Careful sample preparation is crucial for good quality DMA shear measurements. The most important influence factors are discussed with the aid of practical examples.
In the DMA shear mode, two identical samples are clamped between the three clamping plates of the shear sample holder. The two outer plates are fixed in the clamping assembly while the oscillating force acts on the center plate.
The shear mode is widely used for materials with very different physical properties and geometries such as soft elastomers, hard composites, viscous liquids, powders, and plastic films.
The following section presents some general tips on sample preparation concerning
Geometry of samples
Tools for preparing samples
Use the right tools in order to obtain the desired circular disk or rectangular bar from the material you want to measure:
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Many materials with quite different physical properties and geometries can be easily measured in the shear mode. The advantages of the shear measurement are:
Careful sample preparation is crucial for obtaining reliable and reproducible results. The stiffness of the test specimen measured must be at least five times smaller than the stiffness of the sample clamping assembly, that is, not greater than 3 • 107 N/m.
With thin films, this can often be achieved by stacking several layers of the film separated by aluminum disks to form a sandwich. This increases the effective thickness of the film.
The most important point for reliable shear measurements is to make sure that the entire area of the test specimen is in good contact with the shear plates. If the starting temperature of the measurement is significantly lower than the clamping temperature, the test specimen may possibly have to be re-clamped. In any case, at the beginning of the measurement (i.e. at the start temperature), the modulus should have a plausible value.
Elastomers are materials that are typically measured in shear. The uncertainty of the modulus value in the rubbery plateau of elastomers is determined by the reproducibility of the predeformation and the sample geometry. It is about 12% of the modulus value.
With anisotropic materials such as laminates, the orientation of the sample is very important. If the sample is installed so that the layers are parallel to the applied shear force, the soft components determine the mechanical behavior. In this case, the compliance gives the clearest information about the sample. In contrast, if the layers are at right angles to the applied shear force, the hard components determine the behavior of the sample.
Liquids and pastes can be measured with the sample holder for liquids. The viscosity can also be calculated from the loss modulus in such experiments.
It is also possible to measure powders in the shear mode. To do this, two disks are pressed from the powder (typical dimensions: diameter 4 mm, thickness 1 mm) and then clamped between the shear plates.
Sample Preparation for DMA Shear Measurements | Thermal Analysis Application No. UC344 | Application published in METTLER TOLEDO Thermal Analysis UserCom 34