Clamped Bending Geometry

General Information

Clamped three-point bending holds a solid rectangular sample in a horizontal position. The support frame on the motor grips the sample ends, while the transducer clamp grips the center of the sample. Select the small frame with medium clamp when using the clamped three-point bending tool. Note that the length is pre-set.

Available Sizes

The following are the available frame and clamp sizes for the clamped bending single and dual cantilever.

Clamped Bending Single Cantilever

Frame = 34 mm

Clamp = 2 mm

Clamped Bending Dual Cantilever

Frame = 34 mm

Clamp = 2 mm

Applicable Environmental Systems

Furnace (ETC)

Recommended Sample Dimensions

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

• L = 42-45 mm (for dual cantilever)
• W = Up to 6 mm optimal, 12 mm maximum
• T = Dependent Upon E*

Sample Testing Limits

The suggested sample dimensions give the recommended range for sample geometry based on the limitations of the size of the test geometry. Not all samples should be tested at the maximum geometry. Extremely rigid samples with a very high modulus (such as highly-filled polymers or reinforced composites) may require a more modest geometry to obtain an instrument operating range where useful data can be expected. A single point test using 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, narrower, or longer to obtain better results. If the sample still cannot be tested practically, it may be necessary to use a different testing geometry, (such as three-point bending or mixed bending) for very high modulus samples, or a tensile geometry for very thin or low modulus sample.

The ETC is a radiation oven with the solid sample and temperature probe in different locations - thus the actual temperature of sample and probe may be different. The difference varies with the sample thickness and the applied heating rate. In this case the furnace temperature needs to be calibrated in order to monitor the correct sample temperature.(see: Offset and Span Calibration for DHR/AR)

Installing the Geometry and Loading the Sample

Installing the Geometry

1. Use the Raise to loading gap button to raise the head.
2. Install the lower smart swap geometry with the 34 mm frame attached. Insert the alignment tool, but do not tighten the clamps.
3. Attach the upper 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.
4. 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 Experiment tab > Geometry.
5. With the up and down buttons on the keypad, lower the geometry (clamp removed) into the slot in the alignment tool. In TRIOS, set the gap to zero from the Instrument panel using the Tare button.
6. Next, run the axial mapping calibration from the File Manager >Geometries > Calibration. Use the Read Alignment Position to set the alignment position.
7. Manually move the upper geometry to allow it to rotate freely. On the Instrument Motor panel, align the upper geometry with the Move to alignment position button. Then click Calibrate to perform the axial mapping.
8. Raise the stage to provide room to load the sample. Remove the alignment tool and mount the 30 mm frame to the base, if required.

Loading the Sample

Refer to the figure below for an illustration of the geometry with a sample loaded.

1. From the Motor Instrument panel, click the Move to alignment position button to align the upper test fixture.
2. Slide the sample onto the clamps.

• For single cantilever, tighten one outer clamp.
• For dual cantilever, tighten both outer clamps.

3. Tighten the center clamp while watching the Normal Force meter on the Gap Control panel. Adjust the stage position while tightening to maintain force close to zero.

4. Once sample has been fully tightened, adjust stage position to obtain a Normal Force meter indication of zero or activate axial force control to adjust the static normal force to zero.(see also Axial Force Guidelines).

The figure below shows a dual cantilever configuration:

Recommendations for Setting Up the Procedure

1. Before starting the test procedure, make sure that the test fixture is aligned and locked (see also Installing the Geometry)
2. If temperature tests are to be performed, use axial force control from the Instrument Control panel to pre-load the sample before starting the test and insert a Conditioning Options step in the procedure to maintain the axial force control during the test.
3. For low temperature testing, manually re-tighten the clamps when cooled to temperature, before starting the test.

Equations

Single Cantilever

Dual Cantilever

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