Currently, within the Air Force, airframe life predictions are
based on a crack growth damage integration package that uses a data base and
analysis to interrelate the following six elements:
a)
The initial flaw distribution which accounts for size
variations and location of cracks in a given structure;
b)
aircraft usage describing the load spectra data base;
c)
constant amplitude crack growth rate material properties
accounting for stress ratio and environmental effects;
d)
crack tip stress intensity factor analyses which account for
crack size, shape, and structural interactions;
e)
damage integrator model which assigns a level of crack growth
for each applied stress application and accounts for load history interactions;
and
f)
the fracture or life limiting criterion which establishes the
end point of the life calculation.
Prior to describing each of the above itemized elements in
separate subsections, the damage integrating equation will be introduced to
show how the various elements interact.
As expressed in a numerical form, the damage integrating equation is
|
|
(2.4.1)
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where Daj
is the growth increment associated with the jth
time increment. The purpose of Equation
2.4.1 is to determine the life tf. The various elements affect the quantities
in Equation 2.4.1 in the following manner:
1.
acr is
determined interrelating elements b, d, and f.
2.
ao is
determined using element a.
3.
Daj is
determined by interrelating elements a, b, c, d, and e.