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DHR/AR Rheometer: Setting Up a Step (Transient) Creep Test

In this topic
Overview
Test Setup

Overview

The response of a material to a shear stress step s(t) = so h(t) at time t = 0, is the shear strain g(t). The material function is the compliance J(t):

J(t) can be separated into 3 terms:

with Jo the time independent elastic compliance;Jd the retarded compliance and Y(t) the creep function with Y(0) = 0 and Y(∞)= 1

In a creep test, a step stress function is applied to the material and the deformation is recorded over time. During the creep phase, the strain increases to reach a steady state flow eventually.

If the stress is removed after a time t =t1 the recoverable deformation (recoil) g r (t) is obtained.

In the Recovery phase, the elastic energy stored in the material, drives the strain back to an equilibrium strain.

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If steady state flow has been obtained in creep, i.e.,Y(t1) = Y(t)= 1:

The recovered strain for t→∞ is the total recoverable strain gR (t1) at time t1: If no steady state flow has been obtained during creep (Y(t1)#1), gR reduces for t→∞

If steady state flow is obtained during creep, gR(t1) reduces for t→∞ to:

The steady state compliance Je is the sum of Jd and J0 . Creep recovery experiments are inherent to CMT rheometers and conducted in an open loop control.

For elastic fluids, the deformation will recover and a steady state in the compliance will be reached with time. The total compliance at time t1 can be separated into two parts, a recoverable compliance and a non recoverable compliance. From the recoverable compliance the equilibrium compliance is obtained, however, only when steady state flow was reached during creep. From the non-recoverable compliance, the viscosity can be obtained.

Typically a creep recovery experiment is time-based. Steady state sensing algorithms are available to terminate the creep, as well as the recovery, when equilibrium has been reached.

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Test Setup   

To select a Step (Transient) Creep test, see Using Experimental Procedures for detailed instructions.

When you perform a Step (Transient) Creep test, the following parameters need to be chosen.

Environmental Control

Select the following environmental control parameters:

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Test Parameters

Set up the following test parameters.

  1. The % Tolerance default value is five percent of the total number of data points in the test. Change the percentage, if desired.

Data Acquisition 

  1. Save image: Select to store images of the test within the data file for future recall when using the Camera accessory.
  2. Select the Fast sampling option, or manually set the data sampling conditions. In the fast sampling mode, data acquisition decreases logarithmically with time.
  3. If fast sampling is not set, data are acquired linearly.
  4. Enter the Initial time between samples.
  5. If Adjust time between points is selected, the linear sampling rate is adjusted (and time between data points is doubled) after a given number of data points.
  6. Enter the Number of points before time changes.

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Step Termination 

TRIOS software allows you to define conditions in which a step is halted ahead of its normal termination conditions (Limit checking). You can use this to ensure that, for instance, the instrument does not over speed or apply excessive strains.

Rather than running a step for a certain amount of time, you may wish to run it until stable data is obtained. You can set an Equilibrium limit (such as the viscosity value becoming constant when running a single shear with time) that will stop the currently active test.

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