DHR/AR Rheometer: Setting Up a Step Strain (Stress Relaxation) Test

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Overview
Test Setup

Overview

During a step strain or relaxation test, the material is subjected to a step shear strain at the time t = 0.

The response of the material, the stress s(t), is a decreasing function with time. The relaxation modulus is defined as:

G(t) is a monotonic function. G(t → 0) = G0 the glass modulus; G(t → ∞) = Ge the equilibrium modulus, which is zero for thermoplastic melts. For infinitesimal deformation, i.e., if go → 0 , G(t) becomes independent of go , and is referred to as linear viscoelastic shear modulus.

In the rheometer, the strain does not change instantaneously. Instead, the strain is ramped up to the desired value over a very short, nevertheless measurable, period of time. The time required depends on the magnitude of the step, the power of the actuator, the control settings, and the stiffness of the material. The typical response time is in the order of 10 ms for a 10 mrad step.

The material function obtained from a relaxation test is the modulus. If the modulus is obtained from the measured stress and the command strain, the modulus goes through a maximum below 0.1 seconds. Due to the delayed response of the motor, the stress response of the material is not instantaneous as expected. In order to avoid underestimating modulus at short time, the material function is calculated from the measured stress and the actual (measured) strain. It is useful to look at the results in a linear representation also. The modulus in a relaxation experiment drops exponentially after the commanded strain has been reached. From an experimental point of view, this is not the best experiment to perform on predominately viscous materials, because the torque relaxes to fast and the instrument operates most of the time at the low end of the torque range.

Test Setup

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

When you perform a Step (Transient) Stress Relaxation test, 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.
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.
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 specified Temperature, if Wait for temperature is selected. Because of the mass of the sample, test fixtures, and environmental control systems a "soak time" (i.e., time to equilibrate at temperature) is recommended, particularly when starting steps at different temperatures. 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.

Test Parameters

Set up the following test parameters.

Enter the Duration time of your experiment.
Enter the desired Displacement, Strain, or Strain% value. The strain defines the deformation applied to the sample. This value is used, along with the sample geometry and dimensions, to calculate the peak angular deflection to apply 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.
Check the Steady state sensing box if you want to place a condition on the number of data points required for calculation of the steady state. Enter the following parameters:
The % Tolerance default value is five percent of the total number of data points in the test. Change the percentage, if desired.
Over time period: This is the time period in which the tolerance is verified.
Consecutive within tolerance: Number of repeated verification steps required before accepting steady state flow.

Advanced

Strain rise time: The motor cannot perform an instantaneous deformation step. Enter a rise time (default is 0.01 seconds) to optimize the start up and minimize the strain overshoot. For low-viscosity materials, a longer rise time provides better start up behavior.

Data Acquisition

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

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.