Developing an effective blade structural health monitoring (SHM) system is important for the wind energy industry. This has challenged the scientific communities for years, and still, the problem has not been solved. This research aims to find a solution to this problem and provide a basis for further research in this field. The failure rate and the downtime associated with wind turbine blades are relatively high. Within the framework of preventive maintenance, it is necessary to inspect wind turbine blades periodically. Blade inspection needs special tools, skilled manpower, vessels, and lifting equipment; therefore, it is costly. This is the main reason that the wind energy industry would like to minimize the number of periodic blade inspections by using a reliable SHM system. An issue that complicates the SHM of wind turbine blades is the complexity of blade geometry. Different parts of a wind turbine blade are connected using adhesive. These adhesive joints, e.g., leading edge, trailing edge, and spar capshear web adhesive joints, are important parts of the blade structure and play an essential role in the integrity of the blade. These joints are very vulnerable to fatigue damage. Manufacturing flaws and impact can instigate and accelerate fatigue damage in a wind turbine blade. Impacts may happen during the transportation or installation of the blades, especially for offshore wind turbines. In this study, vibration, acoustic, and infrared thermography are used to characterize fatigue damage in test specimens representative of the spar cap-shear web adhesive joint of a wind turbine blade. To create different levels of damage, the test specimens were subjected to fatigue tension tests. To study the effect of impact on damage propagation, the test specimens were subjected to different levels of impact using a Gas Canon machine before being subjected to fatigue tests. During the fatigue tests, a Polytech laser vibrometer, Vallen acoustic system, and FLIR infrared thermographic system were used for vibration, acoustic and thermographic analysis...@en

Wind turbine blades carry the risk of impact damage during transportation, installation, and operation. Such impacts can cause levels of damage that can propagate throughout the structure compromising performance and safety. In this study, the effect of impact damage on fatigue damage propagation in test specimens representative of a spar cap-shear web adhesively-bonded connection of a wind turbine blade was investigated. In addition, the effectiveness of using acoustic emissions to detect early impact-induced fatigue damage was studied. Three impact tests with increasing levels of energy were investigated. The results showed that for an impact test with an average energy of 16.32 J, the fatigue damage accumulation process was not influenced by the size and location of the impact damage. But for impact tests with an average energy of 23.68 J and 32.13 J, greater crack density and accelerated de-lamination and de-bonding of the adhesive from the laminate could be seen in the impact zone. Acoustic emission was shown to identify the position of the damage zone for the higher energy impact tests. It was also effective in showing the progressive accumulation of fatigue damage in this zone during the fatigue test.

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In this paper, a new indicator to localize fatigue damage in a fibre glass composite structure, i.e. spar cap to shear web thick adhesive joint of a wind turbine blade, is presented. This indicator is based on the effect of damping on the phase of the mode shapes of the structure. When fatigue damage occurs, damping increases in the defective area and this leads to an increase in the local energy dissipation. This non-uniformity in the energy dissipation throughout the structure causes the structure to vibrate with mode shapes whose structural elements no longer have the same phase creating complex mode shapes. A visco-elastic finite element (FE) vibration model is developed for a thick adhesive joint of a wind turbine blade. The mass, stiffness, and damping matrix extracted from the FE model are used to determine the complex mode shapes. The results show that the damaged area is located where the spatial derivative of the phase of the components of the mode shapes is minimum. Changes in the phase of mode shapes of the structural elements are strongly dependent on the location of damage. In the locations where the strain modal energy is greater, the change in the phase is also higher.

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Wind turbine blade spar cap to shear web adhesively bonded connections can suffer from damage at the bond-line which can propagate through the structure compromising blade integrity. This study investigates changes in the stiffness and damping of a thick adhesive joint test specimen during a fatigue test. The stiffness is calculated using an extensometer and damping is determined using vibration and thermography. Fatigue tests showed three distinct phases of damage. Firstly, transverse cracks and delamination initiate and grow with little change in stiffness observed while damping increases by 18.6% and 17.8% inferred from the vibration and thermographic methods, respectively. In the second phase, the number of transverse cracks increases reaching saturation and the stiffness reduces by 4.7%. The increase in the loss factor is 65.7% and 95.6% from the vibration and thermographic methods, respectively. In the final phase, the crack density remains constant whilst de-bonding of the joint begins and grows until failure. The loss factor increases by 111.4% and 116.9% from the vibration and thermographic methods, respectively though the stiffness shows a cumulative reduction of only 8.6%. The results show the potential for monitoring changes in damping to infer incipient damage in an adhesively bonded composite joint structure.

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The adhesively-bonded connections in a wind turbine blade, e.g. the spar cap to shear web joins, are key elements for the structural integrity of the blade. These joins can suffer from damage at the bond-line which can propagate through the structure and compromise the operation of the blade. In this paper, we determine the damping properties of a test specimen representative of that joining a spar cap and a shear web during a period of progressive damage. In addition to the experimental damping measurement, an analytical dynamic model based on the visco-elastic properties of the material is developed to relate the damping to the loss factor. The experimental results show that when a crack is initiated in the test specimen, the damping increases by around 5-7%. This value increases with the propagation of transverse cracks in the adhesive and reaches a value of 35% when the adhesive layer experiences crack saturation and the damping reaches 45 % before failure occurs. Although a significant change in the damping is observed, there is no significant change in the natural frequency (<1%) and by association little change in the stiffness of the test specimen.

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