DHR/AR Rheometer: Setting Up an Oscillation Fast Sampling Test

In this topic
Overview
Test Setup

Overview

In an Oscillation Fast Sampling experiment, a sinusoidal strain is applied to the sample while the measured torque and angular displacement signals are collected in a continuous buffer and correlated at a rate of up to 100 times per second, independent of the selected test frequency. The size of the buffer is defined by the number of cycles set for the data acquisition. Although the correlation rate is independent of the excitation frequency, the correlation rate should be set to allow at least 20% renewal of the buffer content for each consecutive correlation. Since one data point averages at least 1/2 of a cycle, the test frequency has to be selected such that the oscillation period is much smaller than the duration of the reaction process. AutoStrain is not active during fast sampling experiments.

Typical applications are fast curing processes such as UV curing or fast structure recoveries after previous shearing.

Test Setup

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

When you perform an Oscillation Fast Sampling test, the following parameters need to be chosen.

Environmental Control

Select the following environmental control parameters after choosing between Isothermal or Ramp control:

Isothermal control

Temperature: The temperature range is dependent on the configuration of your instrument and the installed environment control system. The start and end temperature, as well as the temperature increment, are dependent on the transition(s) being evaluated. The initial and final temperatures should bracket the entire range of interest. Enter the desired start test temperature or select Inherit set point to maintain the previously-specified temperature at the start of this step.
Wait for temperature: Select this option to wait until the entered temperature is reached before beginning the test. If you wish to begin data collection, while achieving the temperature, disable this option.
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.

Ramp control

Temperature: The temperature range is dependent on the configuration of your instrument and the installed environment control system. The start and end temperature, as well as the temperature increment, are dependent on the transition(s) being evaluated. The initial and final temperatures should bracket the entire range of interest. Enter the desired start test temperature or select Inherit value to maintain the current temperature conditions at the start of this step.
Wait for temperature: Select this option to wait until the entered temperature is reached before beginning the test. If you wish to begin data collection, while achieving the temperature, disable this option.
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.
Ramp rate: Enter the desired rate of thermal change that the sample material will undergo during the test.
End temperature: Enter the desired end test temperature.
Soak time after ramp: Enter the desired time period during which temperature is held at the selected End temperature after the ramp has been completed.

Test Parameters

Set up the following test parameters.

Duration: Sets the duration of the test (Isothermal control only).

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, and 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, and 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.

Enter the desired Single point frequency:

Select between Frequency (Hz) or Angular frequency (rad/s).
Enter the desired frequency into the first field.

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 the Data acquisition drop-down arrow.

Correlation Parameters: These options control how data is sampled for use in the dynamic mechanical correlation used to generate the oscillatory data. These settings are typically used to modify the amount of time for the sample to reach steady state oscillation, and to control the amount of raw data sampled for use in the correlation.

Delay cycles: The number of cycles (whole or fractional) to apply the sinusoidal deformation at the specified amplitude and frequency, before beginning data collection.
Delay time: The amount of time, in seconds, to apply the sinusoidal deformation at the specified amplitude and frequency before beginning data collection.

Both Delay cycles and Delay time are used to specify the amount of time used to reach steady state at each measurement. If both Cycles and Time are specified, the shorter of the two time periods is used. This can be particularly useful in the case of a frequency sweep, where, at low frequencies (with very long cycle periods) the delay is determined by the delay cycles, but at higher frequencies the delay can be based on the delay time.

Sampling cycles: The number of cycles to use for collection of raw data, in units of half cycles. The larger the number, the more raw data is collected for the measurement, but the longer the time required for each data point. Note that the maximum data collection rate is 100 points per second.
NOTE: The actual number of data points is dependent on the test conditions (dynamic frequency and correlation settings). For measured frequencies above 79 rad/second, the maximum fast sampling rate is 100 to 150 points/second. For measured frequencies below 79 rad/second, the maximum fast sampling rate is 50 to 100 points/second.
Frequency based correlation: Select this option to use the instrument correlation default settings. The number of cycles to use for collection of raw data changes as a function of the selected frequency. With increasing frequency, the number of correlation cycles increases. (<1Hz, 2hcycl; <2, 14hcycl;<4Hz, 30hcycl;<8Hz, 60hcycl;<16Hz, 120hcycl;<32Hz, 240hcycl;<64Hz, 500hcycl;<100Hz, 1000hcycl).
NOTE: Selecting this option will reduce the maximum sampling rate.