Torsion Rectangular/Cylindrical Geometry for the DHR/AR Rheometer

In this topic
General Information
Applications
Loading the Sample
Clamping Torque
Axial Force Control
Equations

General Information

Torsion measurements are performed either on a rectangular-shaped or a cylinder-shaped sample. Torsion rectangular is the most used test method because sample preparation in compression molding is simple, and temperature equilibrium of the rectangular specimen is faster than for the cylindrical specimen. The disadvantage of the rectangular cross-section is that the constant stress lines are not linear and symmetric to the center of rotation. The maximum stress line is the centerline of the long side of the rectangle. De Saint Vénant was the first to describe the torsion of a non-symmetric test specimen. A schematic drawing of the torsion rectangular and torsion cylindrical test fixture is shown below. Solid or rubbery samples are preformed into rectangular bars and clamped with their long axis coaxial with the rheometer rotation axis. The ETC is used for samples to be run in air or inert gas. For submersion in water or aqueous solutions, a submersion cell is available for use with the Peltier concentric cylinder jacket.

Two pair (upper and lower geometry) of jaws clamp the sample at both ends. In the torsion rectangular clamps, the sample is held between a spacer block and a clamping block; the clamping force is applied on both sides of the clamping block. In the torsion cylindrical, the sample is held top and bottom between appropriate size collets. Since the upper and lower geometry is separated by the long dimension of the sample, the expansion of the sample needs to be corrected for during temperature ramp tests to avoid sample buckling or stretching. The axial force adjustment mechanism compensates for sample expansion or contraction. Near the glass transition, the axial force needs to be controlled even more accurately to avoid undesired sample stretching or compression. Based on the sample length changes as a function of temperature, the coefficient of thermal expansion for the sample under test is also calculated according to:

The typical length of a torsion sample is between 10 mm and 40 mm. The width for rectangular samples is 10 mm to 12 mm. Stainless steel clamping blocks with spacers are used to accommodate various sample thicknesses and to center the rectangular sample.

For cylindrical samples three different diameter collets are provided (1.5, 3.0, 4.5 mm). Samples need to be prepared within ± 0.2 mm of the collect diameter.

Applications

The torsion geometry is used for testing solid materials such as thermoplastics, thermosets, composites, and elastomers below the glass or melt transition. The torsion geometry is used with the furnace (ETC) on DHR/AR Rheometers.

The rectangular torsion submersion clamp available for the DHR/AR is integrated into the Peltier fluid jacket for concentric cylinder systems (see DHR/AR Series Rheometers Getting Started Guide for details on installation and alignment).

Loading the Sample - Torsion Rectangular

When using the torsion submersion cell, follow the general guidelines below to load a torsion sample on the DHR/AR Rheometer. Although the samples used with the submersion clamps on the DHR/AR Rheometer have the same dimensional ranges as those used with the standard ETC solid sample clamps, the same loading procedure cannot be used because the clamping nuts cannot be tightened when the sample is in place in the cup. For this reason, the sample must be mounted in the clamps before being introduced to the cup. This means that, unlike those of the standard ETC, the upper and lower clamps of the submersion cell need to be aligned with the sample before installing the geometry. To allow this, the base of the submersion cup, which has the lower clamps integrally mounted, is removable and can be located centrally on top of the submersion cup. For gap setting and sample loading using the submersion clamps, follow the instructions given in the Wizards. The Zero Gap Wizard is activated by the zero gap routine and the Sample Loading Wizard is activated on the Instrument section of the ribbon by selecting load sample.

Before loading the sample, prepare the sample specimen with the required dimensions. Measure and record the sample dimensions: Width and Thickness. These dimensions have to be used to update the geometry in the experiment panel.
Select a matching pair of clamping blocks/spacers (based upon sample thickness) to center the sample along the torsion axis. The recommended block and spacer thickness will be indicated in the Experimental panel, upon entering the sample dimensions. Select the correct spacer (depends on the sample thickness) and insert with the clamping block onto the two threaded support rods. Insert the clamp front face insert (3.5 mm). Mount the locking nuts loosely; do not tighten.
Place the sample into the lower geometry between the clamp front face and clamping block. Rectangular samples are centered in the geometry using the reference lines scribed in the clamp face and the clamping block. Partially tighten the clamp (using the two clamping nuts on the sides for the DHR/AR) to hold the sample.
Lower the stage until the upper geometry is about 4 mm from the sample.
Radially align the sample with the upper geometry, if necessary.
NOTE: Ensure that the bearing is unlocked before aligning the sample.
While confirming the sample fits into the geometry, lower the stage until the sample is fully inserted. If the sample is not aligned properly, raise the stage again and carefully realign the sample.
Ensure (visually) that the sample is completely inserted into the geometry.
Tighten the lower and upper clamps (using the clamping screw or nuts). If an isothermal test is run at the loading temperature, or if the temperature is varied throughout the test, with testing temperatures being greater than or equal to the loading temperature, then the clamping screw should be tightened to the desired torque level using the torque screwdriver at the loading temperature. If, however, the starting test temperature is less than the loading temperature, then the clamping screw should be tightened to the desired torque level at the test starting temperature to ensure that the sample is securely clamped in the torsional geometry.
Raise the stage until the desired tensile force is applied. Note that the tension level depends on the sample (see guidelines below). The tension level should be set according to the sample characteristics, with thinner and/or lower modulus materials requiring less axial force.
If the temperature is not held constant during the experiment, the axial tension force will change due to sample expansion or contraction. In order to avoid this effect, activate the axial force control from the Axial force Control panel.

Loading the Sample - Torsion Cylindrical

Before loading the sample, prepare the sample specimen with the required dimensions. Measure and record the sample diameter. This dimension has to be used to update the geometry in the experiment panel.
Select a matching pair of collets (based upon sample and fit to the upper and lower clamp assemblies). Attach the upper geometry and zero the gap.
Place the sample into the lower clamp and partially tighten the clamp to hold the sample in place. It may be necessary to remove the upper geometry to allow access. A Tommy Bar inserted in the lower assembly will allow the sample to drop a certain distance while the upper geometry is fitted.

Lower the head until the sample can be fed into the upper collet, leaving enough sample to be clamped by the lower.
NOTE: Ensure that the bearing is unlocked before aligning the sample.
Finger tighten the lower and upper clamps. Before clamping using the dual Tommy Bar method described below.
Raise the stage until the desired tensile force is applied. Note that the tension level depends on the sample (see guidelines below). The tension level should be set according to the sample characteristics, with thinner and/or lower modulus materials requiring less axial force.
If the temperature is not held constant during the experiment, the axial tension force will change due to sample expansion or contraction. In order to avoid this effect, activate the axial force control from the Axial force Control panel.

Clamping Torque

For torsion rectangular clamps, a torque screwdriver is included with the torsion geometry. For each material, some experimentation may be required to find the clamp torque that should be used. Under-tightening (or for softer materials, over-tightening) the clamps will result in erratic data. Once a torque value is established for a specific sample (material and thickness), all subsequent samples should be tightened to the same value. Both clamps should be tightened to the same torque value.

Samples are typically loaded at room temperature and then tested at various temperatures (i.e., temperature is varied in a temperature ramp test). It may be necessary to clamp the sample in the torsion geometry at the test starting temperature. Guidelines for clamping samples are provided below.

Solids in Torsion: Sample Loading Guidelines for Ambient and Subambient Testing on DHR/AR Rheometer
Sample Type	Loading Guidelines
Soft thermoplastic above T_Q at RT (for example PE, PP, PVDF)	Load at RT and tighten to 40cN•m for DHR/AR. Apply axial force (see Axial Force Loading in the table below). Start the test. For subambient testing, set the initial temperature and allow the sample to equilibrate. Re-tighten clamps to 60 cN•m for DHR/AR. Start the test.
Stiff thermoplastic below T_Q at RT (for example PC, PET, PS)	Load at RT and tighten to 50 to 60 cN•m for DHR/AR. Apply axial force (see Axial Force Loading in the table below). Start the test. For subambient testing, set the initial temperature and allow the sample to equilibrate. Re-tighten clamps to 60 cN•m for DHR/AR. Start the test.
Elastomer above T_Q at RT	Load at RT and tighten finger tight. Apply axial force (see Axial Force Loading in the table below). Start the test. For subambient testing, set the initial temperature and allow the sample to equilibrate. Re-tighten clamps to 40 cN•m for DHR/AR. Start the test.
Stiff Thermoset below T_Q at RT	Load at RT and tighten to 60 cN•m for DHR/AR. Apply axial force (see Axial Force Loading in the table below). Start the test. For subambient testing, set the initial temperature and allow the sample to equilibrate. Re-tighten clamps to 60 cN•m for DHR/AR. Start the test.

For cylindrical clamps the collets are tightened using the Tommy Bars as shown in the pictures below.

Warning: Using a single Tommy Bar to tighten either collet could result in fracturing the ceramic heat break.

Axial Force Control

When the test temperature changes, it is critical to maintain axial force control on the sample. For example, if a temperature ramp test is run from low to high temperatures, then the sample and test geometries will undergo thermal expansion as the temperature changes. If the torsion geometries were held in a fixed position, then as the sample and the geometries expand, the sample would buckle. Sample buckling significantly interferes with calculations for the torsion geometry. To avoid buckling problems, always run torsion tests involving temperature changes with axial force control.

Axial force control allows the user to specify an axial force that is applied to a sample, as well as the allowable variation in the axial force (i.e., 1 N of axial force ± 0.1). The axial force can be specified to be either compressive or tensile. In the case of torsion tests, tensile axial forces are used. The following table provides recommendations for the axial/normal force settings. For additional information, see Axial Force Adjustment Guidelines.

Solids in Torsion: Sample Loading Guidelines for Testing at Room Temperature and Above on the DHR/AR Rheometer
Sample Type	Axial Force	Axial Force Tolerance	Gap Change Limit Up	Gap Change Limit Down
Soft thermoplastic above T_Q at RT (for example PE, PP, PVDF)	2 N	1.75 N	2 mm	5 mm
Stiff thermoplastic below T_Q at RT (for example PC, PET, PS)	2 N	1.75 N	2 mm	5 mm
Elastomer above T_Q at RT	1 N	0.5 N	2 mm	5 mm
Stiff Thermoset below T_Q at RT	5 N	4 N	2 mm	5 mm

Equations

NOTE: The equations used for torsion do not take into account clamping effects. Clamping effects reduce the effective free length of the sample and therefore artificially increase the value of the modulus. Sample clamping effects increase with decreasing sample length and become significant below a sample length of 60 mm. For details on how to determine a corrected (effective) sample length, refer to the TA Application-Product Note APN024, Evaluation of the Correct Modulus in Rectangular Torsion.

Geometry Factors for Rectangular Solid Samples

Geometry Factors for Cylindrical Samples

Variables

T = Rectangular sample thickness (mm)

W = Rectangular sample width (mm)

L = Rectangular or Cylindrical sample length (mm)

R = Cylindrical sample radius (mm)