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

H250 Specimen
Fibre bridging
Figure 5.10: Static crack growth path for an STT specimen with H250 core at the beginning
and end of the test.
Table 5.2: Static crack initiation and propagation load.
STT Specimen Static crack initiation load (kN) Static crack propagation load (kN)
H250 core 1.08±0.07 0.63±0.06
H100 core 1.05±0.06 0.56±0.09
H45 core 0.45±0.09 0.31±0.04
Using the static tests results and a few trial specimens, reasonable load levels for the fatigue tests
were designated to have a stable crack propagation. 80% of the static propagation load of the
STT specimens was used as the maximum fatigue load with a loading ratio of R=Fmin/Fmax=0.1
and frequency of 2 Hz. Load controlled fatigue tests were conducted on the STT specimens and
two specimens of each core type were tested. To break the initial resin accumulation at the predefined
crack tip, pre-cracking was performed on the specimens. A cyclic load, at approximately
50-60% of the static crack propagation load, was applied to the specimens to break the blunt
crack tip, which resulted in approximately 5-10 mm crack propagation. When all the specimens
were pre-cracked, the crack tip was located underneath the face/core interface in the core. The
following crack growth paths were observed in the fatigue experiments:
1. For the specimens with the H45 core, unstable crack growth occurs initially and the crack
propagates up to a length of 150 mm in a few cycles, see Figure 5.11. After the unstable
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