Page 393 - Biaxial Multiaxial Fatigue and Fracture

ne Environment Effect on Fatigue Crack Gmwth Rates in 7049 Aluminium Alloy at ... 311

CONCLUSIONS
The fatigue crack growth threshold behaviour of an A1 7049 (UA-OA) alloy was studied by
comparing differently aged materials with identical chemical composition and yield strength,
but different microstructure. These two alloys exhibit the same crystallographic texture and
grain morphology, but differ in precipitate microstructure. Experiments at different R-ratios
and different environment (ambient air and vacuum) showed that thresholds depend on the
different microstructure and the associated deformation mechanisms. These are homogeneous
slip in the overaged (OA) condition and localised slip with crystallographic cracking and a
tendency to crack branching in the underaged (UA) material. The microstructural differences
manifested in terms of planar vs. wavy slip, indicate that the resistance to crack growth in the
planar slip alloy is significantly higher than that of the overaged alloy (OA) due to the
contributions from crack branching and environment in the tension-tension load region. In the
compression-tension region, the underaged alloy shows a loss in the fatigue resistance, which
is probably caused by a change in the slip and fracture modes. The second most important
influence comes from the environment. It shows that the threshold cyclic stress intensity
factor is reduced by approximately 50% probably mainly by hydrogen embrittlement. The
overall behaviour is due to the complex relationship between the effect of environment with
microstructure and loading. The key to the understanding such complex mechanisms lies in
quantifying the role of crack tip chemistry and decoupling the role of time dependent
environmental effects on crack growth from the cycle dependent fatigue loading. Such
understanding is only possible through careful systematic measurements of fatigue data under
high vacuum and in the selected environments. The parameters C and m of the Paris law,
traditionally considered as a specific property of the material, significantly differ for each
microstructure and environment.

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