Creep

The creep as well as the stress relaxation are two methods into quasi-static of characterization of a ductile Matériau .
The creep is the physical phenomenon which causes the irreversible Déformation of a material subjected to a constant Contrainte (noted σ0) for one suffisante.
length of time The deformation (the answer to the constraint) ε (T, σ0) depends on conditions external at piece-rates: time (T), value of the constraint, Temperature, pressure, etc.
Concerning this Mechanical test, two cases arise:

  • for a Test tensile, a Poids is imposed on a test-tube (use of a suspended mass);
  • while for a shear test, a couple is imposed.
For a viscoelastic material, the deformation corresponds to instantaneous elasticity , delayed elasticity and the flow visqueux.
The experiment of creep can be controlled by measuring the variation of the test-tube after having withdrawn the pressure applied. The material underwent a deformation permanent (or residual) connected to the irreversible flow. This phenomenon relates to especially the viscoelastic Fluide S . The longer the duration of the constraint was, the more the permanent deformation is importante.
Creep originates in the phenomenon of slip of the macromolecular chains the ones compared to the others, in the case of a polymère.
To avoid this phenomenon of flow, the slip should be decreased. A solution consists in proceeding to the Réticulation of the polymer, which will create covalent bonds between its chains (bridging). The deformation of the solid viscoelastic obtained will be weaker than that of the material fluide.
Some examples for including/understanding well:
  • the Glaciers run out under their own weight because the ice creeps;
  • a metal part heated with the red can be deformed like a paste. Indeed, at very high temperature, metals behave like viscoelastic fluids;
  • a rack bends after a few years under a constant weight;
  • a paddle of engine of Avion is subjected to the centrifugal force like with an high temperature during operation. The study of the creep of turbine blades is a problem running of industrial research.

See too

External bonds

Techniques of the Engineer: HTTP: /www.techniques-ingenieur.fr/dossier/essais_de_fluage/M140

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