Unit 3 - Fracture under stress

Question 1

What factors determine if a material will fail?

Answer 1

Magnitude of load applied

Speed at which loads is applied

Number of times load is applied

(wear & corrosion can also lead to failure)

Question 2

What 3 types of loading may lead to failure?

Answer 2

Steady loading

Impact loading

Fatigue loading

Question 3

What is tensile testing?

Answer 3

Tensile load applied to a material and increased until material fractures

Throughout, the length is measured and therefore the strain can be calculated

A stress-strain curve can be produced

Question 4

What is the point called when a material fractures?

Answer 4

Rupture strength

Question 5

Why may ultimate strength (max stress calculated) be greater than rupture strength?

Answer 5

Necking (in ductile materials) - material appears to carry less stress when calculated using original CSA rather than the true reduced CSA

Question 6

What are the 2 types of fracture that may occur to a material when it is subjected to a steady load?

Answer 6

Ductile (when it occurs with necking)

Brittle (when it occurs without necking)

Question 7

Describe how a ductile fracture comes about

Answer 7

Occurs after considerable plastic deformation, characterised by necking

Begins with formation of microscopic voids at centre of bar (stress causes separation of metal at grain boundaries/interfaces between metal grains & inclusions)

Voids grow & coalesce. Eventually actual metal to metal contact area within bar is reduced so that it is unable to support applied load –> fracture

Deformation by shearing also contributes

Question 8

Describe how a brittle fracture occurs?

Answer 8

Suddenly without any plastic deformation/necking

Separate crack fronts fan out from origin of crack

Question 9

What is the difference in appearance between ductile & brittle fractures?

Answer 9

Ductile - necking, flat granulated central portion, small shear lip (gives cup-and-cone surface)

Brittle - flat fracture perpendicular to load, granular appearance with chevron pattern

Question 10

Under what conditions may a ductile material undergo a brittle fracture?

Answer 10

If material contains notch/crack

Low temperatures

If exposed to fatigue loading

Question 11

What may sudden changes in shape of an object result in?

Answer 11

Stress concentrations (where stress is higher than average stress of material)

Notches/holes (that result in stress concentrations) are called stress risers

Question 12

What are stress trajectories?

Answer 12

Diagrams used to show locations of stress concentrations

Points with tightly packed stress trajectories = areas of high stress

Question 13

Why are cracks & notches prone to fracture propagation?

Answer 13

Sharp changes in shape have high stress concentrations which means fracture is likely to develop from this point & propagate

Question 14

How may stress concentrations be dissipated & fracture propagation halted?

Answer 14

Build features such as smooth holes into a design in order to blunt a propagating crack (stress no longer concentrated at sharp point)

All materials will contain microscopic defects (scratches, pores, cracks)

Question 15

What is notch sensitivity?

Answer 15

The phenomenon of concentrated stress at tip of a notch

Can be calculated by comparing energy absorbed by notched & unnotched designs

Question 16

How may the resistance of a structure to an impact load be assessed?

Answer 16

Charpy impact test:

• pendulum released from known height & allowed to break specimen at bottom of swing
• height reached by pendulum at and of swing is lower
• impact energy = mg(H1-H2)

Impact energy = energy absorbed by specimen

Question 17

How does temperature effect a materials ability to resist a load?

Answer 17

As temperature increases:

• ductility increases (material is able to absorb more energy)
• strength reduces
• strain increases

Question 18

Describe the appearance of a fatigue fracture

Answer 18

Smooth region:

• concentric clam shell markings (indicate various positions at which crack has stopped as it intermittently propagated through)
• allows origin of fracture to be located

Granular/fibrous region:

• granular: produced by rapid brittle fracture when remaining cross-section can no longer support load (if whole surface, indicates few load cycles were needed)
• fibrous: if material undergoes ductile fracture

Question 19

When are fatigue fractures found in bone?

Answer 19

When load is repeatedly applied in short period of time (faster than bone can remodel)

Question 20

Why is muscle fatigue important in fatigue fractures?

Answer 20

Tired muscles less able to neutralise tensile loads & act as shock absorbers

Question 21

How can corrosion lead to fracture?

Answer 21

Imperfections on metal surfaces are prone to attack by corrosion

Imperfection develops into a crevice as metal ions migrate away from imperfection

Crevice gives rise to stress concentrations –> fracture

Question 22

What is a passivation layer?

Answer 22

Inert film (usually an oxide) on surface of metals

Question 23

What is the difference between ferrous & non-ferrous metals?

Answer 23

Ferrous - contains iron

Question 24

All steels are alloys of what?

Answer 24

Iron & carbon

Steel = most common ferrous alloy

Question 25

What makes stainless steel corrosion resistant?

Answer 25

Chromium - gives protective oxide surface

Question 26

How are steels classed?

Answer 26

Carbon content

Stainless steel 316L (low carbon) used in orthopaedics

Question 27

What is the difference in material property of low carbon & high carbon steels?

Answer 27

High - high strength & hardness but brittle

Low - low strength & hardness but ductile

(Surface hardening of low carbon steel improves strength and hardness of surface but retains ductility of core)

Question 28

What are the advantages of titanium in ortho?

Answer 28

Low density (weight)

High strength-weight ratio

High corrosion resistance

Question 29

What are the disadvantages of titanium in ortho?

Answer 29

High cost

High fabrication cost

Low Young’s modulus (can be compensated for by using thicker sections)

Question 30

What other non-ferrous metal (not titanium) may be used in ortho implants?

Answer 30

Cobalt

Question 31

Name 5 properties of polymers & suggest why they are popular

Answer 31

Lightweight
Corrosion resistant 
Low tensile strength 
Electrical insulator 
Low melting point 

Easy to use in manufacturing

Question 32

How are polymers made?

Answer 32

Polymerisation of monomers

Lots of smaller molecules put into a long chain

Question 33

Describe the stress-strain behaviour of polymers

Answer 33

Non-linear
Time dependent
Elastic & plastic

Question 34

What are the 2 subtypes of polymers

Answer 34

Plastics

Elastomers

Question 35

What are the 2 categories of plastics?

Describe them

Answer 35

Thermoplastic - can be reheated & reformed (polyethylene, polypropylene)

Thermosets - cannot be reformed. They harden at high temps (during formation at high temps a by product is released)

Question 36

What is the most important property of elastomers?

Answer 36

Can deform by enormous amounts without permanently changing shape

Better known as rubbers

Question 37

Describe ceramics

Where may they be used in hip prostheses?

Answer 37

Crystalline structures
Very hard but brittle

Used for head of hips - wear resistant & low friction (would not be used as stem as they would easily snap)

Question 38

What are the 3 categories of composite materials?

Describe & give example

Answer 38

Particulate:

• particles of hard brittle material dispersed in softer ductile one
• concrete

Fibre:

• fibres of strong stiff brittle material withing soft ductile one
• fibreglass

Laminar:

• thick layers laminated (layers)/coating of material over other
• plywood