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8/12/12 Aeroteaching: Aircraft materials
1/6aeroteaching.blogspot.ro/search/label/Aircraft materials
Home The objective About the Author Contacts Aircraft Structural Design
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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Aircraft damage (3)
Aircraft design (5)
Aircraft hardware (7)
Aircraft materials (3)
Aircraft protection and finish (2)
Aircraft requirements (3)
Aircraft stress (1)
Aircraft structures (3)
Aircraft weight (1)
CHAPTERS
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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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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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8/12/12 Aeroteaching: Aircraft materials
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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.
JAA / EASA FCL questions
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