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Thursday, 18 December 2014

AIRCRAFT SKIN MATERIAL SELECTION


Key mechanical properties of materials used for aircraft skins

Application of suitable material with recommended mechanical properties will provide the best result; recommended mechanical properties of materials for the application in aircraft skin are as mentioned below.

Hardness

Hardness is a mechanical property that indicates the ability of materials to resist the abrasion, distortion, penetration & cutting action. Heat treatment process used to increase the hardness.

Strength

Strength is a mechanical property that indicates the ability of a material to resist the deformation.

Density

Density is also mechanical properties of material used in aircraft skin, normally it is recommended to use light density material and also depends according to the balancing of aircraft.

Ductility

Ductility is very important mechanical properties required for aircraft materials as it is desired that selected materials should have ability to absorb shocks.

Elasticity

Elasticity is a mechanical property that provides the ability to material to return to its original size and shape after removal load.

Toughness

Indicates the ability to absorb energy to break this is required mechanical property that should be associated with materials using for aircraft skins.

Fusibility & conductivity

Materials using for aircraft skin should have above properties.

Stiffness

Resistance against bending

Fatigue

Resistance against cyclic load. 

Failure mechanism that must be considered during material selection

Fatigue 

As we know that due to cyclic loading cracks will be developed and process rapidly and failure might be possible due to cyclic loading and this is called as fatigue. We may see in following figure about the relation b/w maximum loads & No. of cycles that a material may endure before processing fatigue- cracks 
Y-Axis --Maximum Stress
X-Axis --Cycle to failure    

Buckling

As we know that long slender structural part or members are used to fail under the compression load. In this type of failure of materials there will be production of bending in middle section of structural member when they are acting between compression loads axially. 

In following figure we may understand how material will be failed under the buckling action. Aircraft skin will also be under the same environment and types of loading as listed below in figure.


Hence we must be sure that materials that we have selected should have ability to withstand against this environment. 


Commonly used materials for aircraft skin

There are following materials those are commonly used for aircraft skins
  1. Aluminum Alloy
  2. Steel
  3. Titanium
  4. Plastics
  5. Glass
  6. Composite materials

Stress strain diagram for above materials

Aluminum alloy – Composition & micro structure

There are lot of grades of aluminum alloy used for aircraft skin, we consider aluminum grade 2024. 


Chemical composition of grade 2024

Elements
Quantity
Si
0.5
Fe
0.5
Cu
0.8-4.9
Mn
0.3-0.9
Mg
1.2-1.8
Cr
0.1
Zn
0.25
Ti
0.15
other
0.15
Balance
Aluminum

Micro structure of aluminium grade 2024

Designation of alloy condition
Micro structure
A
Lathe shaped S.
Mn-rich insoluble dispersoids.
Coarse Fe-Si particles,
Absence of dislocation, substructure,
Elongated grain structure.
B
Aligned spheroidal & slightly elongated particles.
Mn-rich dispersoids.
Coarse Fe-Si particles.
Tangled dislocation cell structure, cold worked, optical microstructure.
C
Some fine S.
Mn-rich insoluble dispersoids,
Coarse Fe-Si particles,
Tangled dislocation cell structure, elongated grain structure
D
Under aged-no detectable S.
Mn- rich insoluble dispersoids.
Coarse Fe-Si particles.
Tangled dislocation cell substructure, elongated grain structure.

Titanium grade Ti-6/4– Composition & micro-structure
Titanium grade material used in aircraft skin will have more than 3 times more hardness as compared to basics grades. 

Chemical composition
Ti-90%
Aluminum- 6 %
Vanadium- 4%
Micro structure
Bi-modal micro structure of titanium ally provides a good result to perform as the genuine material for aircraft skin. 

Composite materials- Composition & micro structure
Composite materials are widely used these days in various types of application such as in case of aviation areas we have observed the wide application of glass fiber. 

Chemical composition 

Constituents
Quantity
SiO2
72
CaO
10
Al2O3
0.6
MgO
2.5
B2O3
-
TiO2
-
Na2O
14.2
K2O
0.15
Fe2O3
SO3
0.7

Micro structure
Micro structure of fiber glass controls its (fiber glass) properties. We may summarize these are as following.
  1. Fiber reinforcement-properties
  2. Matrix of reinforcement –properties
  3. Dimensions & shape of reinforcement
  4. Quantity of reinforcement in matrix
  5. Reinforcement –orientation
Micro structure of used material in aircraft skin
As we have observed that key properties of materials for aircraft skin are hardness, stiffness & strength.

These properties depend on the micro structure of material. Mechanical properties that we have identified above for aircraft skin normally depend on following points
  1. Type of phase
  2. Phase -volume fraction
  3. Size of grains
  4. Shape of grains
These properties determines the suitable application of materials.
Strength of any materials depend on its grain size, if grain size will be finer strength will be good , as material that we have identified have finer grain size and shape and hence these material will have good strength and hardness and will be suitable materials in the area of aircraft skin manufacturing.

Surface engineering approach

Surface engineering is a process to improve the properties of materials such as for increasing fatigue life and fatigue strength ,for increasing resistance power against corrosion.
There are some surface engineering process that will be useful in our application

Shot peeing 

Shot peening is a surface engineering method to increase the fatigue strength and to increase fatigue life of parts or materials.

Isotropic super finishing process

This process is used to provide better finishing and this process will be useful in removing unwanted stress concentrations.

Coating 

In this process of surface engineering ,some other materials are added to the surface of components to increase its surface quality, this process is also called as coating

New materials for aircraft skin which is not widely used 

Aluminium fiber combination

Aluminium fiber combination may be used in manufacturing the aircraft skin as these materials will have good resistance against fatigue loading.

These materials will provide following advantage as listed below
  1. Maintenance cost will be reduced
  2. Fatigue loading effect will be reduced
  3. Energy efficient
  4. Good strength
  5. Weight will be reduced
Aluminium fiber material will have more strength as compared to carbon fiber reinforced plastic.
In aluminium fiber combination, there will be a central layer of fiber metal laminate which is reopened between layers of aluminium of good quality and it will develop such a material that will have high resistance against fatigue loading with good strength.

In this blog we have discussed materials used for aircraft skin and their characteristics .
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