Section 9.5. Life Sensitivity Analysis for Hole Repair
Because holes are stress concentration sites, it is not
surprising that a large number of holes are drilled oversize and repaired to
remove crack indications identified during inspection. It is not possible to conduct a detailed
damage tolerance analysis on every repair of this type; however, engineers can
assess the life of many components before and after
the hole is enlarged using Equation 9.3.1 and its integral counterpart Equation
9.3.12. Detailed evaluations
should always be conducted for critical locations; in some cases, the detailed
evaluations will become the building blocks for other simplified repair
analyses.
Hole repairs are made to remove crack indications from the edge
of the hole. Several example damage
tolerance analyses are presented in this section to summarize the effect of
oversizing the hole to remove some (but not all) of the crack damage. Practically speaking, the objective is to
remove all the crack damage. But,
because non-destructive evaluation (NDE) capability is what it is, the analyst
can not presume that all traces of the crack are removed when the hole is
oversized. From an economics and safety
viewpoint, all traces of the crack should be removed and the aircraft restored
to its original condition. When
conducting a damage tolerance analysis to protect safety, it is wise to error
on the conservative side in defining the initial crack size after a hole
oversizing operation.
Before introducing the example analyses, it is instructive to
review the integral counterpart of Equation 9.3.1, i.e. Equation 9.3.12, which
is presented as Equation 9.5.1
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(9.5.1)
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or
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(9.5.2)
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The parameter b
is the geometry correction factor that is normally a function of crack
length. We again note that the integral
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(9.5.3)
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is dependent of stress effects and is only dependent on the
geometry of the structure and of the crack.
So if the stress parameter, i.e., the stress history, is constant, then
the impact of geometry changes on life can be assessed by studying the
variation of I as the geometry
changes. The following example will be
used to illustrate this point.
then calculate the life required to grow a thru-thickness crack
from several initial crack sizes to a 0.550 inch long radial crack. Assume the stress is 30 ksi.
SOLUTION:
The integral counterpart of the
growth rate equation for this problem is
where b
is associated with the radially cracked hole geometry, (see Section 11):
where
The life results for several initial crack lengths are
presented in the following table.
Crack Growth Life as a Function of Initial Size for af = 0.550 inch
ao
(inch)
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=30 ksi
|
=50 ksi
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Life
(Flight hours)
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Life Ratio
|
Life
(Flight hours)
|
Life Ratio
|
0.001
|
7715
|
1.48
|
2041
|
1.47
|
0.005
|
6720
|
1.29
|
1767
|
1.27
|
0.010
|
6364
|
1.22
|
1696
|
1.22
|
0.025
|
5806
|
1.11
|
1538
|
1.11
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0.050
|
5220
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1
|
1386
|
1
|
0.075
|
4796
|
0.92
|
1270
|
0.92
|
0.100
|
4395
|
0.84
|
1164
|
0.84
|
0.125
|
4023
|
0.77
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1070
|
0.77
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It is important to note that the life ratios generated by
dividing all the life values by the life value associated with ao = 0.050 inch is
independent of stress level, as shown by the results for of 30 ksi and
50 ksi.
Equation 9.5.3 can also provide a simplified method for
determining the effect of increasing the diameter of a cracked hole. Consider Figure 9.5.1,
which defines the three stages associated with increasing the hole diameter to
remove a pre-existing crack. One of the
first steps in the analysis is to obtain an estimate of the initial structural
life (this life is referred to as the DTA result or the Blueprint life). For purposes of this analysis, the DTA
result is presumed available for the region of interest.
Figure 9.5.1. Three Stages in the Life of a Cracked Hole
As indicated in Example 9.5.1, the larger the initial crack size, the
shorter the life. Thus, the decision of
choosing the initial flaw size after over-sizing is an important one - both for
economy and for safety. For consistency
of analysis with JSSG-2006 requirements, it is recommended that crack sizes be
no smaller than that associated with initial manufacturing. An example problem is presented later in the
section to consider the influence that the initial post rework crack size has
on the remaining structural life.
First, let us consider the influence that the reworked oversized hole
has on life relative to that of the initial hole.
EXAMPLE
9.5.2 Effect of Rework Hole
Size on Life
In
this example, the blueprint diameter is 0.250 inches and the final crack length
is 0.550. For comparative purposes, the
initial crack length (both manufacturer’s and post rework) is 0.050 inches and
is assumed to be a through-thickness crack.
The figure shows a description of the geometrical conditions both
initially and post-rework.
Geometrical Parameters
Associated with Blueprint and Post Rework Crack Configurations
Present a comparative life
analysis that defines the effect of enlarging the 0.250 inch diameter hole to
larger sizes during repair of hole crack damage. Allow the crack growth rate exponent p to vary from 2.5 to 3.5.
Assess the effect of the exponent p
on the results.
where Ir
is the value of Equation 9.5.3 for radius r.
Comparative Analysis To
Determine the Effect Of Enlarging the Hole
(Initial Crack Length = 0.050 Inch)
Initial Hole Radius (inch)
|
Rework Change in Radius (inch)
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Final Crack Hole Radius (inch)
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% Life Reduction
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p = 2.5
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p = 3.0
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p = 3.5
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0.125
|
0
|
0.125
|
0
|
0
|
0
|
0.125
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1/64
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0.140625
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9%
|
10%
|
14%
|
0.125
|
1/32
|
0.15625
|
17%
|
20%
|
20%
|
0.125
|
3/64
|
0.171875
|
23%
|
27%
|
29%
|
0.125
|
1/16
|
0.1815
|
27%
|
31%
|
35%
|
0.125
|
5/64
|
0.203125
|
34%
|
39%
|
46%
|
0.125
|
3/32
|
0.21875
|
38%
|
43%
|
53%
|
0.125
|
7/64
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0.234375
|
41%
|
46%
|
59%
|
0.125
|
1/8
|
0.250
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44%
|
49%
|
66%
|
0.125
|
9/64
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0.265625
|
47%
|
53%
|
72%
|
0.125
|
5/32
|
0.28125
|
50%
|
56%
|
77%
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