Creep (Rheology)

Creep is one of the earliest “controlled stress” rheometer tests that quite literally “creeps” the material, i.e. we measure over a relatively prolonged period the small movement (the creep defined as creep compliance, J) of the sample by applying a small constant stress.

The benefit of this type of measurement is that even with small applied stress, the typically small resultant deformation (J) builds up to be significant over time.

From a rheological point of view is that this also starts to indicate the “viscoelastic” properties of the material as they are resolved with time into the following regions:

  • G1 which is the initial fast “elastic” response where the elastic structure stretches
  • G21 is now the “viscoelastic” response which still has some rapid elastic stress, but slowed down a little with the slower viscous response
  • η2 is now the “viscous” region, where all the elastic structure (JE) has been stretched and we are left with pure viscous flow

As such, early generation rotational rheometers often used creep testing as a form of “high resolution” viscosity measurement to ensure that each data point of a flow curve was at “steady state”, i.e. pure viscous flow.

However, as next generation rheometers such as the Kinexus by NETZSCH has “live data” for each data point in a flow curve, steady state can also be checked without using a specific creep test.

Creep therefore now tends to be used for more specialist testing, such as looking at the prolonged effects of small applied stresses (such as gravity) on a material, or mimicking applied processes such as the MSCR (Multiple Stress Creep and Recovery) test for bitumen/asphalt samples.

The Recovery part of the Creep test is a common extension to validate the creep results. Now the rheometer turns off the small applied stress, to literally measure the recovery with time, where the creep elastic compliance (JE) should be the same as the recovery elastic compliance (JR). Again the “elastic” response is the fastest (G1), followed by the “viscoelastic” response (G21). As viscous flow does not have any recovery we have the irrecoverable flow region which is the same as how far the sample viscous flows.