The effect of mean stress and environment on corrosion fatigue behavior of 7075-T6 aluminum |
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Authors: | L V Corsetti D J Duquette |
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Affiliation: | (1) Combustion Engineering, 06095 Windsor, Conn.;(2) Materials Division, Rensselaer Polytechnic Institute, 12181 Troy, N. Y. |
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Abstract: | Axial fatigue tests were performed on a 7075-T6 aluminum alloy in tension-compression and under superimposed positive mean
stresses in dry air and in aqueous 0.5N NaCl solution. Both corrosive environments and positive mean stresses resulted in
lower fatigue lives but no interaction between these variables was observed. Crack initiation in air occurred at electropolish
pits at inclusion/alloy interfaces and propagated primarily in a Stage I (crystallographic) mode. Crack initiation in NaCl
solutions occurred at heavily corroded regions surrounding non-metallic inclusions and propagated in a cleavage mode normal
to the direction of applied stress. The relative number of cycles to crack initiation is shown to be a function of the magnitude
of cyclic stress but not of mean stress. Similarly, the percentage of reduction in fatigue life due to corrosive environments
is approximately constant at all mean stress levels. These data indicate that fatigue crack initiation is primarily related
to mobile dislocations associated with cyclic deformation. Crack propagation on the other hand appears to be controlled by
the maximum applied stress. A model for environment assisted cracking is presented which suggests that hydrogen induced cleavage
is responsible for the degradation in fatigue properties of this alloy.
Formerly Research Assistant, Materials Division, Rensselaer Polytechnic Institute, Troy, N. Y. 12181. |
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