The trend within the windindustry is towards highercapacity turbines. As blades getlonger and more complex to manufacture,the number of minor defectssuch as voids or dis-bonds withinthe composite structure invariablyincreases. Over its typical lifetime,a blade may see over 60 millionrotational cycles, and any defectleft untreated during its manufacturemay give rise to stressconcentrations leading to crackingand premature failure.Having a robust and reliable solutionto repair these is vital to ensure thelongevity of the blades in operationand to reduce the number of fieldservice repairs.
Material Selection
In an ideal world, the solution selected to repair voids or dis-bondswould have final properties that are the same as the original blade. However, the properties that make bonding pastes suitable formanufacturing large composite structures, such as long open time,high sag resistance and relatively long thermal curing profiles, arenot suited for repair injection where low viscosity and full roomtemperature curing capability are beneficial or preferred.
The material used should have little or no shrinkage as this willrequire further remedial work to fully fill the voids. Shrinkage voidswithin an existing repair cause localized stress concentrationswhich can also initiate detrimental cracking. This is a well-knowndisadvantage of existing fast-curing solutions on the market.
It is therefore inevitable that an alternative to the blade bondingadhesive is required for injection bonding and ideally, it shouldhave as similar properties as possible in the other areas. Sika hasdeveloped SikaPower®-800, a low-shrinkage epoxy repair injectionadhesive that is chemically identical to current blade bondingadhesives. It offers very similar mechanical properties, a highertoughness to resist cracking and can also fully cure without theneed for external heating.
Key Features and Benefits
- Excellent injection properties
- Chemically identical to originalblade materials
- Similar mechanical properties
- Low shrinkage
- Improved toughness
- Ambient temperature curing
- No post curing required
Application and Curing
INJECTABILITY
SikaPower®-800 has been formulated to offer improved injectability propertiescompared to current commercially available solutions. This saves time during injectionrepairs and thus increases customers' production efficiency
Injection volume vs. time
CURING
It can be very time consuming and expensive to reach the required post-cure temperatureduring the repair process. The physical mass of the blade acts as a large heatsink drawing heat away from the repair area, increasing the time it takes to reacha minimum level. Field repairs also have the added issue of the prevailing ambienttemperature adding to this effect, especially in the colder months.The optimal solution is to therefore have a material that can fully cure without theneed for subsequent post curing. SikaPower®-800 has been developed to reach ahandling strength of 1-2 MPa after 3hrs and reaches 95% of final strength after 24hrs.Subsequent post curing has a limited effect on increasing the mechanical propertiesand is therefore not necessary.
Strength vs. time