12-designallowables

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


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


(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


Stre

ss, k

si

Yield stress


(c) Some clad aluminum alloys


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


Time, hours

Allowables are Required by FAA toCertify all Aircraft Structures

FAA requirements

FAR 25-613

Material strengthgand design values


Material Design Allowables

Strength properties (reduced)Ftu, Fty, Fsu

Elastic properties (typical)F, G, u

Mechanicalproperties

Physical properties (typical)W, k


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


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.


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


(initial designs)

Material Property Population Distributioin

Frequency

Typical PropertyA B


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


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


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


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


Preferred DFR Utilization

Use servicedemonstrated DFRMost preferred

TestTestdemonstrated DFR

Adjust above DFRs forsimilar details withmodification factors

Determine DFR


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


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


panel model

Material Crack Growth Rating

Material A

Material BMaterial BGrowthrate

dL

Standard1

pdLdN

KmaxZp

M= n 10-4dN

MA MB


Stress intensity, K

M - Measures relative material resistance to crack growth

12-designallowables

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