The Meaning of Life
Jamie Johnson
BEng MSc CEng MIMechE
C 130 P
C-130
Program Di
Director
t
Cascade Aerospace
[email protected]
Scope
•
•
•
•
•
•
Concept of fatigue
Regulations
g
& fatigue
g management
g
Usage monitoring
Fatigue testing
Damage patterns
Options
Structural Fatigue
• Degradation of strength & stiffness of a
structure as a result of repeated application of
loads during in-service operations
Fatigue
• Due to load ‘cycles’ rather than ‘steady state’
loads
– Rate of load application not important
– Damaging loads can be much less than static limit
loads
• Fatigue damage is ‘cumulative’
Fatigue
• ‘Damaged’ item may retain original static
strength until:
– Local stress concentration causes crack to grow
rapidly
– Crack causes significant loss in area
Fatigue
• ‘Fatigue exhibits ‘scatter’
– Failure of a given component under a given load
will not always occur at the same time
– Distribution tends to be ‘Log Normal’
Fatigue
• Local Cracks
– Single cracks appearing at areas of local stress
concentration
– Behaviour can be observed from test or predicted
analytically
l i ll
– If crack location is known then it can be inspected
f [or
for
[ partt replaced]
l d]
– Effect on structural integrity can be analysed
Fatigue
• Widespread Fatigue Damage (WFD)
– Multiple cracks appearing at numerous locations
– Behaviour can be observed in tests, but is almost
impossible to predict analytically
– Cracks can ‘interact’ such that small flaws can
become critical rapidly
– Effect on structural integrity can be equally
difficult to predict
Regulations & Fatigue
Management
• Regulations applied to prevent local cracks
becoming critical & to retire aircraft before the
occurrence of WFD
• Fatigue Management policies generally based
on Full Scale Test (FST)
Regulations & Fatigue
Management
• Safe Life based on retiring aircraft/components
before a significant probability of WFD exists
– Valid up to 1/3 Test Life
Regulations & Fatigue
Management
• Damage Tolerance: an inspection-based
method of finding cracks before they become
critical
– Initial Inspection based on half the time for initial
flaw (ai) to grow to critical crack length (aCR)
– Repeat Inspection based on half Initial or half the
time
i for
f a detectable
d
bl crackk (a
( NDI) to become
b
critical
ii l
(whichever is least)
Regulations & Fatigue
Management
Regulations & Fatigue
Management
Regulations & Fatigue
Management
FAA Limits
• AD 2011-09-04 Centre Wing Inspection
– 40K AFH repeat 10K AFH
• Rule $121.1115 WFD LOV
– 50K AFH/20K Cycles
y
Military Usage
• Mission by Mission;
– Usage is defined in Equivalent Baseline Hours (EBH)
– EBH = Airframe Hours (AFH) x Severity Factor (SF)
• Flight
g byy Flight
g
Usage; Flight by Flight
Measured Flight Data
Crack Growth Algorithm
Inspections
Usage;
Mission by Mission
Sample Flight Data
Define Mission Profiles
Define Mission Mix
Single Mission Usage
Mixed Usage “Baseline”
Mission Crack Growth
SEVERITY
FACTOR
Baseline Crack Growth
Inspections
Usage
•Severity Factor dependant on;
–Take Off Weight
g
–Altitude
–GAG
GAG Cycles (Mission Length)
–[Airspeed]
–[Environment]
–[Manoeuvre]
Usage
Mission Type
Utilization
%
AFH
Severity
Factor
EBH
Long Range Logistics
50
8 000
8,000
0 75
0.75
6 000
6,000
Short Range Logistics
25
4,000
3
12,000
o g Range
a ge Tactical
ac ca
Long
12.5
5
2,000
,000
2
4,000
,000
Short Range Tactical
12.5
2,000
4.5
9,000
Total
100
16,000
1.94
31,000
Usage
• LM/USAF Baseline becoming C-130 standard
• SB 82-788 Wing
g Operational
p
Usage
g & Service
Life Assessment
• Full LM Usage Assessment (or DaDTA)
USAF Wing
Durability Test
• Conducted by Lockheed Martin on behalf of
USAF:
–
–
–
–
–
1998-1992
Tip to tip
B/E/H center wing
H84 outer wing
USAF spectrum
Economic Life
• As aircraft ages more cracks are likely to
occur:
–
–
–
–
More inspections
More rectification
Higher costs
Lower availability
Options
•
•
•
•
Inspect
Rework
Refurbish
Replace
Inspections
• Aim to detect smallest flaw possible; in order
to gain largest interval
• Best techniques are most intrusive
• Practical limitations on how many repeat
inspections can be carried out
• LoV provides backstop
Rework
• Rework or replace elements where localized
cracking is known to occur:
–
–
–
–
–
Reduces future inspection and maintenance burden
Reduces probability of major repairs
Increases availability
Does NOT increase Service Life
Improvement factors not recognized by all
certification agencies
WASP
• Wing Availability &
Sustainment Program
(WASP)
• SB82-771: recommends
rainbow fitting
replacement at or before
25,000 EBH
Refurbish
• Replace all fatigue-prone structure (up to 90%)
• Assembly
y techniques
q
and processes
p
are critical
• Final product has to certified and given
Service Life
• NRE is significant; low volume costs will
approach or exceed replacement assemblies or
aircraft
Replace
• New center wings in production
– Incremental design improvement incorporated
– Extended Service Life wing
• Installation p
process and equipment
q p
defined
– Reduced NRE
• OEM-certified product
• Used aircraft and structural assemblies readily
available
Replace
Case Study
L
Lower
Fwd
F dS
Spar C
Cap
Splice Tee
Engine Mount Truss
Lower Cap
AL Web
CWS 174.30L
Steel Web
VIEW LOOKING AFT - LH
CWS 178.78L
Summary
• Structural Fatigue principle driver of Service
of Life
• Usage Evaluation critical for comparison
• Rework activities preventative measure against
significant repairs
Questions?