Material Allowables

Material Allowables

Class Objectives 

• Classes of Allowables 

• Property Distribution 

• Material Specifications 

• Test Requirements 

q 

Design and Analysis of Aircraft Structures 

14-2


14-3


14-4

Investigation of Methods and Allowables 

$ Fullscale 

tests 

Component 

tests 

$ Analysis 

$ 

$ 

Sub Sub-component component tests 

Structural elements tests $ 

Allowable development 

Material specification development 

$ 

Material screening and selection $ 


$ 

verification 

Design-value 

development 

Material 

property 

evaluation 

14-5

Allowables Ensure the Structural 

Integrity of the Final Product 

• Allowables Design Requirements 

• Static Allowables Properties and Bases 

• Allowables Development and Publication 

• Presentation of Design Manuals and 

Examples of Use 


14-6

Allowables Are Required by FAA 

to Certify All Aircraft Structures 

FAA requirement 

FAR 25-613 25 613 


MMaterial t i l strength t th 

and design values 

14-7

Within Boeing, Allowable Development Is the Responsibility of 

the Stress Methods and Allowables (SMA) Group 

Structural Damage 

Technology (SDT) 

Central 

Engineering 

SMA 

Methods Allowables 


Boeing Materials 

Technology (BMT) 

14-8









14-9

Allowables and Design Value 

Definitions 

• Allowables: 

Mechanical properties having a level of statistical 

assurance – general case 

• DDesign i value: l 

A mechanical property value based on the allowable 

adapted p to the criteria and analysis y method used – 

specific case 


14-10

Strength Is an Allowables Domain 

Mechanical 

Properties 

St Strength th properties ti ( (reduced) d d) 

Ftu, Fty, Fsu Elastic properties (typical) 

EE, GG, u 

Physical properties (typical) 

w, k 


14-11

Classes of Allowables 

“A” BBasis i • 99% probability b bilit with ith 95% confidence fid 

“B” BBasis i 

• 90% probability with 95% confidence 

Point 

Design 

• Associated with a unique q structure and 

valid only for that particular configuration 

“S” Basis 


• Specification acceptance value 

with no statistical significance— 

this is not an allowable! 

14-12

The Allowable Need is Dependent 

Upon the End User 


controlled by - 

Vendor 

Preliminary 

specifications: 

XBMS, XBAC 

Firm specifications: 

BMS, BAC, 

AMS*, MIL*, QQ* 

Allowables Design/analysis 

Estimated 

design values 

Trade studies 

initial design 

Preliminary Preliminary 

allowables design 

Firm 

allowables 

*No longer Government monitored 


Firm design g 

drawing release 

- hardware 

fabrication 

14-13

Material Property Population Distribution 

Frequency 

A B Typical Property 


14-14

Probability Distribution 

A formula that gives the probability that a value fall 

within prescribed p limits 

– Weibull distribution 

– Normal distribution 

– Lognormal distribution 


14-15

Unknown Distribution 

Nonparametric analysis 

• “A” A basis allowable: lowest of 299 test points 

• “B” basis allowable: lowest of 29 test points 


14-16

Allowable Need Is 

Dependent Upon the End Use 


controlled by 

Vendor Estimated design 

values 

Preliminary 

specifications: 

XBMS, XBAC 

Allowables Design/analysis 

Preliminary 

allowables 

Trade studies’ 

initial design 

Preliminary design 

Firm specifications: Firm allowables Firm design drawing 

AMS*, MIL*, QQ*, 

release hardware 

BMS, BAC fabrication 


14-17









14-18

Allowables Development Requirements 

• Program g request q 

• Firm specification 

• Test program planning 

• Specimen fabrication and testing 

• Allowables development 

• DDocument t D6 D6-57037, 57037 “P “Procedure d ffor MMaterial t i l 

Selection and Development of 

Design/Producibility Data” 


14-19

Requirements for Development 

of Metal Allowable 

• Existence of a material specification 

• TTen lots l t of fmaterial t i lf from at tl least tt two production d ti heats h t 

for each test 

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

the thickness range covered by the material specification 

• All tests are performed in accordance with the applicable 

standard specifications 

• Specific standard properties (tension (tension, compression 

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 2 0 for each grain direction and each lot 


14-20

Allowables 

Typical Allowables Test Program for Metallic Materials 

Tension 

(L) 90 + 10 

(LT) 

10 

Compression 

(L) 

10 

(LT) 

10 

Shear 

(L) 

10 

(LT) 

10 

Bearing, e/D=1.5 (L) 

10 

(LT) 

10 

Bearing, g, e/D=2.0 (L) ( ) 

10 

(LT) 10 


14-21









14-22

BDM-4090 

7075 


14-23

BDM-4090 

7075 


14-24

BDM-4090 

7075 


14-25

BDM-4090 

7075 


14-26

Sample Problem 

• What is the tensile ultimate strength B allowable of 

.5-inch thick 7075-T651 bare plate p at 250°F service 

temperature after exposure to 350°F for 1,000 

hours? 


14-27

Example 

Solution 

38% 


14-28

Example 

Solution 

Answer = 

(79 ksi)(38%) = 

30 ksi 


14-29

Joint Strength 

BDM-4301 


14-30

Joint Strength 

Single Shear 


14-31

Sample Problem: Find the Joint Capability 

With BACR15BB6D Fastener 

2024-T3 

P 

BACR15BB6D 

t = 0.050 

t = 0.050 


0.43 

(minimum 

typical) 

P 

14-32

Example 

Solution 


14-33

Example 

Solution 

Answer = 910 lbs 


14-34

Tension 

Fasteners 


14-35

Material Allowables

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