Mechanics unit 5 - Fracture under stress deck 2 Flashcards
Because of a materials depednancy on temp for absorbing energy what should be done to provide a more complete picture of its behaviour ?
A series of impact tests performed at different temperatures
Describe the difference between a catastrophic fracture and a fatigue fracture
- A catastrophic fracture is when a large load which exceeds the ultimate strength will cause a fracture
- A fatigue fracture is when a fracture occurs due to repeated loadings less than the ultimate strength
A fatigue fracture is the result of a combination of what?
The number and magnitude of loadings
The number may vary from a few to a few million depending on the magnitude of the loadings
Describe the appearance of the 2 main regions present in fatigue fractures
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
What is particularly prone to fatigue fractures ?
Orthopaedic implants in the lower limbs
Do SAQ 9&10
One part of this unit covers the exact same stuff about fatigue fractures as in the orthopaedics module so just use that
Do SAQ 11
What causes corrosion of a metal ?
Chemical reaction of the metal with its environment
How does the corrosion process acting on a metal affect it ?
It will serverly limit the fatigue life and ultimate strength of the material. It will not show an endurance limit
How are certain metals and alloys often used in orthopaedic implants resistant to corrosion ?
They form an inert layer called the passivation layer that covers the surface and protects them
What are metal and alloys which form a passivation layer known as ?
Passive metals
Why is the junction between 2 metals/ alloys used prone to corrosion?
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
Do SAQ 12&13
What 2 groups can metals be divided up into and what is this division based on ?
- Ferrous metals - contain iron
- Non-ferrous metals - contain no iron
What are metal alloys ?
They are formed when a metal has other elements added to it to improve the mechanical and corrosive properties of it
What is the main ferrous alloy used in biomedical settings and what is it formed from ?
Steel - formed from alloys of iron and carbon +/ - other elements called alloying elements which alter the mechanical properties of steel
What are steels classed based upon ?
Their carbon content
What is the specific alloying element which is present in stainless steel and what specific mechanical property is it needed for ?
Chromium - this gives stainless steel its corrosion-resistant property as it forms a passivation layer
Do SAQ 14
What are the most common non-ferrous alloys used in biomedical applications and what specifically is it usually used for?
Titanium and titanium based alloys
Used for - heart valve replacements, fracture fixation and joint replacement endoprostheses
What are the main advantages and disadvantages of titanium based alloys ?
Advantages:
- Lower density than steels
- Higher strength to wait ratio than aluminum
- Excellent corrosion resistance
Disadvantages:
- High cost to buy and manufacture compared to aluminium and steel
- Low youngs modulus compared to steel (more flexible)
Do SAQ 15
