Compression Geometry

In this topic
General Information
Recommended Sample Dimensions
Installing the Geometry and Loading the Sample
Coefficient of Thermal Expansion
Test Procedure Recommendations
Equations

General Information

The Compression geometry is used for testing soft thermoplastic and elastomer bulk materials in compression. Parallel plate fixtures for ETC and Peltier plate can be used for compression testing.

Applicable Environmental Systems

Furnace (ETC) and Peltier.

Recommended Sample Dimensions

To prepare samples that fit within the physical constraints of the geometry, the following are the recommended sample dimensions:

Bulk Samples

(Depends on plate size)

L = 0.5–5 mm
D = 8–40 mm

Sample Testing Limits

The suggested dimensions for typical samples allow testing in the recommended range based on the limitations of the size of the test geometry. Note that the sample modulus at test temperature has also a significant effect on the choice of the sample dimensions. A single point test with the desired geometry should be used to fine-tune sample parameters and geometry selection. If too much force is required or the measured strain is significantly lower than the commanded value (which indicates that the transducer compliance may be too large to accurately correct), the sample should be made thinner or narrower to obtain better results. If the sample film or fiber cannot be tested practically by itself, a series of films may be stacked, or fibers may be bundled for testing. In this case, geometries may have to be approximated. Any such slight errors in geometry measurement can produce errors in modulus data, but will not affect temperatures of transitions within the sample.

Installing the Geometry and Loading the Sample

Installing the geometry

Follow these instructions to install the geometry. Parallel plate geometries for compression testing do not require any alignment; use the following loading procedure for parallel plate geometry.

Use the Raise to loading gap button to raise the stage to the loading position.
Install the lower smart swap geometry (Peltier plate or lower plate for ETC).
Attach the upper plate geometry and tighten the draw rod. See also Fitting a Geometry on the DHR/AR. Note that the upper clamp is not a smart swap geometry.
If this is the first time you are using this geometry, use the Geometry Wizard to configure and define the parameters for this geometry. Otherwise, select the appropriate geometry from the Geometry toolbar on the Experiment tab.
With the Zero Fixture command in the TRIOS Gap Control panel, zero the linear test geometry.
Run the axial mapping calibration from the File Manager > Geometries > Calibration. Use the Read Alignment Position so that the instrument reads the current position.
Make sure the upper geometry is in the alignment position (if not, use the Move to alignment position button on the Motor Instrument panel), then click Calibrate to perform the axial mapping.

Preparing the Sample

Use a punch to cut round-shaped samples.

Loading the Sample (films, fibers)

Position the upper test fixture to accommodate the desired sample thickness.
Place the sample in the middle of the plate.The diameter of the plates can be larger than the diameter of the sample.
Using the stepper motor control buttons on the test station, adjust the stage to provide a very slight tension, as indicated by the NORMAL FORCE meter on the front panel display, or activate axial force control to adjust the static normal force (see also Axial Force Guidelines).

Coefficient of Thermal Expansion

When testing at other than ambient temperatures, the coefficient of thermal expansion for the Compression geometry compensates for geometry expansion according to the following equation:

With a = Coefficient of thermal expansion [1/°C]

Dt = Change in temperature [°C]

L₀ = Original length of sample [mm]

DL = Change in length of sample [mm]

Positive DL indicates increasing of sample length.

Note that when the box is checked in the geometry, the upper fixture is adjusted to compensate for the expansion such that the actual gap remains constant. Refer to Calibrating DHR/AR Geometries to determine the coefficient of Thermal Expansion for the geometry.

Test Procedure Recommendations

Before starting the test, make sure that the sample is loaded correctly and the test fixture is aligned and locked.
If temperature tests are to be performed, use axial force control from the Control panel to stretch/compress the sample before starting the test and insert a Conditioning Options step in the procedure to maintain the axial force control during the test. Use the force tracking mode to ensure that the static force is higher than the dynamic force.
For low temperature testing, manually re-tighten the clamps when cooled to temperature before starting the test.