The behaviour of model materials in creep tests

If we now perform a creep test on a Maxwell model, the behaviour can be described by the simple equation 

If a creep test is performed on a Kelvin-Voigt model, the strain gradually builds up to a constant value as described by 

Here t is called the retardation time, since it characterises the retarded response of the model, and its value is again given by r /G. At very short times, the response is viscous, and y ta/r. At f = t, the strain has risen to 63 % of its final, asymptotic value of a/G. 

If we now stress the Burgers model in a creep test, it is easy to see what will happen, see figure 3 first, the unrestrained spring Gi will instantaneously deform to its expected extent, while the isolated dashpot will start to deform at its expected rate. However, the spring in the Kelvin-Voigt element cannot immediately respond, being hindered (i.e. retarded) by its dashpot. Nevertheless, it does begin to deform, and eventually comes to its expected steady-state deformation. It is possible to show that the overall deformation can be written down as  

Some examples of real materials whose creep deformation at low stresses are described quite well by the Burgers model are 

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