Structures 1

Question 1

Adequate Strength

Answer 1

The ability to withstand stress; should not break or permanently deform.

Question 2

Adequate Rigidity (Stiffness)

Answer 2

The ability to resist deflection under load; can deform elastically, but not plastically.

Question 3

Service Life

Answer 3

A/C design requirements:
1. Total Flight hours
2. Total number of flights
3. Total number of landings AND types of landings

Must possess adequate durability - resist cracking, corrosion, thermal degradation, wear, FOD effects

Question 4

Minimum Weight/Growth Factor

Answer 4

Aircraft: 5-20 to 1 (5-20x the weight of the additional component)

Question 5

Structure

Answer 5

Those parts of an aircraft, the primary purpose of which is to insure the integrity of the aircraft and to carry the loads encountered in flight and on the ground. (skin, bulkhead)

Question 6

Loads to the Aircraft

Answer 6

Gravity, aerodynamics, inertia, and pressure.

Question 7

Stress

Answer 7

A measure of the resistance to force (internal resisting force per unit area)

Question 8

Concentrated Load

Answer 8

A Load distributed over a small area or at a point (ie. bomb or tank attachment point, landing gear attachment point)

Question 9

Distributed Load

Answer 9

Distribution of a load over a given area (ie. Lift over a wing, fuel in a wing)

Question 10

Axial Load Directions

Answer 10

Tension is considered positive, compression is negative

Question 11

Shear Load

Answer 11

Load that causes the fracture surface to slide across each other as they come apart

Question 12

Torsion

Answer 12

Twisting moment

Question 13

Limit Load

Answer 13

The maximum load on a structure expected in service. (NATOPS limit)

Question 14

Ultimate Design Load

Answer 14

The limit load x 1.5.

Question 15

Ultimate Load

Answer 15

The highest load that will not cause failure; anything above will fail.

Question 16

Class of EI Material Damage

Answer 16

1. Mechanical
2. Chemical

Question 17

Mechanical damage causes

Answer 17

1. Fracture event
2. Carless handling
3. Mating of fracture surfaces (worst thing to do)

Question 18

Chemical damage causes

Answer 18

Corrosion

Question 19

Precaution when cutting parts to avoid influencing fracture surface

Answer 19

1. Don’t cut too close to the fracture surface.
2. Avoid thermal damage.
3. Heat from torch. Use coolant.

Question 20

Types of Stress

Answer 20

1. Normal - stress perpendicular to the surface
2. Shear - stress parallel to the surface

Question 21

Torsional Stress Min/Max

Answer 21

Stress is 0 at the center of a shaft, maximum at the edge (based on radius)

Question 22

Beam Bending

Answer 22

Outer radius: Tension (+)
Inner radius: Compression (-)

Question 23

Bending Stress

Answer 23

Center of the beam: Shear stress is max, normal stress is zero

Edge of beam: Shear stress is zero, normal stress is max

Question 24

Strain

Answer 24

Unit deformation of a deformable body

Question 25

Longitudinal Strain

Answer 25

Change in length per unit length in the direction of interest (delta/original length)

Question 26

Shear Strain

Answer 26

Change in angle of an initial right angle

Question 27

E (Young’s Modulus)

Answer 27

Measure of a materials stiffness (stress divided by strain)

Question 28

Elastic vs Plastic

Answer 28

Elastic - recovers to original size and shape when unloaded

Plastic - Does not recover to original size and shape. There will be some permanent deformation.

Question 29

Stress Strain Diagram

Answer 29

-Slope = E
-Yield Stress Point - Point at which the material becomes permanently deformed
-Elastic region - Follows slope + .2% (0.002) offset up to the yield stress point
-Plastic region - Any point past the yield stress point
-Ultimate Stress Point - Maximum Stress a material can handle before decreasing
-Fracture Point - point where the material fractures into two pieces
-Plastic Strain - The strain the is permanently deforming the material
-Elastic Strain - The region where the material slightly returns to its original shape

Question 30

Strain Hardening

Answer 30

The difference in stress between the yield stress point and a point further down the diagram (must exceed yield point)

Question 31

Ductile vs Brittle Materials

Answer 31

Ductile - total strain to fracture is >5%

Brittle - total strain to fracture is < or equal to 5%

Question 32

Impact Loading

Answer 32

Impact loading causes brittle fractures

Question 32

Strength vs Toughness

Answer 32

Strength - the ability to withstand stress

Toughness - the ability to absorb energy (area under stress/strain diagram curve)

Question 33

Critical Strain Rate

Answer 33

The strain rate at which the energy absorbed before failure is maximum

Question 34

Creep

Answer 34

Time-dependent deformation produced in solids subjected to stress

Question 35

Requirements for Creep

Answer 35

1. Constant load
2. Time
3. Heat

Question 36

Stages of Creep

Answer 36

1. Initial Stage - initial elastic, plastic (if any), and thermal strain, at time zero
2. Constant Rate Creep - strain hardening from Stage 1 is balanced by softening due to elevated temperature
3. Third Stage - increasing creep rate, necking, leading to stress rupture

Question 37

Planes of Failure

Answer 37

Ductile Materials fail at 45 degree plane (Shear Stress)

Brittle Materials fail at 90 degree plane (Normal Stress)

Question 38

Failure appearances

Answer 38

Ductile - microscopic dimples causing a dull surface

Brittle - bright, granular appearance on a flat face

Question 39

Publication for EIs

Answer 39

COMNAVAIRFOR INST 4790.2

Question 40

Ductile to Brittle Transition (DTBT)

Answer 40

Ductile materials under high stress rates act brittle

Question 41

Ductile strength

Answer 41

Allows you to fly to NATOPS limits with no plastic deformation or anything breaking