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

Load (kN)
2
1.5
1
0.5
0
Panel 1
Panel 2
Crack propagation point
0 1 2 3
Displacement (mm)
Figure 5.39: Axial piston displacement vs. load diagram from the static tests.
To obtain stable crack growth 80% of the static crack propagation load was applied in the fatigue
tests as maximum cyclic load with a loading ratio of R=0.1 and a frequency of 2 Hz. Load
controlled fatigue tests were performed on the debonded panels and a DIC system was used to
monitor the debond shape and the crack growth. Four images every second were captured from
the surface of the panel every six hundred cycles by the DIC cameras. The debond front position
was located using the out-of-plane displacement contour from the surface of the panels. Locating
the debond front from the out-of-plane displacement of the debond is not as accurate as the
method used in the previous section for the measurement of the crack growth in STT specimens,
which was based on monitoring the crack tip and the strain concentration at the crack tip.
However, since it was not possible to gain access to the crack tip due to closed debonding, the
best possible option was to use the out-of-plane displacement contours and iso-surfaces to
monitor the debond growth, as shown in Figure 5.40. Pre-cracking was performed to penetrate
the resin accumulation at the predefined crack tip by loading the specimens up to 50-60% of the
static crack propagation load by cyclic loading. Pre-cracking was stopped after approximately 5-
7.5 mm crack growth. Following the pre-cracking the specimens were loaded up to 100000
cycles until the debond reached the edges of the panels. Fatigue crack growth vs. cycle diagrams
for the debonded specimens are presented in Figure 5.41. It is seen that initially the crack growth
rate is large, but decreases as the crack propagates due to development of membrane forces
resulting in a decreasing energy release rate at the crack tip.
To investigate the crack growth paths after the panel tests, all tested panels were cut. Figure 5.43
illustrates the crack growth path in the sandwich panels at zero and ninety degrees, as shown in
Figure 5.42, along the debond front. During the pre-cracking and initial loading cycles the crack
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