Test Method for Determining Imminent Failure in Metals
Author: Soules, Jack
Source: US Patent 2004/0036470A1
Doc ID: 2004050
Year of Publication: 2004
Abstract:
A method for determining a "safe-operation" point for a
metal structural element subjected to repeated loading, the
same or different, generating variable levels of strain and
residual stress in the worked element; and, predicting the
imminent failure of the structural element. The surface of the
metal element is worked to provide a residual strain, for
example, by shot-peening. Measurements of electrical conductivity
are compared at various chosen frequencies corresponding
to different depths in the "near-surface" of the
element. Similar measurements are made in the near-surface
of a "standard" and a first difference is computed between
the conductivity of the shot-peened surface and the "standard"
surface. This first difference provides a basis for
comparison of the effects of residual stress after successive
loadings of the shot-peened metal element. The effect of
residual stress on the peened portion after it is subjected to
a number of successive loadings is measured and compared
to the reference. When the effect of residual stress is
measured as conductivity or resistivity, it is found that there
is a progressive increase in conductivity (and decrease in
resistivity) as the metal element begins to succumb to
fatigue. It is found that when the resistivity decreases to a
value in the range from about 50% to 60% of the measured
first difference in resistivities, the metal element has reached
its 50% "safe-operation" point beyond which it rapidly,
progresses to fatigue failure.
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