Section 9.3.3.1. Generation of Crack Growth Curves
Crack growth life curves were
generated for the three transport wing stress histories using a crack growth
analysis computer code. The material
chosen for the study was a 7075-T651 aluminum alloy; the associated constant
amplitude crack growth rate curve was
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(9.3.9)
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with KC = 68 ksi Öin
and R = -0.12. The Willenborg-Chang retardation model
embedded within the software was used to account for load-interaction
effects. These modeling choices affect
the absolute accuracy of the crack growth predictions but not the implications
of the analysis which are presented in a relative sense.
Rather than
dealing directly with the actual structural geometries for the three wing
locations, it was decided that the crack growth analysis would be applied for a
common geometry for all three stress histories. This choice does not affect the crack growth rate analysis as
will be further discussed below. The
choice of common geometry for all three stress histories makes it possible to
evaluate the relative effects of per flight and per cycle damage for the
analyses. It was decided also to choose
a simple geometry of a four (4) inch wide center cracked panel, giving a stress-intensity
factor coefficient of
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(9.3.10)
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The initial and final crack length chosen for the
configuration were 0.11 and 1.25 inch, respectively. Figure 9.3.3 summarizes the common
configuration employed in this analytical study.
Figure 9.3.3. Common Geometry Used to Evaluate Stress
History Effect on Crack Growth Behavior
Figure 9.3.4 presents both the crack
growth life curve and its crack growth rate counterpart for the center wing
stress history. The crack growth rate
curve was generated by forming the secant defined slope for consecutive points
on the life curve and relating this slope to the stress-intensity factor
calculated using the mean crack length and the RMS maximum stress values (given
in Table 9.3.3). The stress-intensity
factor in Figure 9.3.4 is given by
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(9.3.11)
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where (smax)RMS
= 8.0 ksi, and (K/s) is given by Equation 9.3.10. The curve through the center of the points
in Figure 9.3.4 is the mean trend curve that
connects all the points.
Figure 9.3.4. Crack Growth Behavior for the Center Wing Location
Figure 9.3.5 presents the crack
growth life curves generated for the other two wing locations, again using the
computer code. Figure
9.3.6 summarizes the crack growth rate behavior associated with all three
stress histories. The inner and
outboard wing crack growth rate data points were also generated by the secant
method of analysis. The RMS maximum
stresses used for the stress multiplier in Equation 9.3.11 were 7.24 and 8.01
ksi for the inner wing and the outer wing location, respectively.
Figure 9.3.5. Flight-by-Flight Crack Growth Life Behavior
for Inner Wing (WS-733) and Outboard Wing Stress Histories
Figure 9.3.6. Flight-by-Flight Fatigue Crack Growth Rate
Behavior for Three Transport Wing Histories