Blade materials, testing methods and structural design - Wind Power Generation and Wind Turbine Design - page 418

number of rotations (blade-tower passings) is of the order 10 8 to 10 9 , which is
approximately two orders of magnitude higher than the load cycles experienced
by composite materials used in other highly loaded structural applications such as
helicopter blades.
The main trends in the development of wind turbine blades are towards longer
and optimized blades; this is particularly the case for offshore wind turbines. The
weight of a large wind turbine blade also increases the loads on the rotor input
shaft and bearings as well as the wind turbine tower and mechanisms used to con-
trol yaw and pitch of the blades. Weight savings is therefore of great importance,
and signifi cant efforts are devoted by wind turbine companies in the selection of
materials. To ensure that the blades can meet the required design life, the materials
must have high stiffness, be fatigue resistant, and be damage tolerant.
As for other low-weight-driven designs, the material considerations thus involve
the specifi c stiffness (i.e. the stiffness divided by density), specifi c strength
(strength divided by density) and specifi c fatigue limit (fatigue limit stress divided
by density).
This chapter provides an overview of the interconnection between the blade
design process, material properties, materials testing and sub-component testing
and full-scale blade testing. As shown in Fig. 1, the design of modern wind turbine
blades involves an understanding of material behavior and failure modes at many
length scales. This design process requires close collaboration between engineers
involved in modeling of aerodynamic loads, structural analysis and composite
materials and technicians responsible for the manufacturing process, quality con-
trol and on-site inspection and monitoring of blades. This chapter starts with a
review of manufacturing processing (Section 2), followed by a description of full-
scale blade testing including some results for blades tested to failure (Section 3)
that leads to a classifi cation of common failure modes (Section 4). The material
properties that control the development of the various failure modes are presented
in Section 5. Section 6 describes experimental methods for determination of these
properties. Various examples of the use of the modern design methods that make
use of these strength-controlling material properties are given in Section 7. Finally,
Section 8 contains a discussion and closure.
2 Blade manufacture
The design of the wind turbine blade is a compromise between aerodynamic
and structural considerations. Aerodynamic considerations usually dominate the
design for the outer two-thirds of the blade, while structural considerations are
more important for the design of the inner one third of the blade.
2.1 Loads on wind turbine rotor blades
The rotor blade is loaded in a combination of fl apwise and edgewise loads. Basi-
cally the blades are exposed to three different load sources. One is the wind load
that through the lift and drag on the aerodynamic profi le loads the blade primarily