Mechanics unit 5 - Fracture under stress deck 2

Question 1

Because of a materials depednancy on temp for absorbing energy what should be done to provide a more complete picture of its behaviour ?

Answer 1

A series of impact tests performed at different temperatures

Question 2

Describe the difference between a catastrophic fracture and a fatigue fracture

Answer 2

1. A catastrophic fracture is when a large load which exceeds the ultimate strength will cause a fracture
2. A fatigue fracture is when a fracture occurs due to repeated loadings less than the ultimate strength

Question 3

A fatigue fracture is the result of a combination of what?

Answer 3

The number and magnitude of loadings

The number may vary from a few to a few million depending on the magnitude of the loadings

Question 4

Describe the appearance of the 2 main regions present in fatigue fractures

Answer 4

The frist region is described as a smooth region with concentric clam shell markings - each marking indicates where the crack has stopped and then started again as propagates throughout the component in response to the repeated loads, these markings also allow the origin of the fracture to be located

The second region is either granular or fibrous in appearance:

• Granular appearance is produced by a rapid brittle fracture
• Fibrous appearance is produced if the material undergoes a ductile fracture

Question 5

What is particularly prone to fatigue fractures ?

Answer 5

Orthopaedic implants in the lower limbs

Question 6

Do SAQ 9&10

Question 7

One part of this unit covers the exact same stuff about fatigue fractures as in the orthopaedics module so just use that

Question 8

Do SAQ 11

Question 9

What causes corrosion of a metal ?

Answer 9

Chemical reaction of the metal with its environment

Question 10

How does the corrosion process acting on a metal affect it ?

Answer 10

It will serverly limit the fatigue life and ultimate strength of the material. It will not show an endurance limit

Question 11

How are certain metals and alloys often used in orthopaedic implants resistant to corrosion ?

Answer 11

They form an inert layer called the passivation layer that covers the surface and protects them

Question 12

What are metal and alloys which form a passivation layer known as ?

Answer 12

Passive metals

Question 13

Why is the junction between 2 metals/ alloys used prone to corrosion?

Answer 13

This is because there will be slight movement permitted between the two metal/ alloy components which results in a small crevice between them which will not be protected by the passivation layer which is therefore ripe for attack by corrosion

example shown in pic

Question 14

Do SAQ 12&13

Question 15

What 2 groups can metals be divided up into and what is this division based on ?

Answer 15

1. Ferrous metals - contain iron
2. Non-ferrous metals - contain no iron

Question 16

What are metal alloys ?

Answer 16

They are formed when a metal has other elements added to it to improve the mechanical and corrosive properties of it

Question 17

What is the main ferrous alloy used in biomedical settings and what is it formed from ?

Answer 17

Steel - formed from alloys of iron and carbon +/ - other elements called alloying elements which alter the mechanical properties of steel

Question 18

What are steels classed based upon ?

Answer 18

Their carbon content

Question 19

What is the specific alloying element which is present in stainless steel and what specific mechanical property is it needed for ?

Answer 19

Chromium - this gives stainless steel its corrosion-resistant property as it forms a passivation layer

Question 20

Do SAQ 14

Question 21

What are the most common non-ferrous alloys used in biomedical applications and what specifically is it usually used for?

Answer 21

Titanium and titanium based alloys

Used for - heart valve replacements, fracture fixation and joint replacement endoprostheses

Question 22

What are the main advantages and disadvantages of titanium based alloys ?

Answer 22

Advantages:

1. Lower density than steels
2. Higher strength to wait ratio than aluminum
3. Excellent corrosion resistance

Disadvantages:

1. High cost to buy and manufacture compared to aluminium and steel
2. Low youngs modulus compared to steel (more flexible)

Question 23

Do SAQ 15