DHR/AR Rheometer: Setting Up a Step (Transient) Stress Growth Test

In this topic
Overview
Test Setup

Overview

In a step strain rate test, the deformation rate is stepped from zero to g₀ at t = 0.

The response of the material to the strain rate step is the stress s(t). The material function is the time-dependent viscosity function:

The linear viscoelastic viscosity function h(t) is independent of g₀. In a step rate test (stress growth), a step strain rate is applied and the stress is recorded over time. At a constant strain rate the sample strain increases linearly with time. The stress response increases from zero, and exponentially approaches a steady state value. (stress growth).

When the step rate experiment is performed using a cone and plate geometry, viscoelastic fluids generate a normal stress, which is measured as normal force. The normal stress increases from zero to reach a steady state value, similar to the shear stress.

The first normal stress coefficient can be obtained from first normal stress difference

The first normal stress coefficient reduces for t→∞ (steady state flow) to:

The start up curve of the transient shear stress and predominately the normal stress depends on the response time of the actuator and the instrument compliance. Below 0.1 seconds, the stress growth is significantly affected. Similar to the stress relaxation test, better short time response is obtained when the viscosity function and the normal stress coefficient are evaluated, based on the actual rate and not the command rate.

In the linear region at low rate, the viscosity curves superpose, and the viscosity is independent of the shear rate. With increasing rate, the viscosity becomes non linear and at high rates exhibits a stress over-shoot. The normal stress shows the same behavior. The stress growth experiment is predominately used to measure the non linear viscous (viscosity) and elastic (normal stress) behavior of a viscoelastic material.

Test Setup

To select a Step (Transient) Rate i.e. Stress Growth test, see Using Experimental Procedures for detailed instructions.

When you perform a Step (Transient) Rate test, the following parameters need to be chosen.

Environmental Control

Select the following environmental control parameters:

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.

Test Parameters

Set up the following test parameters.

Enter the Duration time of your experiment.
Enter the desired Shear rate or Velocity value. The Shear rate is the specified rate of steady shear deformation in reciprocal seconds.

Data Acquisition

Save image: Select if you want to collect images using the furnace camera or any webcam connected.

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.