Shared Flashcard Set

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C15
Creep
19
Science
Undergraduate 3
06/04/2011

Additional Science Flashcards

 


 

Cards

Term
what is creep? describe the shape of a graph that describes it
Definition

time-dependent plasticity occuring at stresses below the flow stress and at temperatures in excess of Tm/3

[image]

Term
describe the stages of creep
Definition

primary: decreasing creep rate as disloation microstructure develops to reduce strain rate

secondary: equilibrium is established between deformation and recovery mechanisms to maintain a steady state strain rate

tertiary: increasing creep rate as the effective cross section reduces leading to failure

Term

define the following creep data:

creep strength

creep life

minimum creep rate

stress rupture life

Definition

creep strength: stress to produce nominal strain in a given time

creep life: time to nominal strain at given stress and temperature

min creep rate: f(stress, temperature) tests are long and expensive

rupture life: time to break the specimen at particular stress and temperature (quick, cheap, inaccurate strain measurements)

Term
Monkman-Grant Relationship
Definition

considers time to rupture and minimum creep rate and states it is a constant

assumes that the bulk of life is in phase 2

Term
temperature and stress dependence and self-diffusion
Definition
the activation energy in the temp/stress relationship correlates well with the self-diffusion coefficient for the bulk material
Term
what is the larson miller parameter
Definition

it allows one to estimate the creep life of a material by extrapolating higher termpature data to much longer times at lower temperatures thereby accelerating the creep test

 

t-time to rupture, Q-activation energy for creep, C-constant

Term
what effects creep
Definition
stress used and temperature
Term
What are the mechanisms of creep and some of the categories
Definition
[image]
Term
What are some methods of diffusion creep?
Definition

Herring-Nabarro: occurs when stresses are too low for dislocation motion can be activated and at high temperatures where diffusion is fast. atoms can move through bulk

desired to have small grain sizes because there is a L-1/2 dependence on grain size

[image]

Coble: at lower temperatures the main diffusion path is along grain boundaries therefore compared with Herring Nabarro, D is replaced with grain boundary diffusivity.  additionally there is a greater dependence on L due to path

Term

Creep deformation maps

what is the effect of work hardening on these maps

Definition

shows under what conditions certain creep mechanisms will occur.  also has contours of constant strain rate

with work-hardening:

  1. yield stress is higher
  2. diffusional creep dominates to higher stresses (dislocations are inhibited by small grains)

 

Term
discuss dislocation creep mechanisms
Definition
  1. power law creep: very strong dependence on stress, usually due to the glide of dislocations which is limited by climb of them around obstacles
  2. pipe diffusion
  3. power law breakdown: stresses overcoming activation energy, reaching realm of plasticity
Term
what do dislocations form as creep progresses
Definition
cellular networks
Term
what methods can be used to design creep resistant materials
Definition
  • use FCC alloys
  • high melting point materials
  • increase grain size or elongate grains in direction of tensile stress
  • add grain boundary precipitates to prevent slididng
  • dispersion hardening
Term
what material is ideal for turbine blade type situations and why is it ideal
Definition

Ni based superalloys

  • diffusion creep is reduced by producing elongated grains with minimal transverse boundaries
  • FCC structure
  • strain minimized during thermal cycling
Term
what feature indicates creep failure
Definition

the formation of voids, microcracks

 

these are usually at the site of grain boundaries or precipitate boundaries

Term
what is Robinson's law and what law is it analogous to?
Definition

similar to miner's law

exposure is broken into intervals at a particular stress and temperature for which the rupture time is known from tests directly

Term

Describe the use of damage parameters

what is the effect of grain size

Definition

failure begins at the onset of void/crack formation

a damage parameter can therefore relate the initial area and the effective area (area lost to voids) to then develop values for the effective stress and Young's modulus

 

 

large grain size means a longer time before tertiary.  useful because you may want to delay the onset of final failure

Term

fatigue + creep : this is a possibility at high temperatures

what equation models this and what are some of the cases that involve material behavior

Definition

this can be modeled by miner + robinson's laws

if there is no itneraction, the lifetimes are unchanged by damaage of the other sort

linear addition leads to direct correlation betweent miner's and robinson and these can be added

if tehre is strong interaction, the lifetimes for both creep and fatigure are much shorter

Term
what are some mechanisms of interaction between creep and fatigue
Definition
  1. blunting of crack tip by creep>>reduce fatigue damage rate
  2. cutting of grain boundary porosity>>increase growth rate of pores
  3. increasing dislocation density>>decrease primary creep
  4. increase in dislocation density>>icnrease creep rate
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