Page 392 - Biaxial Multiaxial Fatigue and Fracture

3 76 M. FONTE ETAL.

threshold f& values in vacuum and humid air decrease with increased extent of aging, over
the entire range of load ratios.
The thickness of oxidation products (which has not been determined in this paper) on the
near-threshold fiacture surfaces in the overaged structure may be considered as an indication
of conventional corrosion fatigue processes, i.e. active path corrosion and hydrogen
embrittlement, which will tend to accelerate crack growth. The extend of such embrittlement
is found to depend on both the rate of transport of water vapour to the crack tip and on the
surface reactions kinetics [50,54,56]. It has also been pointed out that the cathodic hydrogen
produced concomitantly with the crack tip oxidation process may be a significant source of
embrittlement in 7075-OA structure [57].
For both aluminium alloy conditions (UA, OA), the crack growth curves, Figs 3 and 4, are
shifted towards lower AK values with increasing R-values. It is interesting to note that both
microstructures show the R-effect on AK, even at higher crack growth rates of io-' dcycle.
Extending these results to much higher growth rates probably leads to observe the R-effects to
be independent of AK which is commonly observed in many alloys. This experimental
observation is consistent with early investigations on the same type of alloy [21-241.
The threshold stress intensity range Mth value of the 7049-UA and 7049-OA material,
which was fatigue tested in ambient air, decreases with increasing load ratio, as mentioned
already. At all ratios, the magnitude of A& decreases with increased aging. Comparison of
the near-threshold fatigue crack growth behaviour obtained in ambient air with the data for
vacuum, however, shows that the presence of humidity leads to a larger reduction of A&, for
the UA microstructure than for the OA condition, at all load ratios. The apparent differences
in the resistance to near-threshold fatigue crack growth of the two aging conditions are
attributed to a complex interplay among several concurrent mechanisms involving moisture-
induced embrittlement, slip characteristics, crack deflection processes and crack closure due
to environment and microstructures factors [18]. This favourable property of the UA alloy
seems to arise from its capacity to produce a highly nonlinear crack profile.
The microstructural differences (UA-OA) manifested in terms of its deformation slip
mode of planar versus wavy, indicate that the resistance to crack growth in planar slip alloy is
significantly better than that of the overaged alloy due to the contributions from crack
branching and environment in the tension-tension load ratio region. In the compression-
tension region, the underaged alloy shows a loss in the fatigue resistance due to a change in
the slip and fracture modes. The apparent differences in fatigue crack growth resistance of the
two aging conditions are ascribed to a complex interaction of several mechanisms: the
embrittling effect of humid air resulting in conventional corrosion fatigue processes, the role
of microstructure and slip mode in inducing crack deflection, and - in an unknown extent -
crack closure arising from a combination of environment and microstructural contributions.
Crack tip branching, deflection and secondary cracking observed in 7049-UA affect crack
tip driving force because Mode I1 and Mode I11 components are superimposed on Mode I
[35]. The mechanisms are important for materials with significant planarity of slip and these
mechanisms can be accentuated by certain environments or microstructures. Thus one can
infer that the role of environment is strongly more significant in the near-threshold regime
than any mechanical contibutions such as plasticity, roughness, oxide, closure, etc. From this,
one may conclude that decreasing with R is an intrinsic fatigue property of the material
for that environment [53].
Results in Table 5 show that the C and rn parameters in the Paris law which are
traditionally considered as a specific property of each material, are significantly different for
each microstructure and environment, either in ambient air or in vacuum.