Fatigue behaviour of composite tubes under multiaxial loading

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Fifth International Conference on Fatigue of Composites
Fig. 6. Influence of the environment on NR cracking.
Fig. 7. Combined influence of the temperature and of the environment 3- Propagation mechanisms of cracks within natural rubber.
By fractography, different cracking mechanisms due to fatigue were observed. The evolution of the
micro-structure is important depending on the propagation rates. When the cracking rates are low (at the
cracking threshold), the mechanisms become fragile. Secondary micro-cracks get multiplied. For faster
rates, we can observe more facets and a rougher relief. The existence of fatigue striations can be
observed for high cracking rates (10 -3 mm/cycle) (see Figure 8).
Fracture mechanisms obtained when R = -1 present some small transformed zone, due to the high
compression levels obtained, leading to a significant chemical deterioration.
The more we increase ratio R, the more wrenching we can observe. Nevertheless, their distribution is
more homogeneous. We can also notice the presence of many cupules of about 1 µm.