Creep Failure Flashcards
Define creep
Creep is time-dependent and permanent deformation of
materials subjected to a constant load at elevated
temperatures
When is creep observed for metals?
- For metals, creep is observed at temperatures above 0.4Tm
What are the three regimes of creep?
- Primary creep
- Secondary (steady-state) creep
- Tertiary creep (rupture)
Which is the most important stage of creep and why?
Secondary creep (Stage II) is the most important stage from
an engineering design standpoint, as it persists for the
longest period
Describe Primary Creep
Primary or instantaneous
creep: continuously
decreasing creep rate
due to strain hardening
Desribe secondary or steady-state creep
constant creep
rate due to balancing
of strain hardening and
recovery; longest stage
Tertiary creep
acceleration of creep
rate until rupture at tr
What are the strain components of creep?
- Instantaneous elastic strain
- Instantaneous plastic strain
- Time-dependent creep strain
What is the relationship between stress, temperature, and rupture time during steady-state creep?
As stress and/or temperature increase, the rupture time tr decreases
What type of metals demonstrate greater resistance to creep?
Metals with high melting temperature, high elastic
modulus and large grain size demonstrate greater
resistance to creep
How are grain boundaries related to high-temperature deformation?
Grain boundaries play an important role in high-temperature deformation.
How do large grains impact surface area for grain boundary sliding?
- Large grains result in smaller surface area for grain boundary
sliding
How does small grain impact strength and toughness of a material at low temperature conditions?
at low temperature conditions,
small grain size increases strength and toughness of a material
When does grain boundary sliding occur?
At high temperature and strain.
Why are single crystals desirable? Provide an example
Single crystal Ni superalloys can help make high performance turbine blades.
What is a common extrapolation procedure for creep data?
The Larson-Miller parameter
What is the problem with the timescales for creep experiments?
They are impractical to collect from normal laboratory tests (eg. years of creep exposure).
What is an alternative to the impractical timescales for creep experiments?
One alternative is to perform tests at comparable
stress, but higher temperatures and shorter
times, and then to extrapolate the results
How does the rupture lifetime relate to operating temperature?
Generally, the rupture lifetime of a material at some
specific stress level will vary with T such that this m
parameter stays constant.
What are the design considerations of jet engines discussed in class?
-Fuel efficiency
-Noise
-Weight
-Cost
Rank the following structures/coatings for the greatest creep resistance to the smallest creep resistance: Single crystal, columnar grain, and conventional casting.
Single crystal (Highest creep resistance) > Columnar grain > Conventional casting (Lowest creep resistance)
What is the relationship between instantaneous strain, creep rate, steady-state creep rate, temperature, and creep rupture lifetime?
Increasing temperature leads to:
-Increase in instantaneous strain
-Increase in creep rate
-Increase in steady-state creep rate
-Decrease in rupture lifetime
Why do metals with larger grain size demonstrate greater resistance to creep?
Increasing the grain size reduces the surface area for grain boundary sliding at high temperature and stress.
