Adhesives are widely used material on wind turbine blades, as they are used to bond various blade components during manufacturing process. During blade component assembly, adhesive materials based on design specifications are applied to joints in between shear web to spar cap, trailing edges, leading-edge bites, root, and tip joints, according to wind turbine manufacturer’s application method and blade design. It is important that proper bonding is achieved in these joints to ensure proper load transfer in between adherends and to ensure blade’s structural integrity in the long run.
Common errors in adhesive joint
Common errors in adhesive application arises from improper surface preparation (surface cleaning and treatment to avoid contaminants), incorrect cure and mixing ratios, insufficient/inconsistent adhesive material thickness, incorrect adhesive material selection (poor wetting ability), and poor adhesive storage and handling.
Not all of these common errors can be visually inspected, therefore proper documentation of manufacturing steps for adhesive applications must be specified, including the list of material suppliers. OEMs must also conduct a separate quality check on their suppliers as material deviation causes unintended deviation from blade design and certification.
Insufficient quality control leads to undetected adhesive defects. Therefore, proper quality control measures and documentation (QA procedures including visual, NDT inspection, etc.) are required to ensure that quality standards are achieved according to blade design and acceptable tolerances (validated by blade test and finite element simulations), before putting the blades into service
Classifying failure modes in adhesion joints
In a blade failure, the blade’s adhesive joint quality is inspected and checked if the joint failed in the following main failure modes defined by ASTM D5573 [1]:
- Adhesive Failure: Rupture of the adhesively bonded joint, such that the separation appears to be at the adhesive-adherend interface. Fibre reinforced polymers (FRP) and adhesive surfaces may have shiny appearance and there is no evidence that any adhesive or FRP, or both, have transferred to the surface [1].
- Cohesive Failure: The separation is within the adhesive [1]. Occurs due to high thickness and/or material deviation of adhesive, making longitudinal strain in the mid-plain section high and fail in that location.
- Fibre-Tear Failure: Failure occurring within fibre reinforced polymers (FRP) matrix, characterized by the appearance of reinforcing fibres on both ruptured surfaces [1].
If fibre-tear failure is observed after blade failure, it is likely that these regions of fibre-tear are secondary effect of main cause of failure due to strong adhesion bond in between adhesive-adherend interface pointing to good adhesion. Chemical testing and microscopic analysis can be required to classify adhesive and cohesive failure type (or mixed failure mode) in order to understand and validate if manufacturing/design related defect exist.
A proactive approach is required in order to lessen the risk of adhesion failure in blades, and this includes following points of recommendation:
- Request of detailed quality control documentation that includes information such as quality control procedure that must adhere to standard practices which includes information on type of control procedures applied and blade locations where QA is applied.
- End-of-warranty internal and external blade inspection to catch, claim, and repair (if necessary) adhesion defects in advance.
- Continuous internal and external inspections of a subset of the blades within a windfarm to track and detect any defect development before significant effect to the blade integrity.
Reference:
[1] | ASTM D5573-2019: Standard Practice for Classifying Failure Modes in Fiber-Reinforced-Plastic (FRP) Joints |
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