The shear rheometer is a measuring device that applies a shear deformation and thus serves to determine the deformation and flow behavior of matter (see rheology ). The term viscometer is also used colloquially ; however, this expression should be restricted to the devices listed in the relevant article.
The high-priced shear rheometer differ from low-priced rotational addition to higher measurement accuracy primarily by the ability in an oscillating mode, wherein the sample of a sinus shaped load is subjected to not only determine the "classical" rheological data, but also continuously also the parameters of viscoelastic samples. In most cases of rheological practice, however, the rotating devices are sufficient.
Structure and functionality
As with the rotary viscometers, there are also two main types of measurement system for the shear rheometers:
- Coaxial cylinder measuring system: The cylindrical measuring cup and measuring body have the same axis of rotation. The measuring cup can stand still and the measuring body rotate (Searle system) or vice versa ( Couette system).
- Plate / plate or plate / cone measuring system: A second plate or a flat cone (cone angle <3 °) rotates on a fixed, flat plate at a certain distance.
During the measurement, the sample is sheared between the rotating or oscillating and the stationary part of the arrangement. The shear rate results from the geometry of the measuring arrangement and the speed of the moving part . The torque required to maintain the movement is measured, from which the shear stress and thus the viscosity and other rheological parameters can be determined.
The plate / plate and plate / cone design is particularly preferred for shear rheometers, since temperature programs can be carried out very efficiently due to the small amount of sample. The plate / cone design has the advantage over the plate / plate design that the outwardly increasing gap width compensates for the outwardly increasing circumferential speed, so that a homogeneous shear speed prevails in the entire measuring arrangement. The plate / cone arrangement is less suitable for filled samples.
The coaxial cylinder measuring systems have the advantage that the sample cannot emerge from the side and any sedimentation of particles in the sample has less of an effect on the measurement result. Instead, there are not clearly defined shear conditions at the two ends of the measuring body, which lead to measurement inaccuracies. In addition, the cleaning effort is higher.
Temperature control unit
As the rheological properties are mostly influenced by the temperature, rheometers are equipped with temperature control units to bring the measurement geometry to defined temperatures or to run temperature profiles. Liquid baths, convection ovens and cooling systems, and Peltier elements are common for this purpose .
Drive and sensors in the direction of rotation
The moving part of the measurement geometry, usually the upper part, is set in a rotating or oscillating motion by an electric motor. Either speed or torque or the corresponding amplitudes can be specified. A rotary encoder records the angular deflection; the torque can be measured using the motor's input current.
The measuring accuracy depends above all on the exact transmission of the torque measured in the drive unit to the measuring geometry. In the case of low-viscosity samples and / or low shear rates, the shear stress that occurs and thus the torque is so small that it can be of the order of magnitude of the torque caused by friction in the drive mechanism. This is why high-priced shear rheometers are equipped with air or magnetic bearings for a wide measuring range in order to minimize friction.
Drive and sensors in the axial direction
For handling processes such as B. sample introduction, the upper measuring arrangement must also be movable in the axial direction. In the case of plate / plate and plate / cone arrangements, the measuring gap must also be set precisely. Modern devices have a device for measuring the gap so that it can be set automatically and readjusted if necessary, especially when there is a change due to thermal expansion due to temperature changes.
Some devices also have a sensor for measuring the normal force . This allows a normal force to be adjusted on the sample to be specified, with which, for example, a possible shrinkage of the sample can be compensated and measured. Also, tensile and compression tests are thus possible to a limited extent.
- Viscosity (depending on various parameters such as temperature, time, shear rate, etc.)
- Yield point
- complex viscosity
- Loss factor ( )
- complex shear modulus (G ', G ")
- If there is a normal force sensor, it is also possible to measure shrinkage and expansion processes.
In the case of oscillating measurements, plotting the values for load and deformation as a curve over time generally results in two time-shifted sinusoids, i.e. H. the maximum deformation occurs later than the maximum load. The time difference between the maxima is measured in relation to the duration of a full oscillation and is referred to as the phase angle δ (unit degree (°) or gon , a full oscillation corresponds to 360 ° or 400 gon). If the phase angle is zero, i. H. If the deformation appears at the moment the load is applied, it is a purely elastic material such as B. rubber. If the tested material also has viscous properties, the deformation appears later than the load. In extreme cases, a material shows purely viscous properties such as B. water. Then at the moment when the applied load is greatest, the deformation equals zero, but here the deformation speed is greatest. This corresponds to a phase angle of 90 ° = 100 gon.
These relationships only apply to oscillating loads in the steady state, but the value determined largely describes the material behavior under short-term loads.
In some cases, mechanical extensions are also available which allow solids to be clamped or which convert the rotary motion into a linear movement, with which expansion tests can also be carried out on solids ( dynamic mechanical analysis ).
Rheometers are sometimes used in quality assurance , but primarily in product development and research. So z. B. Curing processes of reactive adhesive systems can be investigated. A rheometer can also be used to examine more complex effects such as thixotropy . They can also be used to characterize the molecular architectures of materials with very small amounts of sample.
Test preparation and procedure using the example of a bitumen sample
Shear rheometers are used to determine the phase angle and the complex shear modulus of bitumen used in road construction. Due to a high, complex shear modulus, there is hardly any deformation on roads that are driven on at high speeds, since there is no such great deformation due to the additional shear resistance during the short load duration of the rollover.
To create the bitumen sample, the material is pressed at 80 ° C to a thickness of 2 mm, then punched to the test size and placed in the middle of the lower measuring plate. After lowering the upper measuring plate and setting the test gap of one millimeter, the protrusions are cut off. A water bath (60 ° C) is created around the sample to maintain a constant test temperature.
For the test, the two plates are shifted or rotated against each other while oscillating, with the prescribed frequency of 1.59 Hz and a shear deformation of approximately 12% at the maximum points. The applied load τ and the resulting deformation γ are recorded separately over a test duration of several minutes.
- Lothar Gehm: Rheology - Practice-oriented basics and glossary . Vincentz Network, Hannover 1998, ISBN 3-87870-449-6 .
- Thomas Mezger: The rheology handbook: for users of rotation and oscillation rheometers . 2nd edition, Vincentz Network, Hannover 2006, ISBN 3-87870-567-0 .
- Gebhard Schramm: Introduction to Rheology and Rheometry . 2nd edition, Thermo Fisher Scientific, Karlsruhe 2004.