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

propagation, the crack continues to propagate in the core underneath the resin rich-cells
approaching the interface, which indicates the existence of negative mode-mixity at the
crack tip. At negative mode-mixity the crack tends to kink towards the interface but since
the interface is tougher than the H45 core, the crack is forced to remain in the core unable
to penetrate through the resin-rich cells, see Figure 5.11.
2. For the specimens with the H100 core, the crack propagates initially in the core up to a
length of approximately 120 mm, but eventually kinks into the interface and continues to
propagate directly in the interface, see Figure 5.12. It is seen that the static and the fatigue
crack growth paths for H100 STT specimens are different. No fibre bridging is observed
during the fatigue tests even though a similar mode-mixity exists at the crack tip for a
given crack length for both static and fatigue experiments. The difference in the crack
growth paths can be addressed to the smaller maximum fatigue load level compared to
the critical static propagation load, which fails to provide enough energy at the crack tip
to penetrate the first layer of the face sheet.
3. For the specimens with the H250 core, the crack propagates initially in the core up to a
length of 5-8 mm and then kinks into the interface. The interface crack eventually kinks
into the face sheet, which results in large-scale fibre bridging, see Figure 5.13 and 14. As
the crack continues to propagate in the face sheet, fibre bridging becomes more and more
extensive, resisting the crack growth, and eventually results in crack growth seizure, see
Figure 5.15.
H45 Specimen
Crack underneath the resin-rich
Figure 5.11: Crack growth path in the specimens with H45 core.
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