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DHR/AR Rheometer: Setting Up an Oscillation Time Sweep

In this topic
Overview
Suggested Use
Test Setup

Overview

In an Oscillation Time Sweep, the time is the variable. Temperature, frequency, and strain or stress amplitude are typically held constant throughout the test. Strain and stress amplitude, as well as the phase, are recorded to monitor the evolution of a material as a function of time. This test mode can be used to study the stability (thermal, mechanical, etc.) or the evolution of materials with time at constant temperature. As the material evolves, it may be necessary to change the gap due to volume changes using the Axial Force function.

The Axial Force needs to be set in the Gap Control panel for pretest control and in a preceding Conditioning step for control during the test. For fast processes, such as UV curing, use the fast data sampling mode Fast sampling - Isothermal. Since one data point averages at least one cycle, the test frequency has to be selected such that the oscillation period is much smaller than the duration of the reaction process.

Suggested Use

For complex fluids, the oscillation time sweep can be used for pre-testing and to evaluate the effect of strain and time. Since the test parameters frequency and strain can be changed manually during the test via the motor control panels, structure changes can be easily monitored and the testing parameters for oscillation frequency and strain sweeps tests validated.

Time sweeps are performed at constant frequency. However, multiple frequencies are possible using the Multiwave data acquisition option. Multiple frequencies are not possible with fast data acquisition.

For non-linear testing at high strain amplitude, the magnitude higher harmonics can be evaluated.

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Test Setup 

To select an Oscillation Time Sweep test, see Using Experimental Procedures for detailed instructions.

When you perform an Oscillation Time Sweep experiment, the following parameters need to be chosen:

Environmental Control

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Test Parameters 

Set up the following test parameters:

  1. Select between Torque, Stress, Displacement, Strain, or Strain%. This test can be run using either torque/stress or displacement/strain as the controlling variable.

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Frequency Sweep Parameters

Four types of frequency sweeps can be run. Choose the desired method from the list below. The available frequency range is dependent on your instrument type and configuration.

If you want to edit the values already displayed in the table, place the cursor in the table at the desired point and make your editing changes. The entries will be used in order until the first 0.00 entry is encountered, which is recognized as the end of the table.

Controlled Stress/Strain Advanced

Select from the parameters listed below:

Data Acquisition

There are additional data collection options that can be adjusted to control how data is obtained and what additional information is collected during the measurement. To access these options, click on the “arrow” to display these test fields. 

First select the acquisition mode, ‘Correlation’ or ‘transient’. In the correlation mode the transient torque and displacement data are evaluated by the firmware in real time and only the transformed frequency dependent test results are provided. This is the typical mode for most applications. In the transient mode, in addition to the frequency dependent test results the transient torque and displacement data are saved. The transient data are attached to the data file one step for each oscillation data point. The transient mode is typically selected for additional evaluation of the raw data (DFT) or for generating Lissajous plots. Transient data acquisition is only available in continuous (stress and strain) oscillation mode.

Following settings are available in ‘correlation’ mode:

  1. Select the desired Sampling time:

See Also

Viewing the Point Graph

Following settings are available in ‘transient’ mode:

  1. Select the desired Sampling time:
  1. Save image: Select to store images of the test within the data file for future recall when using the camera accessory.

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Controlled Flow

This is used to superpose a continuous flow onto the oscillation signal (parallel superposition). This option is not available for continuous oscillation in controlled stress and strain mode. Select the flow control below: 

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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