Brookfield DV3T Rheometer - Manual (Page 52)

Brookfield Engineering Labs., Inc. Page 52 Manual No. M13-2100
Pre-Shear Speed and Time
An optional Pre-Shear step can be included in the test parameters. The user must supply the
pre-shear speed (0.01 - 250 rpm) and pre-shear time (1 second - 5 minutes 59 seconds).
Pre-shearing is the shearing of sample before measuring its yield properties. This process
breaks down the sample’s structure. It is particularly useful if the investigator wants to eliminate
previous shear history (e.g., bumping, transferring) of the sample before testing and observe
the structural rebuilding of the sample. This may simulate the following: ketchup pumped out
of a bottle will rebuild after it comes to rest on the French Fries. There are materials whose
measured yield stress will be lower after pre-shearing than if tested without pre-shearing. This
may be used to compare the rate at which different materials rebuild. The yield stress measured
in a pre-sheared sample is the “dynamic yield”, while the yield stress measured for an originally
undisturbed material is the “static yield”. Zeroing after pre-shearing is performed on the sample
is highly recommended for every test. The next section explains how to do this.
TIP: Set Pre-Shear Speed and Pre-Shear Time to zero if no Pre-Shear is required.
Zero
An optional, but highly recommended, torque Zero step can be included in the test parameters.
A zero speed will cause the rheometer to rotate the motor in the appropriate direction until 0%
torque is achieved.
This may be necessary because the spindle sometimes twists a small amount during insertion
into the sample. This results in a small, although possibly significant, torque applied to the
sample. Slow zeroing speeds are used to eliminate this initial torque and minimize effects on
the sample’s structure before the test is started.
Zeroing is an essential step after pre-shearing.
A zero step ensures a consistent starting point for each test. A speed for the step must be supplied
in the appropriate box (0.01 - 0.50). Faster speeds achieve a “zero” quicker but may cause a
more variable starting point. The DV3T will reset zero speed to 0.01 rpm when the %Torque
value reaches 0.20% Torque.
Wait Time
An optional Wait Time step can be included in the test parameters. Wait Time creates a time
delay after zeroing before the actual test run begins. During this delay, the motor shaft will be
at zero (0) RPM.
Wait Time is the time the sample is allowed to rest between the completion of zeroing and the
start of the yield measurement. Some samples rebuild structure more slowly than others after
shearing, such as during handling, pouring sample into a beaker, etc. Certain low-viscosity
paints may also have a low yield stress. Waiting 30 seconds, for example, after immersing the
spindle may allow the sample to rebuild, producing a more consistent test method.
Run Speed
Run Speed is the motor speed for the DV3T at which the material is tested. It is common for
materials to appear stiffer when tested at higher speeds. That is, the slope of the stress-vs.-
strain curve increases with increasing speed. This is because the material structure has less
time in which to react to dissipate the applied stress. Increasing the speed will, in most cases,
increase the yield stress measured by the instrument. Most yield tests are conducted at relatively
low speeds (<1 rpm) to minimize any inertial effects when using vane spindles. The range of
acceptable run speeds is 0.01 - 5.0 RPM.