14 Material Allowables

Material AllowablesMaterial Allowables

Class Objectives

• Classes of Allowables• Property DistributionProperty Distribution• Material Specifications• Test Requirementsq

Design and Analysis of Aircraft Structures 14-2

Investigation of Methods and Allowables

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 $

$$


Material screening and selection $

Allowables Ensure the StructuralIntegrity of the Final Product

Allowables Design Requirements• Allowables Design Requirements• Static Allowables Properties and Bases• Allowables Development and PublicationAllowables Development and Publication• Presentation of Design Manuals and

Examples of Use


Allowables Are Required by FAAto Certify All Aircraft Structures

FAA requirement

FAR 25-613FAR 25 613

M t i l t thMaterial strengthand design values


Within Boeing, Allowable Development Is the Responsibility of the Stress Methods and Allowables (SMA) Group

Structural Damage Technology (SDT)

Boeing Materials Technology (BMT)

CentralEngineering

SMASMA

Methods Allowables


Methods Allowables



Examples of Use


Allowables and Design Value

Definitions• Allowables:Allowables:

Mechanical properties having a level of statistical assurance – general caseD i l• Design value:A mechanical property value based on the allowable adapted to the criteria and analysis method used –p yspecific case


Strength Is an Allowables Domain

St th ti ( d d)Strength properties (reduced) Ftu, Fty, Fsu

MechanicalProperties Elastic properties (typical)

E G uE, G, u

Physical properties (typical)Physical properties (typical)w, k


Classes of Allowables

“A” B i 99% b bilit ith 95% fid“A” Basis

“B” B i

• 99% probability with 95% confidence

• 90% probability with 95% confidence“B” Basis

Point

• 90% probability with 95% confidence

• Associated with a unique structure andDesign

“S” Basis

qvalid only for that particular configuration

• Specification acceptance value “S” Basis with no statistical significance—

this is not an allowable!


The Allowable Need is Dependent Upon the End User

Design/analysisMaterial

Allowables Design/analysis

Trade studies

controlled by -

Vendor

Allowables

Estimatedinitial design

Preliminary

Vendor

Preliminaryspecifications:

design values

Preliminary Preliminarydesign

Firm design

specifications:XBMS, XBAC

Firm specifications:

Preliminaryallowables

gdrawing release

- hardware fabrication

Firm specifications:BMS, BAC,

AMS*, MIL*, QQ*

Firmallowables


fabrication

*No longer Government monitored

Material Property Population Distribution

Frequency

A B Typical Property


Probability Distribution

A formula that gives the probability that a value fall within prescribed limitsp

– Weibull distribution– Normal distribution

Lognormal distribution– Lognormal distribution


Unknown Distribution

Nonparametric analysis• “A” basis allowable: lowest of 299 test pointsA basis allowable: lowest of 299 test points • “B” basis allowable: lowest of 29 test points


Allowable Need Is Dependent Upon the End Use

MaterialMaterialcontrolled by Allowables Design/analysis

Vendor Estimated design values

Trade studies’ initial design

Preliminary specifications: XBMS, XBAC

Preliminary allowables

Preliminary design

XBMS, XBAC

Firm specifications: AMS*, MIL*, QQ*,

Firm allowables Firm design drawing release hardware


BMS, BAC fabrication



Examples of Use


Allowables Development Requirements

• Program requestg q• Firm specification• Test program planning• Specimen fabrication and testing• Allowables development

D t D6 57037 “P d f M t i l• Document D6-57037, “Procedure for Material Selection and Development of Design/Producibility Data”


Requirements for Developmentof Metal Allowable

• Existence of a material specification

T l t f t i l f t l t t d ti h t• Ten lots of material from at least two production heats for each test

Thi k f th 10 l t t b t t d h ld• Thickness range of the 10 lots to be tested should span the thickness range covered by the material specification

All tests are performed in accordance with the applicable• All tests are performed in accordance with the applicable standard specifications

• Specific standard properties (tension compression• Specific standard properties (tension, compression, shear, and bearing) are tested at room temperature to determine Ftu, Fty, Ein, Fcy, Csu, Fbru, and Fbry for e/d = 1.5 and e/D = 2 0 for each grain direction and each lot


and e/D = 2.0 for each grain direction and each lot

Allowables

Typical Allowables Test Program for Metallic Materials

Tension

Compression

(L)(LT)(L)

90 + 101010Compression

Shear

(L)(LT)(L)(LT)

10101010

Bearing, e/D=1.5

Bearing, e/D=2.0

(LT)(L)(LT)(L)

10101010g, ( )

(LT) 10




Examples of Use


BDM-40907075


Sample Problem

• What is the tensile ultimate strength B allowable of .5-inch thick 7075-T651 bare plate at 250°F service ptemperature after exposure to 350°F for 1,000 hours?


Example Solution

38%


Example Solution

Answer = (79 ksi)(38%) =(79 ksi)(38%) =

30 ksi


Joint Strength BDM-4301


Joint Strength Single Shear


Sample Problem: Find the Joint Capability With BACR15BB6D Fastener

BACR15BB6D P

t = 0.0502024-T3

0.43(minimumtypical)P

t = 0.050


Example Solution


Example Solution

Answer = 910 lbsAnswer 910 lbs


Tension Fasteners


14 Material Allowables

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