Creep Failure

Question 1

Define creep

Answer 1

Creep is time-dependent and permanent deformation of
materials subjected to a constant load at elevated
temperatures

Question 2

When is creep observed for metals?

Answer 2

• For metals, creep is observed at temperatures above 0.4Tm

Question 3

What are the three regimes of creep?

Answer 3

• Primary creep
• Secondary (steady-state) creep
• Tertiary creep (rupture)

Question 4

Which is the most important stage of creep and why?

Answer 4

Secondary creep (Stage II) is the most important stage from
an engineering design standpoint, as it persists for the
longest period

Question 5

Describe Primary Creep

Answer 5

Primary or instantaneous
creep: continuously
decreasing creep rate
due to strain hardening

Question 6

Desribe secondary or steady-state creep

Answer 6

constant creep
rate due to balancing
of strain hardening and
recovery; longest stage

Question 7

Tertiary creep

Answer 7

acceleration of creep
rate until rupture at tr

Question 8

What are the strain components of creep?

Answer 8

• Instantaneous elastic strain
• Instantaneous plastic strain
• Time-dependent creep strain

Question 9

What is the relationship between stress, temperature, and rupture time during steady-state creep?

Answer 9

As stress and/or temperature increase, the rupture time tr decreases

Question 10

What type of metals demonstrate greater resistance to creep?

Answer 10

Metals with high melting temperature, high elastic
modulus and large grain size demonstrate greater
resistance to creep

Question 11

How are grain boundaries related to high-temperature deformation?

Answer 11

Grain boundaries play an important role in high-temperature deformation.

Question 12

How do large grains impact surface area for grain boundary sliding?

Answer 12

• Large grains result in smaller surface area for grain boundary
  sliding

Question 13

How does small grain impact strength and toughness of a material at low temperature conditions?

Answer 13

at low temperature conditions,
small grain size increases strength and toughness of a material

Question 14

When does grain boundary sliding occur?

Answer 14

At high temperature and strain.

Question 15

Why are single crystals desirable? Provide an example

Answer 15

Single crystal Ni superalloys can help make high performance turbine blades.

Question 16

What is a common extrapolation procedure for creep data?

Answer 16

The Larson-Miller parameter

Question 17

What is the problem with the timescales for creep experiments?

Answer 17

They are impractical to collect from normal laboratory tests (eg. years of creep exposure).

Question 18

What is an alternative to the impractical timescales for creep experiments?

Answer 18

One alternative is to perform tests at comparable
stress, but higher temperatures and shorter
times, and then to extrapolate the results

Question 19

How does the rupture lifetime relate to operating temperature?

Answer 19

Generally, the rupture lifetime of a material at some
specific stress level will vary with T such that this m
parameter stays constant.

Question 20

What are the design considerations of jet engines discussed in class?

Answer 20

-Fuel efficiency
-Noise
-Weight
-Cost

Question 21

Rank the following structures/coatings for the greatest creep resistance to the smallest creep resistance: Single crystal, columnar grain, and conventional casting.

Answer 21

Single crystal (Highest creep resistance) > Columnar grain > Conventional casting (Lowest creep resistance)

Question 22

What is the relationship between instantaneous strain, creep rate, steady-state creep rate, temperature, and creep rupture lifetime?

Answer 22

Increasing temperature leads to:

-Increase in instantaneous strain
-Increase in creep rate
-Increase in steady-state creep rate
-Decrease in rupture lifetime

Question 23

Why do metals with larger grain size demonstrate greater resistance to creep?

Answer 23

Increasing the grain size reduces the surface area for grain boundary sliding at high temperature and stress.