Blade materials, testing methods and structural design - Wind Power Generation and Wind Turbine Design - page 422

and strength, there are some drawbacks beyond higher cost. The compressive
strength can be lower than that of glass fi ber composites. In addition, the compres-
sive strength of carbon fi ber composites is very sensitive to the presence of manu-
facturing-related defects such as fi ber misalignment and fi ber waviness [ 3- 5 ]. For
certain regions of the blade, such as the aeroshell and internal sandwich structures,
wood or other natural materials which typically have a much lower density than
glass fi ber composites can be used. For example, Vestas UK utilizes birch wood
which is laminated with carbon fi bers to produce the aeroshell of large (40 m and
longer) wind turbine blades. Because of their low mass, these blades can be pro-
duced in thinner profi les which further reduce rotating mass. Other materials,
including bamboo are also being developed as green alternatives for use in wind
turbine aeroshells and for sandwich structures within the blade [6].
The composite matrix is also an important consideration for wind turbine blades.
Currently, polyester is the most common choice as the matrix for glass fi ber lami-
nates, but epoxy resins and vinylester are also used because of their superior
mechanical properties. Each matrix material has a set of cost, manufacturing and
mechanical behavior tradeoffs that must be considered. For example, in compari-
son with vinylester, the use of epoxy resins can increase the compressive strength
of glass fi ber laminates by as much as 10-15%. For carbon fi ber composites, epoxy
is the most common matrix material. Current research efforts are aimed at the
development of matrix materials with improved toughness, cure profi les and the
development of improved low-viscosity infusion grades for permeating thick fi ber
stacks. From an environmental viewpoint, the use of thermosetting polymers like
polyester and epoxy poses many problems and additional attention is being given
to the use of thermoplastics such as polybutylene teraphthalate (PBT). A key envi-
ronmental advantage of thermoplastics is their potential to be recycled at the end
of the turbine life cycle.
Since most wind turbine blades are bonded together, the adhesives used to join
blade segments will have a direct infl uence on the reliability of the blade. The
adhesives utilized in blades are primarily epoxy, polyurethane and methacrylate-
based adhesives. The quality and mechanical properties (e.g. strength, creep,
defect tolerance) of blade adhesives are critical since they are used to bond very
large areas of the blade aeroshells and box girder and are subjected to complex
cyclic loading histories. Moreover, adhesives must maintain adequate bond
strength over a wide range of environmental conditions. As discussed later, the
defect tolerance and fracture resistance of adhesive joints is determined through
fracture mechanics mechanics-based testing.
As noted earlier, sandwich structures are widely used in the blade aeroshell
and shear webs in some spar designs. The core material is low-density materials,
primarily balsa wood and polymer foams.
Finally, the surface of wind turbine blades are painted with gelcoats to protect
the composite materials from damage originating from UV-radiation and to limit
the environmental exposure to the blade, e.g. humidity which may change
(decrease) the mechanical properties of the composite materials. The gelcoats
should maintain their adhesion to the underlying surface despite large changes in