Properties of materials

Question 1

What is the unit of stress?

Answer 1

MPa

Question 2

What is the unit of strain?

Answer 2

No units (it is a ratio)

Question 3

What sort of stress is usually applied when stress strain graphs are plotted?

Answer 3

Tensile (more convienient as in compression tends to buckle and this introduces too many other issues)

Question 4

What is anisotrophy?

Answer 4

When the mechanical properties are different depending on the direction of application of force

Question 5

Examples of materials that are isotropic

Answer 5

Metals

PMMA

Question 6

Examples of materials that are anisotrophic

Answer 6

Bone
Tendon
All biological materials

Question 7

What is the yield stress of a material?

Answer 7

The stress at which behaviour changes from elastic to plastic

Question 8

What is the ultimate stress of a material?

Answer 8

Point at which a material ruptures

Question 9

What is meant by strength?

Answer 9

Load per unit area before failure - the load required to break the object - how high a stress you can put on an object before it breaks

Question 10

What is toughness?

Answer 10

Energy absorbed before rupture = energy to fracture

also used to refer to resistance to fracture propagation

Question 11

What is the difference between a brittle and ductile material?

Answer 11

Brittle materials do not distort before failure (after failure the pieces of a brittle material can be fitted together and the fracture surfaces match)

Question 12

Can the fatigue properties of a material be deduced from the stress strain graph?

Answer 12

No

Question 13

What is fatigue toughness?

Answer 13

Work done to failure after repeated loading

Question 14

What is hardness?

Answer 14

Surface property
Resistance to scratching
Important for bearing surfaces

Question 15

List 5 causes of failure of a material

Answer 15

"Brittle fracture"
Fatigue fracture
Creep fracture
Corrosion 
Fretting

Question 16

What are the advantages of new ceramic?

Answer 16

Increased toughness

Less prone to fracture (microstructures prevent fracture propagation)

Question 17

What is new ceramic made of?

Answer 17

Combined alumina 74% and si=zirconia 25%

Question 18

What is visco-elasticity?

Answer 18

The behaviour of materials whose mechanical properties are time or rate dependent

Question 19

3 examples of visco-elastic materials

Answer 19

Polyethylene
Articular cartilage (fluid and solid phases - ‘biphasic’)
Bone

Question 20

What is the difference between ductility and visco-elasticity?

Answer 20

Ductile = change behaviour with load

Viscoelasticity = change in behaviour with time

Question 21

Name 3 visco-elastic properties

Answer 21

Hysteresis
Creep
Stress relaxation

Question 22

What is creep?

Answer 22

Deformation that occurs under constant load
Property of all visco-elastic materials
Rate varies with time
Does not reverse when the load is removed

Question 23

What is stress relaxation?

Answer 23

Stress required to maintain a constant deformation decreases with time (internal material changes its shape)

Question 24

Example of when creep is beneficial

Answer 24

Modular knee component

Question 25

What is a brittle fracture?

Answer 25

When a single load exceeds the ultimate stress

Question 26

What is a fatigue fracture?

Answer 26

Repeated loads above endurance limit (need to plot an S/N graph, where N is the number of loading cycles and S is stress to failure)

Question 27

What is a creep fracture?

Answer 27

Constant load below ultimate stress but above a threshold

Question 28

Can fluids be visco-elastic?

Answer 28

Yes

Question 29

What is viscosity?

Answer 29

The relationship between shear stress and shear rate

Question 30

What is a non-newtonian fluid?

Answer 30

Has visco-elastic properties and can store energy

Question 31

What is shear thickening?

Answer 31

Time independent | Viscosity increases with shear rate

Question 32

What is shear thinning?

Answer 32

Time independent | Viscosity decreases with shear rate

Question 33

What does rheopectic mean?

Answer 33

Time dependent | Viscosity increases with stresss over time (constant shear

Question 34

What does thixotropic mean?

Answer 34

Time dependent | Viscosity decrease with stress over time (constant shear)

Question 35

What is the difference between stress/strain and load/deformation?

Answer 35

``` Stress/strain = applied to a standardised piece of material Load/deformation = applies to a specific object and is individualised ```

Question 36

Are strength and flexibility related?

Answer 36

No they are independent properties and are not interrelated

Question 37

How is the area calculated in a stress/strain graph?

Answer 37

Area under the graph

Question 38

What is work hardening?

Answer 38

Disclocations pilin gup against the grain boundary - repeating this process after time material will already have work hardened and you won't be able to introduce the same amount of ductility to the material

Question 39

How is hardness measured?

Answer 39

Measured by a standardised weighted material and dropping it onto the material to see how much it indents

Question 40

Which metal is a very bad bearing surface because of its poor hardness?

Answer 40

Titanium (has low wear resistance and its wear particles will ultimately lead to failure)

Question 41

What are the 2 phases of articular cartilage?

Answer 41

Solid phase - stress in solid matrix | Fluid phase - hydrodynamic pressure

Question 42

Describe the changes in articular cartilage as it is loaded

Answer 42

Initial load taken by fluid phase Load gradually transferred to solid phase After 2.5-6 hours the load is entirely supported by solid matrix Under normal circumstances fluid/phase/solid phase support = 20/1

Question 43

What are the advantrages of a stainless steel alloy?

Answer 43

Easy to manufacture | Cheap

Question 44

What is the disadvantage of stainless steel alloy?

Answer 44

Susceptible to corrosion (galvanic between batches also stress corrosion cracking)

Question 45

What is the best stainless steel to use? Why?

Answer 45

Ortron 90 - superior in yield stress, ultimate tensile

Question 46

What are the advantages of a titanium alloy?

Answer 46

Good biocompatibility God corrosion resistance (oxide surface) Young's modulus similar to cortical bone

Question 47

Why is it beneficial that titanium alloy has a young's modulus similar to cortical bone?

Answer 47

Improves load transfer to bone which reduces stress shielding

Question 48

What are the disadvantages of a titanium alloy?

Answer 48

``` Notch sensitivity (will break easily if there is a crack) Low wear resistance ```

Question 49

What is the advantage of forged cobalt chrome allow?

Answer 49

Stronger

Question 50

What are the advantages of cobalt chrome alloy/

Answer 50

Excellent wear resistance Excellent corrosion resistance Acceptable biocompatibility Satisfactory fatigue life

Question 51

What are disadvantages to cobalt chrome/

Answer 51

Some patients have allergies to chromium and nickel

Question 52

What are the advantages of new ceramics?

Answer 52

Microstructures to prevent fracture propagation | Increased toughness and less prone to fracture

Question 53

What is hysteresis?

Answer 53

If a material is loaded then the load is taken off progressively it won't take the same route back to original shape - this generates heat because energy is being lost from the system - once the material is loaded past its elastic limit it will undergo plastic behaviour and won't return to its original shape

Question 54

Why does the meniscus of he knee show marked anisotrophy?

Answer 54

Due to arrangements of the collagen fibres

Question 55

How is polyethylene manufactured?

Answer 55

Extrude resin under heat and pressure

Question 56

What is delamination?

Answer 56

Happens over time Occurs in the sub-surface area Is a form of failure in shear

Question 57

What factors affect the mechanical properties of polymers?

Answer 57

Average molecular weight | Crystallinity

Question 58

Describe the structure of a polymer

Answer 58

Linear Branches that can cross link (makes the materail harder and more brittle) Inclusions (e.g. preservatives)

Question 59

What is delamination and where does it occur?

Answer 59

Is a form of failure in shear | Occurs in the sub-surface area