Environmental Engineering Reference
In-Depth Information
gives the energy release rate, G as a function of the applied load level. Next, using
the Paris-Erdogan relation, eqn (1), the crack growth rate can be calculated. The crack
growth rate is then used to estimate the size of the crack after some, say 1000 cycles.
The structure is analyzed again with the new, larger crack size, and a new crack
growth rate is calculated, etc. This is repeated until the calculated energy release
rate equals the fracture energy; this corresponds to rapid crack propagation. By
summing the number of load cycles, the reminder fatigue life can be calculated.
Buckling-driven delamination is a particular type of delamination fracture.
Buckling-driven delamination can occur in laminates and sandwich structures
used for blade aeroshells and load-carrying girders. The compressive stresses dur-
ing operation of a wind turbine can drive a stable or unstable delamination. The
overall structural strength will usually be reduced by the presence of delamina-
tions [65, 66]. In general, there are two macroscopic buckling modes observed for
compressively loaded panels that contain delaminations: local buckling and global
buckling (Fig. 20).
When a panel with a delamination is subjected to compressive loading, the plies
on one side of the delamination may buckle. This buckling will then introduce
bending in the remaining plies on the other side of the delamination. The remain-
ing plies will then be subjected to both bending and the compressive loading
resulting in higher stresses than observed without the delamination [66, 67]. This
type of buckling is referred to as a local buckling mode and will typically occur
when the delamination is large and close to one of the surfaces.
When a delamination is small and deep within the laminate, compressive load-
ing can cause global buckling, where the plies buckle in one direction (i.e. on the
same side of the panel).
Buckling-driven delamination growth can only take place when two criteria are
fulfi lled: (1) the transverse constraint is suffi ciently low that a layer adjacent to the
delamination is able to displace in a transverse direction and (2) the energy release
Figure 20: Local and global modes of delamination induced buckling. In the local
buckling mode, transverse displacement of the buckled region occurs
in opposing directions on either side of the delamination. Global buck-
ling involves transverse displacement of the buckled region generally
occurs in one direction (i.e. on the same side of the panel) [68].
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