Fracture Under Stress

Question 1

What 3 factors are taken into account when deciding whether a material is likely to fail?

Answer 1

magnitude of applied load
rate at which the load is applied
number of times the load is applied

Question 2

How is a material tested under a tensile load?

Answer 2

a material is stretched gradually under a tensile load until fracture - length measurements are taken at intervals and the applied load can be tested along with the cross-sectional area to calculate stress.

thus, a stress-strain curve is plotted

Question 3

When will a material rupture?

Answer 3

when subject to a load greater than its ultimate strength

Question 4

Explain the mechanism of a ductile fracture

Answer 4

Ductile fractures results from the application of a tensile load beginning with microscopic void formation at the centre. as the local stress increases these voids grow and connect with others, producing large cavities. eventually the load is too much to be supported and fracture occurs.

Question 5

Describe the characteristics of a ductule fracture

Answer 5

necking
flat granulated central portion
shear lip

(gives the fracture a cup and cone surface)

Question 6

Explain the mechanism of a brittle fracture

Answer 6

occurs without any necking or plastic deformation.

Question 7

Describe the characteristics of a brittle fracture?

Answer 7

the fracture is flat and perpendicular to the load and has a granular appearance. Commonly a CHEVRON pattern is seen

Question 8

What kind of materials do brittle fractures occur in?

Answer 8

glass
ceramics
high strength metals

Question 9

What do stress trajectories represent?

Answer 9

The pathways of stress through a material - they deflect to accommodate the shape of a material

Question 10

IS a bone more likely to fracture under fast or slow loading?

Answer 10

slow

Question 11

Where are fractures most likely to propagate>

Answer 11

sites of weakness e.g. cracks, pores, notches.

Question 12

How does stress concentrations cause fracture?

Answer 12

stress concentrations are highest at the tips of cracks or notches - this high concentration at one point means a fracture will develop from it

Question 13

What is stress concentration?

Answer 13

sudden changes to material structure that result in points in the material at which stress is higher than the average within the material.

Question 14

What is the Charpy impact test?

Answer 14

a common test for testing structures resistance to impact loading

a pendulum is swung through an object and the potential energy difference was the energy required to fracture a material

Question 15

what happens to a materials ability to absorb energy as temperature increases?

Answer 15

increases

Question 16

What is a fracture caused by repeated loading known as?

Answer 16

fatigue fracture

Question 17

A large load will cause a material to fail catastrophically, what is this called?

Answer 17

catastrophic failure

Question 18

Describe the appearance of a fatigue fracture on a metal component?

Answer 18

concentric clam-shell markings where the cracks have stopped previously - allows trace back to the origin of the ripples

Where final rupture occurs, the rest of the material appears granular/fibrous depending on brittle/ductile fracture respectively

Question 19

What is indicated if all of the material has a granular appearance?

Answer 19

only a very small fatigue fracture was required to cause complete failure

Question 20

Why does the time period over which the loads are applied have an effect on bone stress fractures?

Answer 20

bone has the ability to remodel according to stresses - so a greater time period means the bones can remodel and no fracture occurs

Question 21

under what circumstances do stress/fatigue/march fractures occur in bone?

Answer 21

marching in new army recruits
strenuous physical activity
muscle fatigue

Question 22

Which form of weakening are metals particularly vulnerable to?

Answer 22

chemical corrosion

Question 23

What is wet corrosion?

Answer 23

corrosion in the presence of water

Question 24

Briefly describe how corrosion occurs

Answer 24

imperfections on the surface of a material are particularly prone to corrosion
the process accelerates and imperfections become crevices quickly
crevice = stress concentrations
stress concentrations = limited fatigue life and ultimate strength of the material

Question 25

What is the passivation layer and when is it used?

Answer 25

the passivation layer is an inert oxide layer used on top of orthopaedic implant materials to prevent corrosion by body fluids

Question 26

Where are implants particularly vulnerable to corrosion?

Answer 26

joints - small gaps can be exploited

in plates, the edges at screws can become damaged and corrode