Properties of materials

Question 1

What is anisotropy?

Answer 1

when the mechanical properties of a material differ depending on the direction of loading

Question 2

What is the unit of stress?

Answer 2

Pa

Question 3

Name some anisotropic materials

Answer 3

bone
tendon
biological materials

Question 4

Name some isotropic materials

Answer 4

metals
PMMA
UHMWPE

Question 5

What are dislocations?

Answer 5

microscopic changes in material structure as a result of deformation

once introduced they dont go way

cannot cross the border so pile up against it

heating can remove dislocations

Question 6

What is the yield stress?

Answer 6

the stress at which behaviour of a material changes from elastic to plastic

Question 7

What is the ultimate stress?

Answer 7

point at which the material ruptures

Question 8

Describe a brittle material

Answer 8

a material which does not undergo plastic deformation before failure

after fracture the pieces of a brittle material can be fitted back together

Question 9

Describe failure of a ductile material

Answer 9

undergo plastic deformation before fracture

Question 10

What is toughness?

Answer 10

energy required to fracture

Question 11

What is fatigue failure? and fatigue toughness?

Answer 11

failure due to repeated cyclical loading

work done to failure after repeated loading

Question 12

What are the 3 types of fracture?

Answer 12

brittle
fatifue
creep

Question 13

Name 5 causes of failure

Answer 13

brittle fracture 
fatigue fracture 
creep fracture 
corrosion 
fretting

Question 14

What is viscoelasticity?

Answer 14

describes material whose properties are time or rate dependant

Question 15

Name some visco-elastic materials

Answer 15

polyethylene
articular cartilage
bone

Question 16

What is the difference between ductility and disco-elasticity

Answer 16

ductility - change in behaviour with load

disco-elasticity - change in behaviour with time

Question 17

What are the 3 viscoelastic properties?

Answer 17

hysteresis
creeo
stress relaxation

Question 18

What is hysteresis?

Answer 18

a dynamic lag between input and output

Question 19

What does the area in the middle of a hysteresis loop represent?

Answer 19

energy dissipated - usually heat from internal friction

Question 20

What is creep?

Answer 20

increased deformation under constant load

Question 21

What is stress relaxation?

Answer 21

stress required to maintain a constant deformation decreases with time

Question 22

How does creep fracture occur?

Answer 22

constant load below ultimate stress but above a threshold

Question 23

What is viscosity?

Answer 23

relationship between shear stress and shear rate

in newtonian fluids the relationship is linear and through the origin

Question 24

Describe a rheopectic fluid and give and example

Answer 24

time dependant

viscosity increase with stress over time

lubricants

Question 25

Describe a thixotropic fluid and give an example

Answer 25

time dependant

viscosity decrease with stress over time

mud in a land slide, synovial fluid

Question 26

What is shear thinning/thickening? Give examples

Answer 26

thinning - viscosity decreases with increased stress - joint fluid, paint

thickening - viscosity increases with increased stress - corn starch, water

Question 27

Describe the biphasic nature of articular cartilage

Answer 27

solid phase - stress in solid matrix

fluid phase - hydrodynamic pressure

permeability decreases non-linearly under pressure

initial load taken by the fluid phase and gradually transferred to solid. 2.5-6 hours for the load to be entirely solid matrix. normally fluid phase: solid phase = 20:1