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

clamped approximately 5–10 mm from the crack tip to limit the crack extension and ensure a
straight crack front during pre-cracking. The upper debonded face sheet was loaded using a
sinusoidal cyclic loading with a load ratio of R=0.1 and frequency of 2 Hz. A maximum of 50-
60% of the static crack initiation load was applied during the pre-cracking as the maximum
fatigue load to the specimens to avoid large crack increments. During the pre-cracking the crack
always kinked either to the face sheet or to the core. This can be attributed to the tougher
face/core interface compared to the core and face sheet. As to the face sheets it is believed that
the main reason for this is the polyester resin used as the matrix, making the face sheets less
tough than the interface and prone to matrix cracking. To prevent the crack from kinking into the
face sheet and resulting fibre bridging, a downward force is applied to the lower debonded part
of the MMB specimens (core+ lower face sheet) by using a screw type configuration as shown in
Figure 5.24. This small force provokes the crack to kink into the core from the interface by
creating shear forces at the crack tip as shown in Figure 5.25. The crack location and growth was
observed continuously using a calliper with an accuracy of ±0.05 mm during the cyclic loading.
The pre-cracking was stopped after 5-10 mm of crack growth. During pre-cracking the
predefined face/core debond kinked into the core in most of the specimens. However, in few
specimens with H100 core the debond kinked into the face sheet and the specimens were
discarded, see Figure 5.26.
a
Fatigue load
Downward load
a
Figure 5.24: Pre-cracking test setup.
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