Residual Strength and Fatigue Lifetime of ... - Solid Mechanics

Figure 3.19 shows the determined energy release rate vs. load curves for the panels with 100,
200 and 300 mm diameter. The fracture toughness values shown in Figure 3.19 were determined
from the MMB tests at a -20 phase angle, which is close to the numerically determined phase
angle at the experimental debond propagation load, described previously. It appears that the
energy release rate increases significantly at a load level which can be associated with the
buckling of the debond. The horizontal line in the diagrams shows the average fracture toughness
of the interface of the panels for the determined phase angle. The point where the fracture
toughness line and energy release rate curve intersect eachother indicates the debond propagation
load.
(a) (b) (c)
3.19: Energy release rate vs. load for panels with (a) PMI (b) H130 and (c) H250 core.
Numerically predicted and experimentally determined propagation loads for the debonded panels
are listed in Table 3.7. Due to large scatter in the measured interface fracture toughnesses, the
crack propagation load was determined for a maximum, average and minimum measured
fracture toughness level. It is seen that based on the average of the measured fracture
toughnesses level, the FEA predictions are 7-46% higher than the experimental ones. For the
minimum fracture toughness the deviation is between 3-33% and for the maximum 14-65%. This
deviation may be partly due to differing crack tip details and crack growth mechanisms between
the panels and the MMB specimens. The distortions in the debond crack tip are due to
mechanical releasing of the debonds, making the debonds not perfectly circular, as well as some
partial adhesions in the debonded area in the experiments as mentioned earlier, which are not
taken into account in the finite element modelling. A further cause of inaccuracy in the
predictions could be different debond growth mechanisms between the panels and the MMB
fracture toughness characterisation tests. To investigate this issue some of the tested panels were
cut, and debond propagation paths in the 0 debond front location where the debond starts to
propagate were compared with the MMB specimens tested under mode I dominated loading.
Figure 3.20 shows debond propagation paths in the panels and the MMB specimens. In the
panels and the MMB specimens with H130 and PMI cores the debond kinks into the core and
propagates beneath the face/core interface. However, in the panels and the MMB specimens with
H250 core the debond propagates directly in the interface, which can be explained by the higher
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