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

Figure 5.14: Kinking of the crack into the face sheet in an STT specimen with H250 core.
Figure 5.15: Fiber bridging in the rear side of an STT specimen with H250 core.
A 4 Mpix Digital Image Correlation (DIC) measurement system (ARAMIS 4M) was utilized to
monitor 2D surface major strains and locate the crack tip position by the strain concentration at
the crack tip in the measured 2D strain contours. To ensure the accuracy of the measurement a
tape ruler was glued to the bottom side of the specimens to locate the crack tip as well and
measure the crack length by a calliper with an accuracy of ±0.05 mm. Figure 5.16 shows a
majore strain contour from an STT specimen with H45 core used to locate the crack tip position
by the DIC system.
Fatigue crack growth length vs. load cycle diagrams for the STT specimens from both the visual
measurements and the measurements using the DIC system are presented in Figure 5.17. The
crack length measurements by the DIC system agree well with the physical crack measurements,
as shown in Figure 5.17. Crack growth length vs. load cycle results from the two repetitions of
each type of STT specimens are shown in Figure 5.18. It is seen that for all core densities the
crack initially grows fast, but the crack growth rate decreases as the crack propagates further.
This can be attributed to increasing membrane forces and subsequent decreasing energy release
rates for the H100 and H45 specimens and fully developed fibre bridging resisting the crack
growth for the H250 specimens. For both STT specimens with H45 core, unstable crack
propagation was observed during the initial load cycles where the crack propagated unstably
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