ENSC1004 - Lecture 15

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ENSC1004 - Lecture 14
Creep - Slides

Creep

• Creep is the time-dependent and dissipative deformation of materials subjected to constant stress
  • More critical at high temperature
  • Observed for all material types

Stages of Creep:

• Instantaneous deformation $-$ mainly elastic (upon initial load)
• Primary creep $-$ strain rate decreases with time
  • Material increases in creep resistance
• Secondary creep $-$ constant strain rate
  • Competing processes of hardening and recover
• Tertiary creep $-$ strain rate increases with time
  • Formation of internal cracks, voids, etc.

You can observe the strain rates (slopes) on the diagram above

Effect of Temperature on Creep

• Both temperature and the level of applied stress influence the creep characteristics
• Increasing stress or temperature, the following will be noted:
  • The instantaneous strain at time of initial stress increases
  • The steady-state creep rate increases
  • The rupture lifetime decreases
• Cracking that precedes the rupture of the material can be transgranular or intergranular
  
  $\dot{ϵ}=K_1{\sigma }^n$
• $K_1$ and $n$ are material constants

Taking temperature into account:
$\dot{ϵ}=K_2{\sigma }^n\text{exp}(-\frac{Q_c}{RT})$

• $K_2$ and $n$ are material constants
• $Q_c$ is the activation energy for creep
• $R$ is the gas constant ($8.31\text{J}\cdot \text{mol}/K$)

Data Extrapolation Methods for Creep

• Sometimes impractical to conduct creep tests over long periods of time
  • More convenient to conduct tests at excessive temperature over a short period
  • Then extrapolate that data to in-service conditions
• Commonly used Larson-Miller method: $m=T(C+\log t_r)$
  • $C$ $-$ constant (usually ~20)
  • $T$ $-$ absolute temperature (kelvin)
  • $t_r$ $-$ rupture lifetime (typically in hours)

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ENSC1004 - Lecture 16