materials Flashcards
aircraft
- constructed mainly of aluminium and its alloys
- with smaller amounts of steel and titanium for the **major structural components **
- with composite materials used extensively for more lightly loaded structures, the empennage, cabin floor panels, flying control surfaces, engine cowlings and fairings.
Each material is chosen for its particular properties:
fatigue strength
wear resistance
strength to weight ratio
fire resistance etc.
Aluminium
good strength to weight ratio
good fatigue resistance
good **thermal / electric conductivity **
NOTE: cannot be welded
steel
High strength but heavy
used in areas where weight can be ignored.
Titanium
Ligher than steel
strong & corrosion resistant up till 400C
Magnisium
lighter than alumiunm
excellent strength to weight ratio
NOTE: satisfactory elasticity, so limited use in primary structure
what are composite materials
made of more than 1 element to produce **properties different than the original **
consist of Bulk Material & reinforcements
What are REINFORCEMENTS
- **used to increase the strength and stiffness of the matrix **
- in the form of fibre (glass, carbon, kevlar)usually
- on their own they have poor mechanical properties
- when mixed with Matrix , can be formed in complex shapes
composite:
- The matrix will spread the load to the composite between each of the individual fibres and also protects the fibres from damage. This could be caused by impact or abrasion.
-These composites have good resistance to corrosion
- Fatigue behaviour is different to that of conventional metal alloys and is **not generally a consideration at stress levels below approximately 80% of ultimate stress. **
- Metal structures suffering fatigue retain their design strength up to a critical point after which failure occurs rapidly whereas composites lose their properties gradually.
- Many **composites have low electrical conductivity **but specialist coatings can be applied to give the required electrostatic discharge and lightning strike protection.
-retain properties at elevated temperature
cost implication of composites
The manufacturing costs are high due
- a labour intensive and often complex process
-
These factors are outweighed by the reduced operating costs.
Aircraft such as the Boeing Dreamliner are approximately 20% lighter and this gives a large reduction in fuel consumption
Sandwich construction
This is used extensively on aircraft of all types
for **flight control surfaces, flooring, fuselage panels, empennage skin and sound proofing for engines. **
uses a honeycomb core with skins of composite material (GRP or CFP) or aluminium alloy
Benefits:
provide** rigidity and strength**.
good strength to weight ratio and is particularly strong in the direction of the honeycomb openings.
Note:
Parts made of a sandwich material need additional provision to carry concentrated loads.
Types of attachment methods
Riveting:
* Good for Shear - NOT tension
* The most common way of joining materials
* Pllacing a rivet in a pre drilled hole. The tail of the rivet is deformed and this clamps the material together
* Rivets may be set by hand or by a power operated machine
Welding
* the two metals are fused to become one **using gas flame **to heat the meta
* and a filling material is used to fill the gaps
* Types of welding including forge, electric arc and spot welding
Bolting
* For high shear or tensile loads
* Uses steel bolts.
* Must be locked to make sure that they do not loosen in service using: locking wire, split pins or special nuts.
Pinning
*uses pins of various designs to hold the materials together
Bonding/Redux adhesive bonding:
* A sheet of adhesive is placed between the two materials, heat is then applied to cure the adhesive
* One advantage of this method is that compared to say riveted joints, it is easier to seal structures making it particularly useful for fuel tanks.
what is corrosion
it’s the slow distruction of metal by electrochemical reaction
Most metals are unstable, but corrosion is what brings them back to their stable ‘metallic ore’ original format
occurs in conditions that permit the formation of minute electrolytic electrical cells in or on the attacked metal, in the presence of an electrolyte.
It will also occur when a difference in potential exists between the different constituents of an alloy, or where dissimilar metals are in contact.
in corrosion, metal compounds are converted to?
oxides, sulfates, salts, hydroxides and carbonates
explain the corrosion process
- when metal is exposed to air, oxygen reacts to the metal surface
- an oxide layer formes and ahderes to protect the metail
- Oxide film may combines with water that is not imeprevilous to passage of water through it / or happens if oxides are subject to heat
corrosion happens
when a metalic surface is exposed to water in liquid or moisture in the atmosphere
the level of corrosion is related to impurities in the water (sulfates, salts from sea or pulution)
evidence of corrosion
can happen across the entire surface
or show ‘deep pits’
or** follow grain boundaries** inide the core of the metal
can be further aggrevated by external loads, or they may be residual stresses from the manufacturing process or method of assembly
- poweder
- rush
- etching
- flaking
- pitting
- cracking
- paint discolouration
Types of corrosions
One type leads to another.
you will have more than one type in a piece of metal
metal can deteriorate very fast
Rate of corrosion
High
tropical, sea, industrial (high humidity & pollution)
Moderate
moderate climate, suburbs, inland
Low
Cold climate, rurarl areas and inland
What is surface corrosion & its Products
**Attacks the surface slowly **
Reduces the cross-sectional thickness of the sound material, and so weakens the structure.
The attack is recognized by etching or pitting of the surface, the products of corrosion are recognized as:
1) Steel covered with reddish/brown dust
2) Aluminium/magnisium covered in white/grey poweder. Pitted form
3) Copper alloys - blue greek depost. Surface corrosion is least damaging and material can be rectified
Intergranual corriosn
Intergranular corrosion
caused by non-uniform alloy surfaces
affects the core of the metal and the grain boundaries (more concentrated)
- not limited by the lack of oxygen, but accelerated if applied or residual stresses are present.
- -** Repeated fluctuating or tensile stresses cause separation of the grain boundaries accelerating the spread of the corrosion**.
As a result higher stress concentrations occur in the remaining sound material, this production cracks, which spread leading to complete failure.
- It is probably the most dangerous form of corrosion
- Detection is difficult
- Serious weakening may occur before any external evidence is visible.
- The only surface indication is a series of hairline cracks, these are usually only visible through a magnifying glass.
There is no effective method of determining or limiting the loss of strength that will occur, so that when detected, parts must be immediately rejected.
Stress Corrosion/Stress corrosin cracking
A combination of steady tensile load and corrosive conditions/environment
Produce a form of metal fatigue = stress corrosion cracking(SCC).
The stresses may be built in during manufacture of the part, or introduced during assembly, or may be due to operational or structural loads.
A metal under **stress corrodes more rapidly than unstressed parts, **
initially there is pitting of the surface.
**Loss of the metal at the corrosion pit intensifies the stress at this point, producing a crack which extends under the combined action of corrosion and load until failure occurs. There is generally little visible evidence of corrosion and no apparent loss of metal.
fatigue life
fatigue life of an aircraft is caused by repeated cycles of take off, landing, pressurization (LOAD CYCLES)
the more cycles, the more fatigue the AC will become
ex:
so 6 cycles (6 trips) for 30 min each versus 2 cycles (1 hour each)
so if material is continiously loaded and unloaded it will suffer damage even if the load remains the same.
Corrosive substances
Hydraulic fluid
**Pottassium hyroxide **(reacts with water, gives off heat, rapid corrosion and releases toxic fumes) - might come in exam
also murcury
