4102- chap 11 - damage tolerance
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Chapter 11 - Damage Tolerance 1
MAAE 4102
Engineering Materials
Strength & Fracture
Chapter 11
Damage Tolerance
Professor R. BellDepartment of Mechanical & Aerospace Engineering
Carleton University
© Bell
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Chapter 11 - Damage Tolerance 2
Fatigue Design Approaches
Infinite Life
Unlimited safety – design stresses below fatigue limit
Safe Life
Finite life – safety factor = 20 x design life
Fail Safe Cracks will exist – inspection and repair
Damage Tolerant Refinement of fail-safe philosophy
Use of FM to predict crack growth
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Chapter 11 - Damage Tolerance 3
Damage Tolerant Design
Philosophy based on:
Damage will occur despite all precautions taken
Inspection so that cracks can be detected and repairsmade at appropriate time
Adequate residual strength is maintained in the
damaged structure so that during the period between
inspections when damage is undetected ultimate
failure of the structure will not occur
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Chapter 11 - Damage Tolerance 5
Multiple load pathsDepartment of Mechanical &
Aerospace Engineer ing
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Chapter 11 - Damage Tolerance 6
Leak-Before BreakDepartment of Mechanical &
Aerospace Engineer ing
Leak
KI < KC at a = B
Break
KI > KC
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Chapter 11 - Damage Tolerance 7
Crack ArrestDepartment of Mechanical &
Aerospace Engineer ing
Will the crack arrest?
Dynamic effects
If KI ≈ KIC - fracture will continue due to dynamic effects
If KI ≈ 0.85 KIC – crack may arrest
Assessment require the ARREST TOUGHNESSRequire the Residual Strength Diagrams for
- Part through crack with various a/c ratios
- through crack
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Chapter 11 - Damage Tolerance 8
Residual Strength Diagram Department of Mechanical &
Aerospace Engineer ing
C C a K
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Chapter 11 - Damage Tolerance 9
Effect of Stringers on SIF Department of Mechanical &
Aerospace Engineer ing
a K ref
In an unstiffened plate β =1
In a stiffened plate as a center
crack grows Load is transferred
to the stiffener
Thus the stress in the plate isreduced so β is reduced
The increased stress in the
stringer is reflected in an increase
in the stress concentration L
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Chapter 11 - Damage Tolerance 10
Residual Strength of Stiffened Panels Department of Mechanical &
Aerospace Engineer ing
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Chapter 11 - Damage Tolerance 11
Stringer CriticalDepartment of Mechanical &
Aerospace Engineer ing
The stringer failure line isrepresented in the residual
strength diagram
The failure stress is the
stringer Yield strength / L
This is the stress necessary
to fail the stringer as various
levels of stress is transferredfrom the cracked plate
In the diagram shown a crack
of size as with a stress of
will progress rapidly and will
cause the stringer to fail
The residual strength of the
stiffened panel is thus reduced
to the level of the intersectionof the stringer failure line and
the plate fracture line
This situation is stringer critical
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Chapter 11 - Damage Tolerance 12
Panel CriticalDepartment of Mechanical &
Aerospace Engineer ing
If the stringer is made from a
higher strength material
The failure line is shifted upwards
The intersection with the plate
fracture line moves beyond the
maximum at a/b =1
Thus the arrest capability is restoredbecause the crack with a stress <
will be arrested before the stringer
fails
This situation is plate critical
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Chapter 11 - Damage Tolerance 13
Broken StringerDepartment of Mechanical &
Aerospace Engineer ing
Crack develops at rivet hole at stringer Stringer breaks –carries no load
Load transferred to plate
Therefore plate carries higher load
Thus β is greater than 1 as shown
Problem can now be handled as before
Stringer spacing now defined as b
If stringers have higher stiffness or are
more closely spaced they can carry
more load as are thus more effective at
reducing β
The smaller the fastener spacing the
more effective the load transfer to the
stringers
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Chapter 11 - Damage Tolerance 14
Fracture and Arrest in an Aircraft Structure Department of Mechanical &
Aerospace Engineer ing
D f M h i l &
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Chapter 11 - Damage Tolerance 15
Option for Improvements of Arrest Capability
Department of Mechanical &
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D t t f M h i l &
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Chapter 11 - Damage Tolerance 16
Crack Arrest in a Pipeline
Department of Mechanical &
Aerospace Engineer ing
Ductile fractures in natural gas and
other high-energy pipelines could
be arrested by either:
toughness in the pipe body
or by a crack arrestor device
D t t f M h i l &
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Chapter 11 - Damage Tolerance 17
Crack Arrest in a Pipeline
Department of Mechanical &
Aerospace Engineer ing
D t t f M h i l &
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Chapter 11 - Damage Tolerance 18
Crack Arrest in PanelsDepartment of Mechanical &
Aerospace Engineer ing
Department of M echanical &
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Chapter 11 - Damage Tolerance 19
Options for Crack ArrestDepartment of Mechanical &
Aerospace Engineer ing
Department of M echanical &
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Chapter 11 - Damage Tolerance 20
StopholeDepartment of Mechanical &
Aerospace Engineer ing
Department of M echanical &
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Chapter 11 - Damage Tolerance 21
Hole ExpansionDepartment of Mechanical &
Aerospace Engineer ing
Two Side Access One Side Access
Department of M echanical &
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Chapter 11 - Damage Tolerance 22
Damage Tolerance RequirementsDepartment of Mechanical &
Aerospace Engineer ing
1. Residual strength diagram as a function of crack size
2. The permissible crack size
3. The crack growth time H
4. The size of a pre-existing flaw
5. The interval of inspection, proof testing, replacement
To insure damage tolerance the following is required:
Department of M echanical &
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Chapter 11 - Damage Tolerance 23
Damage Tolerance RequirementsDepartment of Mechanical &
Aerospace Engineer ing
Residual strength diagram as a function of crack size
Select the minimum residual strength
The permissible crack size p is obtained
Larger cracks will result in a residual strength < p
Cracks > ap not permitted
The crack growth time H calculated using a life
prediction program The size of a pre-existing flaw is necessary to carry out
this calculation
H is the interval of inspection, proof testing,
replacement
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Chapter 11 - Damage Tolerance 24
Fracture Prone StructuresDepartment of Mechanical &
Aerospace Engineer ing
Many low stress fractures during early years of the industrial era
Improvements made because of better materials and better details
Modern era produced increase in fracture prone structures
hostile environments – Arctic, offshore
aircraft, nuclear plants
high quality materials and high operating stresses
Refined stress analysis (FEM) materials used closer to their limits
Remedy
improve materials
better detail design
Results in a vicious circle leading to increased risk of cracking
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Chapter 11 - Damage Tolerance 25
Options to Improve Damage Tolerance
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The time for fracture control is H
ap is permissible crack size
ad is detectable crack size
If H is too short the following ways are
open for improvement
Use of better material with improved properties
Selection of better inspection Procedure
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Chapter 11 - Damage Tolerance 26
Options to Improve Damage Tolerance
Department of Mechanical &
Aerospace Engineer ing
Use of better material with
improved properties
Selection of better inspection Procedure
Department of M echanical &
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Chapter 11 - Damage Tolerance 27
Options to Improve Damage Tolerance
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Redesign and lower stress
Provision of redundancy and crack arresters
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Chapter 11 - Damage Tolerance 28
Options to Improve Damage Tolerance
epa t e t o ec a ca &
Aerospace Engineer ing
I Lower design Stress
II Smaller Initial flaw size
III Small improvement in
toughness
IV Large improvement in
toughness
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Chapter 11 - Damage Tolerance 29
Options to Improve Damage Tolerance
p
Aerospace Engineer ing
I Lower design Stress
II Smaller Initial flaw size
III Small improvement in
toughness
IV Large improvement in
toughness
Department of Mechanical &
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Chapter 11 - Damage Tolerance 30
Options to Improve Damage Tolerance
p
Aerospace Engineer ing
I Lower design Stress
II Smaller Initial flaw size
III Small improvement in
toughness
IV Large improvement in
toughness
Department of Mechanical &
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Chapter 11 - Damage Tolerance 31
Options to Improve Damage Tolerance
p
Aerospace Engineer ing
I Lower design Stress
II Smaller Initial flaw size
III Small improvement in
toughness
IV Large improvement in
toughness
Department of Mechanical &
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Chapter 11 - Damage Tolerance 32
Options to Improve Damage ToleranceAerospace Engineer ing
I Lower design Stress
II Smaller Initial flaw size
III Small improvement in
toughness
IV Large improvement in
toughness
Department of Mechanical &
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Chapter 11 - Damage Tolerance 33
Aircraft Damage Tolerance RequirementsAerospace Engineer ing
FAR 25
“An evaluat ion o f the strength, detai l design, and fabr icationmu st show that catastrophic fai lure due to fat igue, corros ion,
manufactur ing defects, or accidental damage, wi l l be avoided
throug hou t the op erat ional l i fe of the airp lane. “
Commercial Aircraft Requirement:Damage must be sustained without failure until the component can be repaired
Military Aircraft Requirement:Damage must be sustained without failure until the economic service life has
expired and the aircraft or component is retired
Damage tolerance can best achieved by incorporating fail-safety features
such as: redundancy
multiple load paths
crack arresters
Fail-safe structures can sustain large damage – but if not repaired failure will occur
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Chapter 11 - Damage Tolerance 34
Aircraft Damage Tolerance RequirementsAerospace Engineer ing
Commercial Aircraft – FAR 25
Analysis must include:
Typical loading spectra, temperatures and humidities expected in service
The identification of principal structural elements and detail design points,
where failure would cause catastrophic failure of the airplane
An analysis, supported by test evidence
Inspections or other procedures must be established to prevent catastrophic failure
Inspection thresholds must be established based on crack growth analysis and/or tests
Assumption that the structure contains an initial flaw of the maximum probable size
as a result of manufacturing or service damage
Damage-tolerance evaluation:
The extent of damage for residual strength evaluation at any time within theoperational life of the airplane must be consistent with the initial detectability
and subsequent growth under repeated loads. The residual strength evaluation
must show that the remaining structure is able to withstand loads (considered
as static ultimate loads) – limit loads for maneuvering, gust, cabin pressurization
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Chapter 11 - Damage Tolerance 35
Aircraft Damage Tolerance RequirementsAerospace Engineer ing
Commercial Aircraft – FAR 25
Fatigue (safe-life) evaluation option.
Compliance with the damage-tolerance requirements of this section is not
required if the applicant establishes that their application for particular
structure is impractical. This structure must be shown by analysis,
supported by test evidence, to be able to withstand the repeated loads
of variable magnitude expected during its service life without detectable cracks.
In summary the residual strength must not fall below the limit load PL
The limit load can be considered to be the load anticipated to occur once in the
lifetime of the airplane
Thus Ppermissible = Plimit
PP leads to aP : aP is not the critical crack
For failure to occur aP would have to occur at the time as PP – low probability
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Chapter 11 - Damage Tolerance 36
Damage Tolerance RequirementsAerospace Engineer ing
Residual strength diagram as a
function of crack size
Select the minimum residual strength
The permissible crack size p is obtained Larger cracks will result in a residual
strength < p
Cracks > ap not permitted
The crack growth time H calculated
using a life prediction program
The size of a pre-existing flaw is
necessary to carry out this calculation H is the interval of inspection,
proof testing, replacement
Department of Mechanical &
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Chapter 11 - Damage Tolerance 37
Aircraft Damage Tolerance RequirementsAerospace Engineer ing
Commercial Aircraft – FAR 25
To meet the requirements the airplane manufacturer must design in such a way
that cracks can be detected before they reach aP and specify to the operator
how often to inspect
The operator is obliged to follow the inspection schedule
Fracture control by FAR rules is exercised by inspection
The requirement forces tolerance of damage large enough for detection, which
promotes fail safe design with multiple load paths and crack arresters
In a competitive field it is in the manufacturer’s best interest to ensure easy
Inspection – designs with high residual strength and large aP which leads to
long inspection intervals
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Chapter 11 - Damage Tolerance 38
Aircraft Damage Tolerance RequirementsAerospace Engineer ing
Military Aircraft RequirementsRef: MIL-A-83444 and USAF Damage Tolerance Handbook
2 Categories of Defined Damage
Slow Crack Growth - SCG Fail Safe – FS
Multiple Load Path - MLP
Crack Arrest Fail Safe - CAFS
The guidelines stipulate that damage is assumed to exist in
each element of new structure in a conservative fashion
i.e., critical orientation with respect to stress field and in a
region of highest stress.
The structure must successfully contain the growth of the initialassumed damage for a specified period of service, and must
maintain a minimum level of residual static strength both during
and at the end of this period .
Initial Crack Sizes
Formerly for primary damage assumptions,initial crack sizes were prescribed for
MLP, CAFS as 0.02 in and SCG as 0.05;
they have since become negotiable.
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Chapter 11 - Damage Tolerance 39
Aircraft Damage Tolerance RequirementsAerospace Engineer ing
Slow Crack Growth category, SCG
structures are designed such that initial damage will grow at a stable,
slow rate under service environment and not achieve a size largeenough to cause rapid unstable propagation.
damage tolerance (and thus safety) is assured only by
- the maintenance of a slow rate of growth of damage,
- a residual strength capacity and
- the assurance that sub-critical damage will either
- be detected at the depot or- will not reach unstable dimensions within several design life times.
Fail Safe category, - MLP or CAFS
structures are designed such that propagating damage is safely contained
after failing a major load path by load shift to adjacent intact elements or by
other damage arrestment features.
damage tolerance (and thus safety) is assured by- the allowance of partial structural failure,
- the ability to detect this failure prior to total loss of the structure,
- the ability to operate safely with the partial failure prior to inspection,
- and the maintenance of specified static residual strength through this period
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Chapter 11 - Damage Tolerance 40
Aircraft Damage Tolerance RequirementsAerospace Engineer ing
For each structure, evaluation of the following parameters is required:
Design Category
Degree of In-Service Inspectability
Inspection Intervals
Initial Damage, In-Service Damage and Continuing Damage Assumptions Minimum Required Residual Strength
Damage Size Growth Limits
Period of Unrepaired Service Usage
Remaining Structure Damage Sizes
Design Limit Load
The intent of the guideline is to provide for at-least designlimit load residual strength capability for all intact structure,
i.e., for sub-critical damage sizes in SCG structure
and damage sizes less than a failed load path in FS qualified designs.
This requirement allows for full limit load design capability and thus unrestricted aircraft usage.
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Chapter 11 - Damage Tolerance 41
Aircraft Damage Tolerance RequirementsAerospace Engineer ing
Slow Crack Growth
flaws or defects are not allowed to attain the critical size
required for unstable rapid crack propagation.
Safety is assured for specified periods of usage depending upon
the degree of inspectability.
The strength of slow crack growth structure with sub-critical
damage present shall not be degraded below a specified limit for
the period of unrepaired service usage.
This component has local redundancy at
fork end
but must be classed as Slow Crack growth
because of single load path at point B
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Chapter 11 - Damage Tolerance 42
Aircraft Damage Tolerance RequirementsAerospace Engineer ing
Fail Safe structure designed and fabricated such that unstable rapid propagation will be
stopped within a continuous area of the structure prior to completefailure.
Safety is assured through slow crack growth of the remaining structure
and detection of the damage at subsequent inspections.
Strength of the remaining undamaged structure will not be degraded
below a specified level for the period of unrepaired service usage.
It was recognized that multiple load path and crack arrest type structurehave inherent potential for tolerating damage by virtue of geometric
design features.
Structure could qualify as
Fail-Safe Multiple Load Path
Damage in skin could be arrested at sparBefore coming critical
Design might also tolerate failure of one spar cap
Prior to catastrophic failure
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Chapter 11 - Damage Tolerance 43
Aircraft Damage Tolerance RequirementsAerospace Engineer ing
Inspection Categories and Inspection Intervals
• In-flight evident inspectable - If the nature and extent of damageoccurring in flight will result directly in characteristics which make
the flight crew immediately and unmistakably aware that significant
damage has occurred and that the mission should not be continued.
• Ground evident inspectable - If the nature and extent of damage
will be readily and unmistakably obvious to ground personnel without
specifically inspecting the structure for damage.
• Walkaround inspectable - If the nature and extent of damage is
unlikely to be overlooked by personnel conducting a visual inspection
of the structure. This inspection normally shall be a visual look at the
exterior of the structure from ground level without removal of access
panels or doors without special inspection aids.
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Chapter 11 - Damage Tolerance 44
Aircraft Damage Tolerance RequirementsAerospace Engineer ing
Inspection Categories and Inspection Intervals (cont)
• Special visual inspectable - If the nature and extent of damageis unlikely to be overlooked by personnel conducting a detailed
visual inspection of the aircraft for the purpose of finding damaged
structure. The procedures may include removal of access panels
and doors, and may permit simple visual aids such as mirrors and
magnifying glasses. Removal of paint, sealant, etc. and use of NDI
Techniques such as penetrant, X-ray, etc., are not part of a special visual inspection.
• Depot or base level inspectable - If the nature and extent of damage will be
detected utilizing one or more selected nondestructive inspection procedures.
The inspection procedures may include NDI techniques such as penetrant,
X-ray, ultrasonic, etc. Accessibility considerations may include removal of those
components designed for removal.
• In-service non-inspectable structure - If either damage size or accessibility
preclude detection during one or more of the above inspections.
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Chapter 11 - Damage Tolerance 45
Aircraft Damage Tolerance RequirementsAerospace Engineer ing
Inspection Categories and Inspection Intervals
Typically, these intervals will be approximately
1/4 of the design service life
Visual inspection preferred
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Chapter 11 - Damage Tolerance 46
Aircraft Damage Tolerance RequirementsAerospace Engineer ing
Initial Damage Assumptions
Initial flaws are assumed to exist in the structure.
These flaws are assumed to exist initially in the structure as
a result of material and structure manufacturing and processing.
Small imperfections equivalent to a 0.005“ (0.127 mm) radius corner flaw shall be
assumed to exist in each fastener hole of each element in the structure.
It must then be shown that this initial crack will not grow to a maximum permissible size aP
within the economic service life for CAFS and MLP structures or within twice the life for
SCG structures
Formerly for primary damage assumptions, initial crack sizes were prescribed for MLP,CAFS as 0.02 in and SCG as 0.05; they have since become negotiable.
The flaws are assumed to be located in the most unfavorable orientation with respect to
applied stresses and material properties
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Chapter 11 - Damage Tolerance 47
Aircraft Damage Tolerance RequirementsAerospace Engineer ing
Continuing Damage Assumption
As the primary damage progresses in both the skin and stringer,
eventually the radial crack in the stringer will extend to the edge of
the stringer, shown in cracking sequence (ii).
At this time, a new crack, equivalent to the 0.005 inch radial crack
flaw plus the growth prior to the primary element failure, is assumed
to exist on the diametrically opposite side of the failed hole, asshown in cracking sequence (iii).
This continues the growth process until the complete stringer fails,
shown in cracking sequence (iv).
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Chapter 11 - Damage Tolerance 48
Aircraft Damage Tolerance RequirementsAerospace Engineer ing
The residual strength capability is defined as theamount of static strength available at any time during
the service exposure period considering that damage
is initially present and grows as a function of service
exposure time.
The residual strength guidelines are specified in
terms of the minimum internal member load P xx that
must be sustained. The load P xx is intended torepresent the maximum load that the aircraft might
encounter during the time interval between
inspections.
The required P xx is at least design limit load for all
intact structure whether the structure is being
qualified as Slow Crack Growth or Fail Safe. The
required P xx is also at least design limit load when the
only planned safety inspections are at the depot
Minimum Residual Strength
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Chapter 9 - Damage Tolerance 49
Aircraft Damage Tolerance RequirementsAerospace Engineer ing
Minimum Residual Strength
The value of P xx is established from load spectra data derived from a
mission analysis of the particular aircraftOne way to determine the level of P xx required is to hypothetically increase the
service exposure time for the aircraft between inspections by a factor of M .
The values of M are summarized in the Table
For example, under the ground-evident level inspectability category,
the P GE load is the maximum load expected to occur once in 100 flights
(M x inspection interval = one flight x 100).
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Chapter 11 - Damage Tolerance 50
Aircraft Damage Tolerance RequirementsAerospace Engineer ing
Residual Strength
aaY
A K A R
aaY K
aaY K
ref
IC ref RS
IC R
R IC
)(
)(
1
)(
The residual strength of a component is determined by the crack at the most critical
location.The damage tolerance requirements specify that the residual strength must have a
magnitude exceeding that of the maximum loads, PXX between service inspections.
These loads are determined on the basis of the inspectability and the average
maximum load that occurs once during a period of M times the inspection interval.
The loads, PXX must, however, always be larger than the design limit load.
Thus, the minimum residual strength allowed is given by PXX.
The residual strength, Rs , at any time during a service life may be expressed interms of the crack size, a , and the fracture toughness, KIC . Deterministically:
R is the residual strength per unit
area
Aref is a reference areaY(a) is the geometric factor
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Aircraft Damage Tolerance RequirementsAerospace Engineer ing
Commercial Requirements FAA
Residual strength shall not fall below Limit Load
Promotes Fail Safe design with multiple load paths and crack arrest features
Fracture control exercised by inspectionInspection period <H/2
Arrest of 2 bay crack
Department of Mechanical &
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Aircraft Damage Tolerance RequirementsAerospace Engineer ing
Military Requirements
Multiple Load Path or
Crack Arrest Fail Safe
or
Ai f D T l R i
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Aircraft Damage Tolerance Requirementsp g g
Military Requirements
Slow Crack Growth
Non Fail Safe
or
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References p g g
MIL-A-83444 (USAF), Military Specification - Airplane Damage
Tolerance Requirements, (2 July 1974). Gallagher, J.P., Giessler, F.J., Berens, A.P., Engle, Jr.,
R.M. and Wood, H.A., USAF
Damage Tolerant Design Handbook: Guidelines for the
Analysis and Design of Damage Tolerant Aircraft Structures,
AFWAL-TR-82-3073, Wright-Patterson Airforce Base,
Ohio, 45433, (May 1984).
U.S. FAA, Airworthiness Requirements, FAR 25b, (1978).