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MAY 6, 2012

Aircraft materials - Selecting metallic material allowable for structuralstress analysis with approved design data

When you are developing a structural

analysis, you have the requirement to

know the allowable of the metallic

material that you are using in the

design of the structural part. Certainly

you can't go to the Internet and find

the 2024T3 material allowable

properties and use this values to

design your aircraft structure. You

need a reliable and approved source

of metallic material properties. Then,

where can I find this reliable material

properties source? This information

is defined in the aircraft airworthiness certification requirements. Go to the CS25 or

FAR25 in the CS25.613 or FAR25.613 requirement paragraph and you can find the

answer. The material strength properties must be based on enough tests of material

meeting approved specifications to establish design values on a statistical basis. This is

the requirement, you need to make coupon testing regarding the material that you what to

use based on authority approved specification. But I want to use aluminium alloys, this is

a material well know by the aircraft industry, must I make tests? If you are from a small

aircraft company is difficult to make test just to use aluminium alloys in the aircraft. For

that reason you must use authority approved material data. According to CS25.613 or

FAR25.613 approved metallic material data is given by the MMPDS handbook or the

ESDU00932. This two references for material properties are the reliable and approved

source for metallic material properties in aircraft application. If you work in a small

company and don't have a material properties handbook approved by the authority you

must use MMPDS or ESDU00932 handbooks. I generally use the MMPDS handbook. And

what type of properties can we find in the MMPDS handbook? The required mechanical

properties to do your analysis.

Material properties are presented in a table format. Each table are referring to the material

designation name. As example I will use the 2024 material table figure. In order to

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correctly select the material properties you must select first the

Manufacturing Specification, Form, Temper, thickness and data Statistic basis. Only after

this selection you obtain the correct allowable for the material that you are using in the

aircraft. The Specification, Form , Temper and Thickness of the material is defined in the

manufacturing drawing that you produce as design engineer.

As stress engineer you must find the manufactured part properties from the material.

Now, what type of statistical basis I must use? The table present the A basis and B basis.

The A basis defines that 99% of the samples fail at superior strength values with 95% of

confidence. The B basis defines that 95% of the samples fail at superior strength values

with 95% of confidence. If you are developing a one loading path structural part (landing

gear) the A basis must be used. If you are developing a redundant structure (two or more

loading paths, stringers as example) the B basis can be used. If weight is not a problem,

use the conservative value of the table: the lower value.

Other parameter that we can find in

the table is the applicable

grain direction. The L, LT and ST

designation define the properties of

the part according to the

manufacturing grain direction of the

material. If you check, you can find

that the grain direction can influence significantly the material properties. For yield stress,

the influence can go up to 5 Ksi of stress resistance. For that reason if you are not

controlling the grain direction in the installation process, be conservative and use the

lower value of all directions.

As presented is the table the direction

of the material grain will present

higher mechanical properties the L

direction. The perpendicular direction

of the grain as lower mechanical

properties, the LT direction. The

ST direction is the vertical axis of the

grain direction and for sheets and

plates this propertie are not required.

Remember selecting material allowable:

1. Go to MMPDS

2. Select material designation chapter (2024)

3. Select table for the material specification, form and temper

(AMS-QQ-A-250/4, Sheet, T3)

4. Select the thickness of the used (0.063 in)

5. Select the A or B column according to the airworthiness

requirements (use conservative, A basis)

6. Select the proper grain direction properties (use conservative,

lower of the two)

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Posted by AeroTeaching at 2:46 PM No comments:

7. Obtain the mechanical properties of the material.

Hope this help to obtain approved metallic material data to your aircraft structural design.

If you work on a manufacturer design company just go to your company design manual

and select the materials allowable from there. The same way that from MMPDS

handbook. Use always approved design data for your analysis, otherwise EASA or FAA

will reprove the modification or repair design.

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APR 17, 2012

Aircraft materials - The aluminium alloys that we use on aircraftstructures

Aluminium alloys are the main

materials used on aircraft structures.

This material is used from 50 to 70%

of the complete aircraft

structure.Why we use aluminium

alloys? Aluminium is the material with

the best properties to use for aircraft

structures. We can find the best

relation between cost, density and

strength. Other major advantages of

aluminium is the easy manufacturing

and metalwork capability of this

material, the abundance and easy

recycling of this metal is other great

advantages.

The aluminium alloys are a mixing of pure aluminium and other elements. The additional

elements that we add to pure aluminium defines the classification and numbering system

of the alloys. The alloys are numbered with four numbers. The fist number defines the

main element added to the aluminium. We can find the following numbering system:

1XXX- pure aluminium

2XXX- main element added to aluminium = Copper

3XXX- main element added to aluminium = Manganese

4XXX- main element added to aluminium = Silicon

5XXX- main element added to aluminium = Magnesium

6XXX- main element added to aluminium = Magnesium+Silicon

7XXX- main element added to aluminium = Zinc

8XXX- main element added to aluminium = others elements

The three numbers after the main number defines the percentage of main element added

and the additional elements of the alloy. The rest of the numbering system is of

importance for the Material Engineer. For the aircraft design engineer we need to now the

main aluminium alloys used on aircraft, its properties and when to use them. The alloys

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used in aircraft structures are the followings:

Aluminium alloy 2024, this alloy is one of the main

alloys used in the aircraft. The 2024 is the best

material for structures where fatigue and damage

tolerance is a problem. Therefore the material used

for the fuselage and part of the wing and stabilizers

is the 2024 aluminium alloy. Do not forget, use this

material when you are working with fatigue critical

structural parts.The fuselage skin, frames, stringer,

doublers, splices and other structural elements that

belong to the fuselage, are made of 2024 aluminium

alloy. Because pressurization load in the fuselage is a fatigue critical load.

Aluminium alloy 2524, this alloy is an evolution of the 2024 alloy, improving the fatigue and

damage tolerance properties of the alloy. Is used in the fuselage skin, frames or other

critical fatigue part. Use when properties vs. cost is improved regarding 2024 alloy.

Aluminium alloy 7075, this alloy is from the 7XXX series alloys. This series is used where

strength is required. Use where fatigue is not the critical load. In this case 7075 is the

best material to use in sheets or plate applications. You can find this material in the upper

wing panels, machined frames, formed frames, stringers, longerons, extruded parts,

etc...mainly applicable to fuselage, stabilizers and wing areas where the ultimate strength

is the design feature.

Aluminium alloy 7050, this is also a 7XXX series alloy, strength is the dominated design

feature. This alloy is mainly required to extruded, machined and forged aircraft parts. If the

requirement is a forged or machined block, where the maximum strength is the target

then you must use this alloy. Used in fittings and connecting elements between fuselage

and wing or stabilizers, extruded stringers, machined or forged frames,etc.

Aluminium alloy 7475, this alloy is

from the 7XXX series. 7475 alloys

represent the evolution of the 7075

alloy improving fatigue properties.

The main advantages of this alloy is

the increasing fatigue properties. The

best 7XXX series alloy to use in

fatigue critical areas. You can find an

excellent combination between

strength and fatigue properties. Use

this alloy when these properties are

required. They are used in fuselage

frames, stringers and critical fatigue connections parts.

Aluminium alloy 6061, this alloy is not structural. Is mainly used in secondary structures

where the welding property is required. If you need to use a material where welding is

required, use 6061 alloy. Do not use this alloy in primary structures, strength capability is

very low regarding the other alloys.

These are the alloys that you must use in the design, repair or modifications in the

aircraft. Select the material according to the requirements of the design, strength or

fatigue are the dominant criteria to use in the selection of the material.

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MAR 10, 2012

Aircraft materials - Aluminium 2024-T3 or 2024-T4 what the difference?

A package of structural modification drawings was sent

to the MOA or part-145 maintenance organization to be

applied to aircraft. After some hours, the maintenance

engineering asked me the following: can I change the

material requested in the drawings? For example the

2024-T3 by the 2024-T4.

The typical structure of and aircraft is made 65% of

aluminum alloys. The aluminum alloys used are the

2xxx series and 7xxx series. From the 2xxx series the

2024 is the king, used in 85% of the cases. The property that make the 2024 invincible in

aircraft application is the fatigue and damage tolerance properties. 7xxx alloys are used

where strength is required, for fittings, bulkheads, beams, frames or splices. The most

commons alloys used are the 7075, 7050, 7475.Then the 2024 alloys is for fatigue

application, where? In fatigue critical structures, the skin, frames, skin splices, pressure

bulkheads, lower wing panels, etc. All structural parts with fatigue issues must use

fatigue material the 2024. The letter T after the alloys defines the heat treatment of the

material. In this case you can find the 2024 in the T3, T4, or T42. There are others

treatments but this is the most commons used. The T3 means that part was heat

treated, cold worked and naturally aged. T4 means heat treated, and naturally aged. The

difference is that T4 has no cold work. A special treatment where the plate is

compressed cold, and the material crystals in the surface of the plate become more

attached to each others, increasing the strength required to separate them. This property

increases the fatigue and damage tolerance life of the plate. The cracks will have more

difficulty to penetrate in the plate. The T42 heat treatment is applied to existing heat

treatments -O, T3 or T4, this treatment is applied by the final user and not by material

manufacturer. That means if we use a -O, T3 or T4 plate and apply a heat treatment with

natural aging to them because of manufacturing propose, the final temper will be T42.

Can you replace a 2024-T3 plate by a 2024-T4 plate the

answer is no. The T3 as better fatigue properties. If you

replace the material you are reducing the fatigue life of

the part. The Design Office has made all calculation for

fatigue and damage tolerance an if they are using T3 is

because is the better material for that particular case. If

you need to change, because of logistics problems

please, please request authorization to the Design

Office to change the material. They will check the

calculation and see that the fatigue life reduction is still

safe for the aircraft. Is not only the fatigue that is

different, the strength of the material is also different, T4

as lower strength than T3 than the margin of safety for

ultimate strength can also be different and this must be checked by the Design Office.

Imagine MS=0.09 with T3 but changing to T4 the result can be MS=-0.01 and this only the

calculation people can check that.

If you want, you can find in the structural repair manual a table approved by the

manufacturer and authority about approved interchangeability of material generally by

increasing or reducing the thickness of the plates. Check that in your SRM.

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