A number of experimental and analytical investigations have
revealed that the flight-by-flight crack growth rate behavior of military
aircraft can be described with a power law relationship (Equation 9.2.3). Specifically, the stress histories
considered were developed to facilitate the design of a new structure or an
analysis of an in-service aircraft for force management purposes. As such, these stress histories represented
an expected average usage based on a force wide composite mission mix; most of
the stresses in these histories repeated after an application of a large block
of flights or flight hours. None of the
histories involved any major mission change during the expected life of the
aircraft. For these histories, one
might say that the operations today will be like the operation next year or
five years from now.
Nevertheless, the generalized observations of power law
flight-by-flight crack growth rate behavior here are immediately applicable to
the study of parameters affecting structural repair. Thus, the results of these studies are summarized in Tables 9.2.1 and 9.2.2 for
bomber/transport behavior and for fighter/attack/trainer behavior, respectively. Table 9.2.1
presents the coefficients for a crack growth rate per flight type equation,
while Table 9.2.2 presents the coefficients for a
crack growth rate per flight hour type equation.
The reader can note from Table 9.2.1
that the exponent p for
bomber/transport aircraft wing stress histories only varies from about 3.0 to
3.5; Table 9.2.2 indicates a wider variation in the
exponent for the aircraft and conditions indicated (2.2 £ p £
3.7). Based on a close analysis of the
results, it can be said that the largest variations in the exponent p are generated due to the wide
variations in spectrum content (load magnitude and frequency).
Table 9.2.1. Bomber/Transport Behavior
Aircraft
|
History
|
Flights/Block
|
(ksi)
|
C+
|
p
|
Aluminum Alloy
|
B1-B
|
Wing pivot
|
100
|
27.3
|
4.91x10-8
|
3.025
|
2219-T851
|
C-5A
|
Upper wing
|
100
|
11.7
|
1.70x10-8
|
3.111
|
7075-T651
|
C-5A
|
Lower wing
|
300
|
12.3
|
1.05x10-7
|
3.183
|
7075-T651
|
B-52D
|
Lower wing
|
200
|
16.4
|
2.61x10-8
|
3.529
|
7075-T651
|
KC-135
|
Proof test, Lower wing
|
200
|
17.8
|
5.97x10-9
|
3.454
|
7178-T6
|
KC-135
|
Lower wing
|
200
|
18.4
|
1.01x10-8
|
3.338
|
7178-T6
|
+
inch/flight, ksiÖin
Table
9.2.2. Fighter/Attack/Trainer Behavior
(Based on 1000 Flight
Hour Block Spectra)
Aircraft
|
History
|
C+
|
p
|
Aluminum Alloy
|
T-38
|
Lower wing (baseline)
|
2.66x10-8
|
2.678
|
7075-T651
|
T-38
|
Lower wing (severe)
|
1.07x10-8
|
3.152
|
7075-T651
|
T-38
|
Lower wing (mild)
|
5.32x10-9
|
2.460
|
7075-T651
|
F-4
|
Lower wing (baseline)
|
1.68x10-8
|
2.242
|
7075-T651
|
F-4
|
Lower wing (high stress baseline)
|
1.77x10-8
|
2.242
|
7075-T651
|
F-4
|
Lower wing (severe)
|
1.76x10-8
|
2.395
|
7075-T651
|
F-4
|
Lower wing (mild)
|
5.77x10-9
|
2.395
|
7075-T651
|
F-16
|
Lower wing (mix)
|
6.92x10-10
|
3.62
|
7475-T7351
|
F-16
|
Tail (mix)
|
1.33x10-10
|
3.67
|
7475-T7351
|
F-16
|
Lower wing (air to air)
|
1.07x10-9
|
2.905
|
7475-T7351
|
F-16
|
Lower wing (air to ground)
|
8.94x10-11
|
3.464
|
7475-T7351
|
+
inch/flight, ksiÖin
Before employing a flight-by-flight crack growth rate type
analysis to estimate the life of a repair, the analyst should be concerned with
the adequacy of such an analysis. The
most important part of the analysis is the definition of the stress history
that the repaired member will experience in the future. If the history is anticipated to be
statistically repetitive as a function of time-in-service then the results from
a flight-by-flight rate analysis will be comparable to a cycle-by-cycle
analysis.
If the mission type or mix is expected to change significantly
as a function of time, then projecting a predefined rate of crack growth
without detailed consideration of how the damage will be changing could lead to
non-conservative errors. One method for
addressing mission type or mix changes is to utilize one rate curve before the
time of change and another rate curve subsequently. A more exact method for addressing mission changes is by using a
cycle-by-cycle crack growth analysis applied to the stress history that
accounts for the changes.
Rate methods have one inherent problem: they tend to minimize
the effects of the infrequently applied large loads. These large loads will cause retardation effects and tend to slow
the growth process (if, in application, failure is not induced). Thus rate methods will normally predict somewhat
shorter (more conservative) lives than the cycle-by-cycle analyses.
Based on Tables 9.2.1 and 9.2.2 the analyst should note that the crack growth rate
equation is a function of material, location, and usage. An equation generated for the horizontal
tail should not be used for the vertical tail (nor wing); an equation generated
for air-to-ground operations should not be utilized for air-to-air operations.