DHR/AR Rheometer: Setting Up an Oscillation Amplitude Sweep

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Overview
Test Setup

Overview

In the Oscillation Amplitude Sweep, the strain (displacement) or stress (torque) amplitude is varied linearly or logarithmically. Strain, stress amplitude, and phase shift are recorded. The main reason for the amplitude sweep is to determine the onset of non-linear behavior. In the non-linear region, the data points are not equally spaced as a function of strain amplitude when the stress amplitude is controlled. In the non-linear region, the material response is not sinusoidal anymore, and odd higher harmonics become important. The magnitude of the higher harmonics can be evaluated.

The onset point of non-linear behavior in a strain sweep of structured fluids is often used to determine the yield behavior. The critical strain is a measure of the yield point, whereas the corresponding stress is the yield stress. Strain sweeps on structured fluids provide meaningful data only when the sample is in equilibrium prior to testing.

Test Setup

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

When you perform an Oscillation Amplitude Sweep, the following parameters need to be chosen.

Environmental Control

Select the following environmental control parameters:

Temperature: The temperature selection to maintain throughout the test. Enter the desired test temperature, or select Inherit set point to maintain the previously-specified temperature at the start of this step. The temperature range is dependent on the configuration of your instrument and the installed environment control system.
Soak time: The amount of time to delay data acquisition at the start of the step, typically to allow for temperature equilibration. This time is measured from the start of the step if Wait for temperature is not selected, or from the point at which the measured temperature becomes stabilized at the commanded Temperature if Wait for temperature is selected. Because of the mass of the sample, geometries, and environmental control systems, a "soak time" (i.e., time to equilibrate at temperature) is recommended, particularly when starting experiments at subambient temperatures or when the temperature is changed significantly between steps. A five-minute soak time is sufficient for most samples in cases where the change in temperature is not too large. This time is also used at each increment temperature.
Wait for temperature: Select this option to wait until the measured sample temperature reaches the specified temperature before beginning data acquisition. If you wish to begin data collection without waiting for the sample temperature to reach the specified value, disable this option.

Test Parameters

Set up the following test parameters.

Choose between Frequency (Hz) and Angular Frequency (rad/s).

Three types of oscillation amplitude sweep tests can be run (Logarithmic sweep, Linear sweep, and Discrete sweep). Choose the desired method from the list. Then choose the desired control variable. Select between Torque, Stress, Displacement, Strain, or Strain%. This test can be run using either torque/stress or displacement/strain as the controlling variable.

Torque: The torque is defined as the specified amplitude of the torque to be applied by the motor at each measurement. This value is used to extract the torque M_s applied to the sample during the measurement. The torque should be selected to be within the linear viscoelastic range of the sample, yet still provide a large enough signal to ensure good data.
Stress: The stress is defined as the specified amplitude of the stress to be applied to the sample at each measurement. This value is determined from the sample torque M_s applied to the sample during the measurement and the sample geometry and dimensions. The stress can be selected to simulate real-life end-use conditions, or it can be a value selected to be within the linear viscoelastic range of the sample, and still provide a large enough signal to ensure good data.
Displacement: The displacement is defined as the specified amplitude of the displacement to be applied to the sample by the motor at each measurement. The angular displacement should be selected to be within the linear viscoelastic range of the sample, yet still provide a large enough signal to ensure good data.
Strain: The strain is defined as the specified amplitude of the strain applied to the sample at each measurement. This value is used, along with the sample geometry and dimensions, to calculate the peak angular deflection to be applied to the sample during the measurement. The strain can be selected to simulate real-life end-use conditions, or it can be a value selected to be within the linear viscoelastic range of the sample, and still provide a large enough signal to ensure good data. Strain may be entered also as a percentage.

The available range is dependent on your instrument type and configuration.

Logarithmic sweep: The logarithmic sweep uses the entered values as the starting and ending range of the test, with intermediate points spaced logarithmically. Logarithmic sweeps can be run in ascending or descending order.
Select between Torque, Stress, Displacement, Strain, or Strain%. then enter the desired range.
Enter the desired Points per decade. This sets the number of points collected in each decade, based on the initial value. The final value is always collected, regardless of whether it is part of the normal pattern.
Linear sweep: The linear sweep uses the entered values as the starting and ending range of the test, with intermediate points calculated by adding or subtracting the increment until the final value is reached.
Choose between Torque, Stress, Displacement, Strain, or Strain%. then enter the desired range.
Choose between Increment or Number of points to be used when collecting test data.
Enter the desired Increment. The increment specifies the change in value between subsequent measurements. If the final value is greater than the initial value, then this value is added at each measurement. If the final value is less than the initial value, then this value is subtracted. The final value is always measured regardless of whether it is generated by the increment or not.
Enter the desired Number of points, which specifies the number of data collected in the specified range. This includes both the initial and final value.

Discrete sweep: The discrete sweep takes a measurement at each frequency in a list of frequency values, with up to 10 discrete frequencies being specified. The test frequencies in the list can be run in any order (i.e., do not have to be monotonic).
- Select between Torque, Stress, Displacement, Strain, or Strain%.
- Enter the initial frequency into the first field.
- Click Add to specify another frequency.
- Repeat step 2-3 until the desired frequency values are specified.

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.

Use the Add button to add a new value at the selected location, shifting all of the other table entries down one position.
Use the Remove button to remove the selected value, shifting all of the table values below the deleted entry up one position.
Use the Reset button to clear all of the table values, leaving only one entry.

Controlled Stress/Strain Advanced

Select from the parameters listed below:

Controlled stress type (Torque and Stress only):
Standard: Oscillation stress/torque is applied to the sample during data acquisition only.
Continuous oscillation: Oscillation torque/stress is applied continuously during the test. Selecting Continuous oscillation will ensure that there is no break in the oscillation. This prevents the motor from drifting between measurements when testing soft or low viscosity samples. Parallel superposition is not available in this mode.
Controlled strain type (Displacement, Strain, Strain% only):
Non-iterative sampling: Determines the torque command for the motor. The software uses the last stress value and predicts the new value required to obtain the target strain based on the previous test results. Note that for the first data point, iterative sampling is used to find the requested strain value. Use this option when it is more important to make rapid measurements (such as monitoring a cure process) rather than accurately control the strain.
Select the initial Torque or Stress to start the sampling process for the first data point. The lower torque limit defines the lowest torque to be applied by the instrument. The iteration procedure stops after the a given number of tries is performed or when the strain is within the tolerance range. The two check boxes allow to save or discard the trial data points with or without all iteration information. Note that if no iteration is desired for the first point, set the number of tries to 1.
Precision sampling: This mode controls the strain by adjusting the motor torque at the end of an oscillation cycle by an iterative approach. Several test cycles are necessary to reach the target strain. If it is more important to make rapid measurements (such as a monitoring cure process), use the Non-iterative sampling option.
Select the initial Torque or Stress. The lower torque limit defines the lowest torque to be applied by the instrument. The iteration procedure stops after the a given number of tries is performed or when the strain is within the tolerance range. The two check boxes allow to save or discard the trial data points with or without all iteration information. Note that if the initial torque (stress) guess is more than one decade off, 4 iteration cycles might not be enough to reach the target strain.
Continuous oscillation (direct strain): In this mode, the motor torque is adjusted during the oscillation cycle to apply the command strain. Parallel superposition and harmonic analysis are not available in this mode.
Motor mode (Displacement, Strain, Strain% only): Choose between Auto, Soft, Medium, and Stiff, depending on the sample stiffness. As a rule of thumb, leave this option set to Auto. Matching the mode to the stiffness of your sample may increase the quality of your data.

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.

Following settings are available in ‘correlation’ mode:

Select the desired Conditioning time:
Time: This is the time, in seconds, during which an oscillation torque is applied before enabling the data acquisition.
Number of cycles: This is the period, in number of oscillation cycles, during which an oscillation torque is applied before enabling the data acquisition.
Select the desired Sampling time:
Time: This is the time, in seconds, during which an oscillation torque is applied and data are acquired. Note that the minimum time is always the time required for one cycle of oscillation and overwrites the user selection.
Number of cycles: This is the period, in number of oscillation cycles, during which an oscillation torque is applied and data are acquired. The minimum value is one cycle.

Save waveform (point display): Select this option to store a snapshot of the data correlation buffer used for the calculation of the oscillatory data, along within the data file, for future recall. Note that this waveform snapshot is a small subset of the actual data used in the calculation, and can be used to provide insight into the quality of the stress and strain signals used based on the shape of the waveforms and noise levels present.

Numbers of points in waveform: Select the size of the data subset. The default is 64 points. The maximum is 1024 (full data set).

See Also

Viewing the Point Display

Save image: Select to store images of the test within the data file for future recall when using the camera accessory.
Use additional harmonics: Select this option to specify that additional correlations be made at the specified harmonics. These calculations result in additional variables to be added to the test. Two modes are possible:
Multiwave: In this mode, up to the 100 first harmonics of the fundamental excitation signal (torque or displacement) can be added. The amplitude for each harmonic in terms of a multiplier of the amplitude of the fundamental excitation signal can be specified. Use this mode to determine the sample response for the selected frequencies simultaneously.
Harmonic analysis only: In this mode, the excitation frequency is not modified and remains sinusoidal. Select up to the 100 first harmonics of the excitation frequency to analyze the response signal (torque or displacement). Use this mode to quantify non-linearities in the sample response.
NOTE: Inertia and compliance correction/compensation is not available for the harmonic analysis. In the multiwave mode, make sure that stress and strain amplitudes are small enough to ensure a linear sample response at all frequencies.

Following settings are available in ‘transient’ mode:

Select the desired Conditioning time:
Time: This is the time, in seconds, during which an oscillation torque is applied before enabling the data acquisition.
Number of cycles: This is the period, in number of oscillation cycles, during which an oscillation torque is applied before enabling the data acquisition.

Select the desired Sampling time:

Time: This is the time, in seconds, during which an oscillation torque is applied before enabling the data acquisition.
Number of cycles: This is the period, in number of oscillation cycles, during which an oscillation torque is applied before enabling the data acquisition.

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

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:

Torque: Enter the desired torque in N.m to control the flow.
Stress: Enter the desired stress in Pa to control the flow.
Velocity: Enter the desired velocity in rad/s to control the superposed flow.
Shear rate: Enter the desired shear rate in 1/s to control the flow.

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.