Blade materials, testing methods and structural design - Wind Power Generation and Wind Turbine Design - page 426

material-related damage modes can develop in a blade. In some instances, these
damage modes can lead to blade failure or require blade repair or replacement.
It is useful to defi ne a few key concepts. The term failure is used here as a
broad term covering various processes that creates damage or cracking. Blade
failure indicates the critical state where the wind turbine blade loses its load-
carrying capability. Failure mode describes classifi cation of the macroscopic
types of failure which can occur. Defi ne damage as non-reversible processes that
occur as distributed phenomena (e.g. multiple matrix cracking or fi ber failure).
Damage modes is the term used to characterize specifi c types of damage at the
material scale. Fracture indicates damage in the form of macroscopic cracks.
Fracture modes indicate specifi c types of cracking (e.g. cracks between different
plies or cracks along interfaces between different materials). Crack initiation is
defi ned as the process of the formation of a sharp crack from a pre-existing fl aw.
The precise occurrence of crack initiation depends on microstructural details,
such as the size and distribution of porosity or other defects and the fracture
resistance of interfaces; all of these depend on the materials and processing
methods used. Crack propagation concerns the growth of sharp crack. Depend-
ing upon the crack size and load level, a crack may extend in a stable manner
(i.e. in small increments), or stop (crack arrest) or rapidly (unstable).
The load-carrying laminates in the blade aeroshell and box girder are made of
composite materials and adhesive joints that are damage tolerant. Damage tolerant
behavior implies that the fi rst mode of damage does not lead directly to failure, but
propagates in a stable manner and gives detectable changes so that the damage can
be detected before it reaches a critical size where it leads to failure. Therefore, failure
of wind turbine blades does not occur as a direct result of crack initiation along an
interface or by progressive damage to the fi bers and matrix. Rather, global failure of
a wind turbine blade involves the progression of several damage mechanisms that
can act in series or in parallel. This hierarchical failure evolution can be thought into
the blade design, creating a damage tolerant design. For example, interface debond-
ing along an adhesive joint can cause a detectable reduction in structural stiffness
while the redistribution of stresses causes corresponding higher cyclic strain ampli-
tudes in the blade or the initiation of cracks in laminates or sandwich structures in the
vicinity of the debond. However, global blade failure will not occur until a damage
type (typically a crack) reaches a critical size leading to unstable fracture.
4.2 Identifi ed blade failure modes
A considerable amount of knowledge is required to assess how damage devel-
ops in a wind turbine blade and to design a blade against failure using analytical
or numerical methods. Therefore, in order to validate the design, and to provide
insight into possible damage modes and their severity, blades are sometimes
tested to failure by full-scale testing. Figure 5 shows sketches of the failure modes
(summarized in Table 1) found in a wind turbine blade tested to failure [2].
The consequences of the various damage and failure modes listed in Table 1
are widely different. For instance, cracking of the gelcoat fi lm is not as severe as