12-designallowables
TRANSCRIPT
Design AllowablesDesign Allowables
Building Block Approach
Full $Analysis verification
Full-scaletests
$$Component
tests
Sub component tests $$
Design-value development
Sub-component tests
Structural elements tests $$
$
Material property
evaluation
Allowable development
Material specification development
Material screening and selection $
$$
Design and Analysis of Aircraft Structures 12-2
Material screening and selection $
Typical Allowables Test Programfor Metallic Materials
Tension (L) 90 + 10Tension (L)(LT)
90 + 1010
Compression (L)(LT)
1010(LT) 10
Shear (L)(LT)
1010
Bearing, e/D = 1.5 (L)(LT)
1010
jBearing, e/D = 2.0 (L) 10j g, ( )(LT) 10
Design and Analysis of Aircraft Structures 12-3
Typical Tensile Stress-Strain Diagrams
Ultimate tensile stress
Stre
ss, k
si
Yield stress
Proportional limit
(a) Material having a definiteyield point (such as same steels)
tress
, ksi
Yield stress
Ultimate tensile stress
(b) Material not having adefinite yeild point (such as
Strain, in/in
p
St
Strain, in/in
Proportional limit
0.002 offset
y p (aluminum alloys, some magnesiumalloys, and some steels)
i
Primary modulus line
Secondary modulus line
Ultimate tensile stress
Stre
ss, k
si
Yield stress
Ultimate tensile stress
(c) Some clad aluminum alloys
Design and Analysis of Aircraft Structures 12-4
Strain, in/in0.002 offset
Typical Creep-Rupture Curve
Secondstage
Thirdstage
Initialstage
Rupturepoint
erce
nt
Transition pointStra
in, p
e
Slope-minimum creep rateCreep intercept
Design and Analysis of Aircraft Structures 12-5
Time, hours
Allowables are Required by FAA toCertify all Aircraft Structures
FAA requirements
FAR 25-613
Material strengthgand design values
Design and Analysis of Aircraft Structures 12-6
Material Design Allowables
Strength properties (reduced)Ftu, Fty, Fsu
Elastic properties (typical)F, G, u
Mechanicalproperties
Physical properties (typical)W, k
Design and Analysis of Aircraft Structures 12-7
Allowable Classifications
Term Description Sub Group ID
Class Identifies level of approval associated with an I, II, IIIppallowable
, ,
Basis Defines an allowables statistical basis A, B, S
Maturity Defines an allowables level of development Firm, Preliminary, Estimated
Design and Analysis of Aircraft Structures 12-8
Class Descriptions
Class Description
I FAA Certified Allowables: Values have been developed using a process where the FAA has approved the test plan and conformance to the plan has been established.to the plan has been established.
II Generic Allowables: Obtained by following MMPDS-01 and 17 guidelines, support
d l i h d dapproved analysis methods, and are applicable to a range of products.
III Program Specific Point Design: A valueIII Program Specific Point Design: A value established for a specific application and is valid only for that application. May be used for similar application if validated.
Design and Analysis of Aircraft Structures 12-9
Classes of Allowables
"A" BasisA value which 99% of the measured values willexceed associated with a 95% confidence levelA Basis exceed, associated with a 95% confidence level(single loadpath structures)
"B" BasisA value which 99% of the measured values willexceed, associated with a 95% confidence level(multiple loadpath structures)
Associated with a unique structure andvalid only for that particular configuration
Pointdesign
A value associated with specification acceptancevalues. No statistical significance may be assumed(initial designs)
"S" Basis
Design and Analysis of Aircraft Structures 12-10
(initial designs)
Material Property Population Distributioin
Frequency
Typical PropertyA B
Design and Analysis of Aircraft Structures 12-11
Maturity Description
Term Description Situation
Firm Analysis which is representative of production has been completed on material/structure.
Material specification in placeConsistent product from stable, repeatable process, p p
Preliminary Analysis has been completed on material/structure which has not yet been brought under control per specifications. Also
Preliminary specifications in placeProduct variability; processg p p
applies to values for which all testing requirements needed to obtain "Firm" values have not been completed.
Product variability; process practices continue to be optimized.
Estimated Values provided based on little or no information which represent best estimates of firm values.
No specific requirements
Design and Analysis of Aircraft Structures 12-12
The Allowable Need is Dependent Upon the End User
Material controlled by- Allowables Design/analysis
Vendor Estimated design values
Trade studies initial designdesign values initial design
Preliminary specifications
Preliminary allowables
Preliminary design
Firm designFirm specifications; AMS, MIL, QQ
Firm allowables
Firm design drawing release
– hardware fabrication
Design and Analysis of Aircraft Structures 12-13
Detail Fatigue Rating (DFR) System
Short, medium, longoperating flight profiles 1. Catalog details
Operating requirements Configuration capability
operating flight profiles
Life goal, and desiredreliability
Standardizeddamage model
based onrating system
gbased on test orservice experience
2. Test data for newdetails
Standardized loadconditions
3. Modification of(1) and (2) usinginfluence factors
4. Calculated detail
Good detail
eDamage analysis
quality based onstandard influencefactors
• Material• Fastener fit
Actualquality
ativ
e sc
ale
Fastener fit• Load transfer• Stress concentration• Etc.Margin
Requred quality
Qua
ntita
Design and Analysis of Aircraft Structures 12-14
Durability Coupon Specimens
Basic material properties fatigue specimens
DFR cutoff Open hole
Structural properties fatigue specimens
Sheet specimens Plate/extrusion specimens
D6-4671-861
12 Fastener dogboneOpen hole
15 Fastener lap splice 4 Fastener dogbone
15 Fastener dogbone15 Fastener dogbone
8 Fastener double shear splice
Design and Analysis of Aircraft Structures 12-15
Preferred DFR Utilization
Use servicedemonstrated DFRMost preferred
TestTestdemonstrated DFR
Adjust above DFRs forsimilar details withmodification factors
Determine DFR
Design and Analysis of Aircraft Structures 12-16
Determine DFRanalyticallyLeast preferred
DFR Calculation
Factor
Test data
KSI)
Factor
ax. s
tress
, K
Log (DFR)
Log
(ma
05 )
SN curve
Log
(10
95/95
Design and Analysis of Aircraft Structures 12-17
Log (N, cycles)
Damage Tolerance Coupon Specimens
Test type Specimen Recommended material form
ApplicationPrimary specimen usageCrack
growthResidual strength
4” wide CCT Extrusions and forgings (all forms)
• Simulates forged and extruded structure
16” wide CCTPlate, sheet(all forms)
• Characterizes environmental crack growth properties
• Fits in standard test chamber
Center cracked tension panel (CCT)
24: wide CCTPlate, sheet (all forms)
• 2xxx: spectrum crack growth• 7xxx: constant amplitude and spectrum
crack growth, residual strength
48” id CCTPlate, sheet (2xxx) • 2xxx: Reliable crack growth allowables
L t ti l id l t th t t48” wide CCT • Largest practical residual strength test for high toughness alloys
92” wide CCTPlate (2xxx) • Selected residual strength verification test
for high toughness alloys
S f fEdge crack tension panel 9: wide
ECT
Plate(all forms)
• Simulates short crack performance of plate material
Built-up Skin/stringer panel model
Extrusion forgings, plate, sheet (all forms)
• Verify interaction of material and geometry factors
Design and Analysis of Aircraft Structures 12-18
panel model
Material Crack Growth Rating
Material A
Material BMaterial BGrowthrate
dL
Standard1
pdLdN
KmaxZp
M= n 10-4dN
MA MB
Design and Analysis of Aircraft Structures 12-19
Stress intensity, K
M - Measures relative material resistance to crack growth