.3 Analog Devices AD5933
4.2.4 HP4192A compared to HP4194A
4.3 Experimental setup
4.4 Results
4.4.1 Initial results
4.4.2 During fatigue test operation
4.4.3 Data collection and baseline damage metrics
4.4.4 Damage detection
4.5 Conclusions
CHAPTER 5 Summary and Conclusions
5.1 Thesis Summary
5.2 Conclusions
5.3 Recommendations and future work
APPENDIX A: IBSHM, indirect damage, impedance plots
APPENDIX B: IBSHM, actual damage, impedance plots
APPENDIX C: HFRF, indirect damage, transfer functions
APPENDIX D: HFRF, indirect damage, transfer functions
APPENDIX E: HP4194A/HP4192A comparison, impedance plots
APPENDIX F: Fatigue test initial impedance plots
APPENDIX G: Data collection instructions for TX-100 fatigue test impedance based SHM
APPENDIX H: Fatigue test impedance plots



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