Fatigue behaviour of composite tubes under multiaxial loading

S Stelzer, G Pinter, etc. / Cyclic interlaminar crack growth in unidirectional and braided composites
3.2 Compression after impact testing
In Figure 14 the residual strength of the braided composites is plotted as a function of the impact
energy. Similar results were obtained for the triaxial braids tested in transversal direction and the biaxial
braids which were tested in longitudinal direction. The values of the triaxial braids which were tested in
the longitudinal direction, however, were found to be at a significantly higher level than the triaxial ones
tested in transversal direction. Hence the addition of 0 o yarns leads to an increase of the compressive
strength after impact values when specimens are loaded in longitudinal direction.
4 Conclusion
Fig. 14. Effect of braid architecture on the residual compressive strength of braided composites.
The procedure for the characterization of the interlaminar crack growth in fiber reinforced composites
proved to be an appropriate way to measure the growth of delaminations. The fatigue crack growth
behavior of unidirectionally reinforced polymer laminates with matrices made of either
polyetheretherketone (PEEK) or epoxy resin could be well compared. The machine parameters
frequency, control mode and type of machine showed no significant influences on the results of the
measurements on UD-reinforced epoxy resins. When varying the specimen parameters initial crack
length and thickness, however, the results indicate the need for guidelines concerning these parameters
to be able to receive reproducible results. An inter-laboratory comparison of the results of this kind of
measurements provided promising results. Round robin measurements were initialized to prove these
findings and will be carried out by members of the ESIS TC4. [2, 18]. The interlaminar crack growth
behavior of UD-PEEK samples could be measured reproducible and the PEEK samples proved to be
much tougher than the epoxy resin samples.
Furthermore an experimental investigation of the interlaminar crack growth in braided composites
under fatigue loading conditions showed that crack branching occurs, which increases the dissipation of
fracture energy in the specimen. All the braided samples showed stick slip effects, leading to crack
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