Creep

Question 1

What is creep?

Answer 1

• Time-dependent material deformation under stress and at high temperature. 𝜀 = 𝑓(𝜎,𝑡, 𝑇)
• Permanent deformation, similar to plastic deformation.
• Occurs in metals at high temperature (>0.4 Tm) and in polymers at room
temperature.

Question 2

What is dislocation Creep?

Answer 2

Dislocations which are locked by precipitates

are unlocked by diffusion of atoms.

Question 3

What is diffusional Creep?

Answer 3

Diffusional creep: Dislocations are not involved. Atoms diffuse
from one grain face (GB) to the other at low ‘T’ and within the
crystal (Bulk) at high ‘T’.

Question 4

Explain the three stages of the creep curve?

Answer 4

Primary: Decreasing slope, strain hardening takes place and deformation becomes difficult.
• Secondary: constant slope due to competition between strain hardening and recovery, i.e. softening of material. Longest duration.
• Tertiary: strain rate increases as failure or rupture occurs. Grain boundary separation, cavities, voids and cracks form.

Question 5

Whats a isochronal curve?

Answer 5

creep curve of stain against stress at a specific time (Log Time Scale).