Aircraft engine components invariably are subjected to mean stresses
over and above the cyclic loads. In monolithic materials, it has
been observed that tensile mean stresses are detrimental and compressive
mean stresses are beneficial to fatigue life in comparison to
a base of zero mean stress. Several mean stress models exist for
monolithic metals, but each differ quantitatively in the extent
to which detrimental or beneficial effects are ascribed. There
have been limited attempts to apply these models to metal matrix
composites. However, since most of the fatigue data have been
limited to tension-tension loading, the range of mean stresses
over which models could be assessed has been limited. In this
work, a unidirectional, SiC/Ti-15-3 composite was tested with
both tension and compressive stresses, thus extending the range
of imposed mean stresses. It was shown that tensile mean stresses
were detrimental and that compressive mean stresses were beneficial
to the fatigue lives.
At the NASA Lewis Research Center, several mean stress models--the Smith-Watson-Topper, Walker, Normalized Goodman, and Soderberg models--were examined for applicability to this class of composite materials. The Soderberg approach, which normalizes the mean stress to a 0.02-percent yield strength, was shown to best represent the effect of mean stresses over the range covered. The other models varied significantly in their predictability and often failed to predict the composite behavior at very high tensile mean stresses. This work is the first to systematically demonstrate the influence of mean stresses on metal matrix composites and model their effects. Attention also was given to fatigue-cracking mechanisms in the Ti-15-3 matrix and to micromechanics analyses of mean stress effects.
Previous articleLast updated May 5, 1997
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