DHR/AR Rheometer: Setting Up an Extensional Test

In this topic
Overview
Test Setup

Overview

The basic deformation applied by a rotational rheometer is the shear deformation. The Universal Testing Platform SER2 transforms the rotational movement of the rheometer into a uniaxial extension deformation in the sample. In contrast to the axial tension tests, huge extensional deformations of Hencky strain of 4 (approximately 50 times the original length) can be achieved. The sample is held in the SER2 by the ends, offering a large free surface to allow a pure extensional deformation. The consistency of the sample must be high enough to prevent sagging. Only viscoelastic materials with a minimum shear viscosity, such as polymer melts, can be loaded and extended without sagging in an early stage of the experiment. Large strain extensional tests can only be performed in conjunction with the SER2 universal testing platform.

In a step Extensional rate test, a constant Hencky rate is applied to the material. The material response is the stress. The material function is the elongation viscosity, the ratio of the stress to the constant elongation rate. Large extensional deformations are typical for many processes in polymer processing, such as film blowing, blow molding, and spinning. The rheological behavior at large elongation deformations cannot be predicted from the shear data. This is due to the fact that structural elements, such as branching of polymers, affect the extensional flow stronger than the shear flow.

The sample specimen is a film 18-mm long, up to 10-mm wide and preferably 0.7-mm thick. The sample dimensions are determined at room temperature and loaded into the rheometer as a solid. At test temperature, the sample melts and changes dimensions. In order to refer the stress calculation to the correct surface area in the rheometer, the specimen dimensions have to be corrected for based on the density difference at room and test temperature.

Test Setup

To select an Extensional test, select Other > Extensional from the Procedure tab option. See Using Experimental Procedures for detailed instructions.

NOTE: Use the Extensional test only in conjunction with the Universal Testing Platform SER2.

When you perform an Extensional 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.
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.
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:

Enter the Solid density and Melt density for your sample. This information is necessary to calculate the correct dimensions of the sample in the melt state from the dimensions of the solid specimen determined at ambient temperature.
Select Prestretch if the density is different at the loading and test temperature. In the Prestretch step, the sample is extended by the length equal to the longitudinal expansion of the sample due to the temperature at the loading and test step. Do not select Prestretch if you add a second extensional test and/or the temperature has not changed and when the density difference is zero.
Prestretch rate: This is the extension rate applied to prestretch the sample. A typical value is 1e-3 1/s.
Prestretch details: This is an information field and is updated as soon as the density or the prestretch rate change. Note that the extensional test fixture has to be the active geometry. Otherwise, no information will be shown.
Relaxation after prestretch: Enter the time desired for the sample to relax after the prestretch.
Select the Duration or Final strain. The Duration is the time of the Extensional test at the selected rate, velocity, or torque. During the extension process, the cross-section of the sample changes and approaches zero with increasing total deformation, or breaks beyond a material specific deformation. In order to stop the test before the sample reaches its breaking point, or in order to avoid multiple rotations of the drums, is usually preferable to terminate the extension step at a Final strain. Note that one revolution of the drums corresponds to a Hencky deformation of approximately 4.
Enter the Extension rate, Extension velocity, or Torque. Extensional tests can be conducted at constant Hencky Extensional strain rate, Extensional velocity (constant rotation speed of the drums), or Torque.
NOTE: Extensional rate, Extensional velocity, and Torque can be positive or negative values. If the instrument is equipped with the ETC camera option, make sure to enter negative values and install the tools as described in the help topic SER Geometry for the DHR/AR Rheometer.
Select the Fast sampling option, or manually set the data sampling conditions. In the fast sampling mode, data acquisition decreases logarithmically with time. Use Fast sampling if you want to record the start-up performance. Fast data sampling starts at the highest possible sampling rate and decreases the sampling rate logarithmically as the experiment proceeds.
Select the sampling mode (Log or Linear) (only available if Fast sampling is not checked).
Select the sampling parameters:
Number of points (Log sampling only): The number of points determines the total number of values collected during the specified range. This includes both the initial and final value.
Sampling interval: The value entered in this field defines the time interval between recorded data points.
Select Post stretch relaxation if you want to continue recording the stress after the deformation stops.

Data Acquisition

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

Save image every: Specify the collection rate for time-based experiments. Select the minimum time in seconds between images. An image is stored with the next test point when the time between images has elapsed.

Advanced

Select the Motor mode. 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.

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.