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AY BMSc Lectures > Properties of materials > Flashcards

Properties of materials Flashcards

1
Q

What is the unit of stress?

A

MPa

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2
Q

What is the unit of strain?

A

No units (it is a ratio)

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3
Q

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

A

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

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4
Q

What is anisotrophy?

A

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

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5
Q

Examples of materials that are isotropic

A

Metals

PMMA

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6
Q

Examples of materials that are anisotrophic

A

Bone
Tendon
All biological materials

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7
Q

What is the yield stress of a material?

A

The stress at which behaviour changes from elastic to plastic

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8
Q

What is the ultimate stress of a material?

A

Point at which a material ruptures

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9
Q

What is meant by strength?

A

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

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10
Q

What is toughness?

A

Energy absorbed before rupture = energy to fracture

also used to refer to resistance to fracture propagation

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11
Q

What is the difference between a brittle and ductile material?

A

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

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12
Q

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

A

No

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13
Q

What is fatigue toughness?

A

Work done to failure after repeated loading

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14
Q

What is hardness?

A

Surface property
Resistance to scratching
Important for bearing surfaces

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15
Q

List 5 causes of failure of a material

A 
"Brittle fracture"
Fatigue fracture
Creep fracture
Corrosion 
Fretting
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16
Q

What are the advantages of new ceramic?

A

Increased toughness

Less prone to fracture (microstructures prevent fracture propagation)

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17
Q

What is new ceramic made of?

A

Combined alumina 74% and si=zirconia 25%

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18
Q

What is visco-elasticity?

A

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

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19
Q

3 examples of visco-elastic materials

A

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

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20
Q

What is the difference between ductility and visco-elasticity?

A

Ductile = change behaviour with load

Viscoelasticity = change in behaviour with time

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21
Q

Name 3 visco-elastic properties

A

Hysteresis
Creep
Stress relaxation

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22
Q

What is creep?

A

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

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23
Q

What is stress relaxation?

A

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

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24
Q

Example of when creep is beneficial

A

Modular knee component

25
Q

What is a brittle fracture?

A

When a single load exceeds the ultimate stress

26
Q

What is a fatigue fracture?

A

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)

27
Q

What is a creep fracture?

A

Constant load below ultimate stress but above a threshold

28
Q

Can fluids be visco-elastic?

A

Yes

29
Q

What is viscosity?

A

The relationship between shear stress and shear rate

30
Q

What is a non-newtonian fluid?

A

Has visco-elastic properties and can store energy

31
Q

What is shear thickening?

A

Time independent

Viscosity increases with shear rate

32
Q

What is shear thinning?

A

Time independent

Viscosity decreases with shear rate

33
Q

What does rheopectic mean?

A

Time dependent

Viscosity increases with stresss over time (constant shear

34
Q

What does thixotropic mean?

A

Time dependent

Viscosity decrease with stress over time (constant shear)

35
Q

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

A 
Stress/strain = applied to a standardised piece of material 
Load/deformation = applies to a specific object and is individualised
36
Q

Are strength and flexibility related?

A

No they are independent properties and are not interrelated

37
Q

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

A

Area under the graph

38
Q

What is work hardening?

A

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

39
Q

How is hardness measured?

A

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

40
Q

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

A

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

41
Q

What are the 2 phases of articular cartilage?

A

Solid phase - stress in solid matrix

Fluid phase - hydrodynamic pressure

42
Q

Describe the changes in articular cartilage as it is loaded

A

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

43
Q

What are the advantrages of a stainless steel alloy?

A

Easy to manufacture

Cheap

44
Q

What is the disadvantage of stainless steel alloy?

A

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

45
Q

What is the best stainless steel to use? Why?

A

Ortron 90 - superior in yield stress, ultimate tensile

46
Q

What are the advantages of a titanium alloy?

A

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

47
Q

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

A

Improves load transfer to bone which reduces stress shielding

48
Q

What are the disadvantages of a titanium alloy?

A 
Notch sensitivity (will break easily if there is a crack) 
Low wear resistance
49
Q

What is the advantage of forged cobalt chrome allow?

A

Stronger

50
Q

What are the advantages of cobalt chrome alloy/

A

Excellent wear resistance
Excellent corrosion resistance
Acceptable biocompatibility
Satisfactory fatigue life

51
Q

What are disadvantages to cobalt chrome/

A

Some patients have allergies to chromium and nickel

52
Q

What are the advantages of new ceramics?

A

Microstructures to prevent fracture propagation

Increased toughness and less prone to fracture

53
Q

What is hysteresis?

A

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

54
Q

Why does the meniscus of he knee show marked anisotrophy?

A

Due to arrangements of the collagen fibres

55
Q

How is polyethylene manufactured?

A

Extrude resin under heat and pressure

56
Q

What is delamination?

A

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

57
Q

What factors affect the mechanical properties of polymers?

A

Average molecular weight

Crystallinity

58
Q

Describe the structure of a polymer

A

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

59
Q

What is delamination and where does it occur?

A

Is a form of failure in shear

Occurs in the sub-surface area

AY BMSc Lectures (8 decks)

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