A microscopic view of creep deformation

A microscopic view of creep deformation

The Big Dig ceiling collapse caused by creep

The Big Dig ceiling collapse caused by creep

Creep is a time-dependent permanent (plastic) deformation under a certain applied load.  Generally, creep occurs at high temperature (thermal creep) but can also happen at room temperature depending on the  material (e.g. lead or glass), although this happens at a much slower rate. As a result, the material undergoes a time-dependent increase in length, which could become quite dangerous while in use [1]. The rate of deformation is called the creep rate. It is the slope of the line in a creep strain vs. time curve (see below) [2].

Creep Strain vs. Time Curve

Creep Strain vs. Time Curve

Creep deformation has three stages:

  1. Primary creep  starts rapidly and slows down with time.
  2. Secondary creep progresses at a relatively uniform rate.
  3. Tertiary creep has an accelerated rate of deformation which terminates when the material fails (breaks or ruptures). It is associated with both necking and the formation of grain boundary voids [3].

There are several design strategies that can be adopted to avoid creeping in materials:

  • Reduce the effect of grain boundaries (use single crystal material with large grains).
  • Add solid solutions to fill the voids in the material.
  • Use materials with high melting temperatures.
  • Consult creep test data during materials selection [4].
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