Understanding Damage Mechanisms in Ferritic/Martensitic Steels Page: 2 of 6
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The LF model is, by far, the easiest method to use. One only needs knowledge of the component
history and temperature-stress-life relationship derived from uniaxial tests at constant conditions.
A damage factor, DLF, ranging from zero to one is calculated by summing the life fraction used at
each service condition:
DLF = t,/tri
Where t; is the time at any temperature and stress and tri is the time to rupture at that temperature
and stress. The remaining life fraction is (1-D). The order of summing is not important. The
time to rupture for each service condition may be interpolated from isothermal stress-rupture
correlations for a specific heat, calculated from a stress-temperature-life parametric fit to the
specific heat, or interpolated from parametric curves representing the average strength properties
for the steel. A consistent multiaxial stress criterion is necessary but creep rate data are not
needed.
The MG model requires that a sample be extracted from the exposed material and subjected to a
creep test at a temperature and stress within the range of interest. The observed minimum creep
rate (mcr) can then be used to estimate the rupture life, tr, from a simple correlation for the
material:
tr = A mer?
where A and p are experimentally determined materials parameters. Here, it is assumed that A
and p do not vary with temperature and stress. A multiaxial stress criterion must be assumed.
The development of the OM model was reviewed by Prager (4) who cites six capabilities of the
model that include the prediction of the creep curve, application to specific heats, prediction of
remaining life without knowledge of history, generalization to multiaxial stress states, selection of
benchmark tests for conditions close to service conditions, and incorporation in finite element
analysis. The Omega concept may be expressed in several ways, but to be consistent with the
notion of life fraction or damage parameter, Dom:
Dom = e Q t / (1 + Qt),
where e is creep rate based on true stress and true strain, t is service time, and Q is a "materials
creep damage susceptibility parameter." The Q parameter represents the combined effect of area
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Swindeman, R. W.; Maziasz, P. J. & Swindeman, M. J. Understanding Damage Mechanisms in Ferritic/Martensitic Steels, article, April 22, 2003; Oak Ridge, Tennessee. (https://digital.library.unt.edu/ark:/67531/metadc785659/m1/2/: accessed April 19, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.