Chapter 2 Material Selection

Question 1

aircraft material composition

Answer 1

A350 example

53% composites (CFRP) (wings, centre wing box and keel beam, tall cone, skin panels, frames, stringers, doublers, doors)
19% aluminum
14% titanium
8% miscellaneous (e.g. sandwich)
6% steel

Question 2

three most important properties of a material

Answer 2

density (weight), stiffness (elastic modulus, stability), strength

Question 3

weight of a large passenger aircraft at take-off

Answer 3

33% fuel
28% structural weight
15% payload
14% equipment and service
10% power plant

Question 4

1D elements

Answer 4

strut/tie and beam

Question 5

strut/tie

Answer 5

strut (compression) / tie (tension) (trusses)

load transmission only along axis (normal force)

axial stiffness: E x A

Question 6

beam

Answer 6

normal force, shear force, bending and torsional moment (all 6 DOFs)

axial stiffness: E x A
bending stiffness: E x I_b
torsional stiffness: G x I_t
shear stiffness: G x A

e.g. floor beam, floor support beam

Question 7

2D elements

Answer 7

membranes (loaded in-plane)

plates (loaded out-of-plane (transversal))

shells (loaded in-plane and out-of-plane)

Question 8

membrane

Answer 8

only in-plane loading

axial stiffness: E x A
shear stiffness: G x A

Question 9

plate

Answer 9

out-of-plane loading

bending stiffness: E x I_b
torsional stiffness: G x I_t

Question 10

shell

Answer 10

all types of loading

axial stiffness: E x A
bending stiffness: E x I_b
torsional stiffness: G x I_t
shear stiffness: G x A

Question 11

aluminum alloys

Answer 11

until recently, the most used material in aircraft structures

density: 2.7 g/cm^3
elastic modulus: 70 GPa
yield strength: 50-500 MPa

+ low density
+ self-protection against corrosion
+ recyclable
+ low price

— low elastic modulus
— low fatigue resistance

Question 12

steel alloys

Answer 12

heavy metal with high strength and stiffness

density: 7.8 g/cm^3
elastic modulus: 210 GPa
yield strength: 250 - 1200 MPa

+ high wear resistance
+ very high isotropic strength
+ hardenable
+ low price

— sensitive to corrosion
— very high density

application: high stiffness and strength, high surface pressure

Question 13

titanium alloys

Answer 13

heavy metal with high fatigue strength

density: 4.5 g/cm^3
elastic modulus: 100 - 120 GPa
yield strength: 200 - 1100 MPa

+ very high isotropic strength
+ low decrease in strength at high temperatures
+ high fatigue resistance
+ corrosion-resistant

— difficult to machine
— very expensive
— low elastic modulus

application: high temperature, high performance

Question 14

fiber composites

Answer 14

fibers usually made of carbon, glass or aramid

matrix usually made of plastics like epoxy, phenolic resin or PEEK

T300 (CFRP UD):
density: 1.55 g/cm^3
elastic modulus 1: 125 GPa
elastic modulus 2 and 3: 8 GPa

+ low density with high strength and stiffness possible
+ high durability, low fatigue
+ anisotropic strength and stiffness provide high design freedom
+ integral design possible
+ lots of possible combinations of materials

– low impact resistance
– complicated damage
– contact corrosion possible (e.g. carbon fiber with aluminium)
– no electrical conductivity
– limited operating temperature (of plastics)

Question 15

don’t combine carbon fibers with […]

Answer 15

aluminum

Question 16

hybrid material (GLARE)

Answer 16

glass laminate aluminum reinforced epoxy (GFRP + aluminium)

+ lighter than pure aluminum
+very good impact behaviour
+ good fatigue behaviour

— slightly lower elastic modulus (57 GPa)
— higher manufacturing complexity

Question 17

breaking length

Answer 17

the maximum length of a vertical column of the material that could suspend its own weight when supported only at the top

L = tensile strength / density x 9.81 m/s^2

Question 18

thermal resistance

Answer 18

= fabric thickness / thermal conductivity