Structures

Question 1

How many times greater than its yield stress is aluminium’s UTS.

Answer 1

About 1.5 times.

Question 2

Which fasteners used on aircraft are designed to take shear loads?

Answer 2

Rivets

Question 3

What factors go into material selection?

Answer 3

Yield and ultimate strength, stiffness, density, fracture toughness, fatigue resistance, creep, corrosion resistance, temperature limits, manufacturing, repair, cost and availability.

Question 4

Discuss wood as a material for making planes.

Answer 4

• 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

Question 5

Discuss aluminium alloys

Answer 5

• 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.

Question 6

Discuss steel as a material

Answer 6

• 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

Question 7

Discuss titanium alloys as a material

Answer 7

• 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

Question 8

Discuss magnesium alloys as a material.

Answer 8

• Good strength to weight ratio
• Tolerates high temperatures and easily formed
• Very prone to corrosion (flammable) and must have a protective finish

Question 9

Discuss nickel alloys as a material

Answer 9

• Inconel, Rene 41, Hastelloy
• Suitable for hypersonic aircraft and re-entry vehicles.
• Substantially heavier than aluminium or titanium
• Difficult to form

Question 10

Discuss composites as a material.

Answer 10

• 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

Question 11

What is sandwich construction

Answer 11

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.

Question 12

What are some modes of failure for aircraft components?

Answer 12

Wing struts: Buckling

Wing spar: Beam shear and bending

Fuselage: Pressurisation

Question 13

Give the three main categories of structural joints.

Answer 13

• Mechanically fastened joints: riveted or bolted.
• Adhesively bonded joints
• Welded joints (less prevalent)

Question 14

When is blind riveting used on aircraft.

Answer 14

Used only when access to the back side of the sheets is prevented. Used for non-critical structural assemblies.

Question 15

When is riveting used? What are two types of rivet heads and their advantages?

Answer 15

Requires access from both sides of a sheet.
Universal (cheaper)
Counter-sunk (lower drag).

Question 16

When and why are bolted joints used?

Answer 16

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.

Question 17

Describe what happens to bolts in tension under cyclic loading.

Answer 17

• 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.

Question 18

When are bonded joints used?

Answer 18

Usually preferred over fastened joints for laminated composite materials due to their relatively poor bearing strength.

Question 19

How can bonded joints be made stronger?

Answer 19

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.

Question 20

When is welding used?

Answer 20

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.