Structures Flashcards
How many times greater than its yield stress is aluminium’s UTS.
About 1.5 times.
Which fasteners used on aircraft are designed to take shear loads?
Rivets
What factors go into material selection?
Yield and ultimate strength, stiffness, density, fracture toughness, fatigue resistance, creep, corrosion resistance, temperature limits, manufacturing, repair, cost and availability.
Discuss wood as a material for making planes.
- Rarely used (save for some homebuilt aircraft)
- Good strength to weight
- Easy to fabricate and repair
- Susceptible to water damage, insect damage, and rot
- Not uniform properties
Discuss aluminium alloys
- Most widely used
- Excellent strength to weight ratio, readily formed, moderate cost, resistant to chemical corrosion.
Most common: 2024 series
High strength: 7075 series
Improved corrosion and strength: 7050 and 7010 series.
In general, the stronger the aluminium, the more brittle.
Discuss steel as a material
- Inexpensive
- Easy to fabricate
- For high strength and fatigue resistance
- High temperature applications
- The properties of steel are strongly influenced by heat treatment and tempering
- Wing attachment fittings
Discuss titanium alloys as a material
- Better strength-to-weight ratio and stiffness than Al and is capable of temperatures almost as high as steel.
- Corrosion resistant.
- Difficult to form
- Affected by impurities
- Modern military aircraft have 10-30% of their structural mass in titanium.
- Expensive
Discuss magnesium alloys as a material.
- Good strength to weight ratio
- Tolerates high temperatures and easily formed
- Very prone to corrosion (flammable) and must have a protective finish
Discuss nickel alloys as a material
- Inconel, Rene 41, Hastelloy
- Suitable for hypersonic aircraft and re-entry vehicles.
- Substantially heavier than aluminium or titanium
- Difficult to form
Discuss composites as a material.
- Weight savings of approx 25% vs aluminium.
- Enables stiffness/strength tailoring
- More expensive
- Not good at high temperatures
- Not good with concentrated loads
- Poor out of plane strength
- Moisture/temperature/UV sensitive
- Difficult to repair
What is sandwich construction
Two face sheets bonded to, and separated by, a core. They can be very structurally efficient. Face sheets carry most of the tension and compression loads due to bending
The core carries most of the shear loads as well as the compression loads perpendicular to the skin.
What are some modes of failure for aircraft components?
Wing struts: Buckling
Wing spar: Beam shear and bending
Fuselage: Pressurisation
Give the three main categories of structural joints.
- Mechanically fastened joints: riveted or bolted.
- Adhesively bonded joints
- Welded joints (less prevalent)
When is blind riveting used on aircraft.
Used only when access to the back side of the sheets is prevented. Used for non-critical structural assemblies.
When is riveting used? What are two types of rivet heads and their advantages?
Requires access from both sides of a sheet.
Universal (cheaper)
Counter-sunk (lower drag).
When and why are bolted joints used?
Superior tensile and shear strengths compared to rivets.
More expensive than rivets.
Nut must be torque speced to preload fastener which ensures the parts do not slip and a more uniform transfer of loads, increasing the fatigue life of the fastener.
Describe what happens to bolts in tension under cyclic loading.
- When the members are preloaded by the bolt with initial load Fi the members are compressed by a small amount.
- When the load P is subsequently applied, it reduces that compression.
- As long as the joint does not separate, then the assembly acts with the stiffness of the whole assembly.
- The whole assembly is much stiffer than the bolt alone.
A greater stiffness means that the stretch of the assembly under load P is much smaller than if applied to the bolt alone. - Consequently, the stretch in the bolt when the load P is applied is much less than it would be if applied to the bolt alone.
- Since the stretch is small, then the cyclic load seen by the bolt will be small also.
- This small cyclic load allows preloaded bolts to work very well in appropriate fatigue conditions.
When are bonded joints used?
Usually preferred over fastened joints for laminated composite materials due to their relatively poor bearing strength.
How can bonded joints be made stronger?
Increase the bond area by increasing the overlap length of the joint or tapering each end of the joint to optimise the adhesive contact area.
When is welding used?
Joining of parts made from identical metals by heating them to a point of surface melting and bringing them together to allow their molecules to coalesce.
Can have HAV and changes material properties.
Commonly used to join parts making up engine mounts and landing gear. Can often lead to warping. Critical structural parts should not be made from welded aluminium due to a reduction in fatigue life.
