Part 8: Failure

Question 1

How does crack propagation differ for ductile and brittle materials?

Answer 1

Brittle cracks propagate rapidly, ductile fractures are slow.

There is a “warning” for ductile fractures

Question 2

What is an inter granular crack?

Answer 2

Crack propagation between grains (along the grain boundary)

Question 3

What is a trans granular crack?

Answer 3

Crack propagation through the grains (along a crystallographic plane)

Question 4

How does the sharpness of a crack impact its stress concentration?

Answer 4

Sharper cracks have higher stress, so cracks propagate faster

Question 5

What are the two methods to increase fatigue life?

Answer 5

Reduce mean stress applied

Reduce stress amplitude

Question 6

What are some ways a metal can be designed with increased resistance to fatigue?

Answer 6

1. Polish the surface to remove stress amplification sites
2. Reduce the number of internal defects
3. Replace sharp corners with rounded corners
4. Harden the surface with treatment such as carburizing or shot peening

Question 7

At what material temperature does creep start to occur?

Answer 7

40% of its melting temperature

Question 8

Creep strain is dependent on what two variables?

Answer 8

Stress and temperature