I often receive questions from users on how to get the most accurate modulus value from a Dynamic Mechanical Analyzer (DMA). You will see that the results are often more dependent on sample preparation and measurement than on the calibration of the DMA. Now don’t get me wrong, proper calibration of your DMA is a requirement for measuring accurate modulus. So please make sure your DMA has been properly calibrated according to the manufacturer’s instructions.

Sample Preparation

First, make sure that your specimen is representative of your complete sample. I prefer to take a center section from a film sheet rather than a piece from the edge. Make sure the edges are all cut parallel. This may be obvious for a film sample, but larger samples may be deceiving in appearance. If you have any doubt, measure the sides at several locations to make sure they are parallel.

click the image to enlarge This is an example from the Modulus Uncertainty Calculator. There are different workbooks for each mode of deformation.

If you are measuring the bending modulus of a stiff polymer or composite sample, you might use a machine shop to prepare the specimen. Make sure the craftsman knows that you do not want the edged ‘relieved.’ This is a standard practice in the world of machining in which the craftsman will lightly touch the edges of the sample to a sander to ‘relieve’ the sharp edge of the specimen. This will make the modulus too low and should not be done for DMA samples.
Next, make sure the faces of your sample are smooth. For composite samples, one face does not need to be machined since the faces are
parallel from the manufacturing process. You might have to sand these surfaces if the weave of the composite fabric is noticeable. If the faces are not smooth, the modulus will be too small since the thickness measurement is made at the high spots, while the bending will occurs mainly in the low spots. Let’s see how to predict the uncertainty in the measured modulus due to these variables.

Modulus uncertainty calculation

I have made a simple EXCEL® spreadsheet that allows you to calculate the modulus uncertainty given the error in measuring the sample dimensions (see figure). This tool is a nice aid to help you determine if the reproducibility of your modulus measurements is due to the DMA (normal instrument variance), the sample preparation or the resolution of the measurement calipers. Let’s finish with a few examples:

Thin-film in Single Cantilever Bending

Let’s say I want to measure a film sample that is 0.1-mm thick, 5-mm long, and 10-mm wide and I have a caliper the has a resolution of +/- 0.01 mm. I simply plug these numbers in the Modulus Calculator in the figure and I get an error of 38%! Why so much error? The calipers for measuring thickness do not have enough resolution for the job. Okay, so I spent the money for calipers that read to +/- 0.001 mm. Is that good enough? Again I plug the numbers in and get 3.8 %. That was money well spent!

Final thoughts

I have shown you that, with a little advance planning, it is easy to get accurate modulus measurements from your DMA. The Modulus Uncertainty Calculator also allows you to reduce the test variance, prior to running samples, so you can examine your sample variance with more confidence. I will send you a copy of the Modulus Uncertainty Calculator if you simply send me a request via e-mail at Steven.sauerbrunn@mt.com.

Steve Sauerbrunn
Technical Manager for the Thermal Analysis Division,
Mettler-Toledo, Inc.