Reifsnider  whom also deeply studied damage development .
The phenomenon cannot be described easily as in metals because of the
complexity of the internal microstructure. The most important consequence of
this complexity lies in the loss of a well-defined damage state, which cannot be
considered like a single crack in homogeneous materials. Reifsnider in his introduction
highlights the difficulty to define the damage condition in composite.
Generally fatigue in composites consists of a wide range of damages like fibre
cracking, matrix cracking, crazing and yielding, debonding, delamination and
void formation, all of them previously seen for fibre and polymers. But what
actually happens is a general condition resulted by their interactions of all of
those rather than a simply collection.
Whilst in an isotropic material the crack growth is controlled by the applied
stress and the geometry, for an anisotropic material the stress distribution at the
crack tip is influenced by the anisotropy, hence the crack propagation can follows
different paths sometimes hard to predict. The different properties between fibre
and matrix, which are approximately represented by a strength ratio 50:1 and a
stiffness ratio 100:1, lead a crack, formed in the matrix, to propagate in the weak phase and to be arrested or diverted as soon as it is stopped by the reinforcing
phase, like a fibre can be. According to this, depending on the bond strength
at the interface, the majority of cracks can be stopped without propagate for a
large distance, thus it is easier to have many small cracks rather than a single
large crack, and so it is not possible to describe the process wi...
... middle of paper ...
...program was to validate the database and the analysis methods used
to predict blade structural performances. Authors developed a composite-beam
structural element representative of wind turbine blades starting from the beam
theory for a general design, via the finite element analysis to the test of many
beam configurations. They also recommended some guidelines for a ply drop
study summarized as follow:
For the same ply drops, thicker laminates are better for resisting delamination.
Dropping more than one ply at the same location increases the delamination
Internal ply drops are more resistant to delamination than external ply
drops. (Single internal ply drops probably will not delaminate prior to
section fatigue failure.)
Beams with ply drops delaminate at similar strain levels, but significantly
higher delamination rates than observed for coupons.
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