Sample Preparation for DMA Shear Measurements - METTLER TOLEDO

Sample Preparation for DMA Shear Measurements

Careful sample preparation is crucial for good quality DMA shear measurements. The most important influence factors are discussed with the aid of practical examples.

 

Introduction

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

  • the geometry of samples
  • tools for preparing samples
  • clamping and predeformation
  • installation in the clamping assembly
  • measurements below room temperature.

 

 

General Tips on Sample Preparation

Geometry of samples

  • Optimum shear samples must have plane parallel surfaces.
  • The geometry factor should be between 20 and 50 m-1.
  • The diameter should be at least three times greater than the thickness in order to make sure that the sample undergoes shear and is not bent.
  • The stiffness of the sample should be at least five times smaller than that of the measuring device. Otherwise, the resulting values for the shear modulus are too low.

 

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:

  • sample punch, carpet knife or scissors for soft materials such as rubber sheets or plastic films;
  • diamond saw for hard samples such as fiber composites;
  • small hydraulic press (KBr press) to press disks from powders.

 



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Summary

Many materials with quite different physical properties and geometries can be easily measured in the shear mode. The advantages of the shear measurement are:

  • no offset force is required as is the case for bending or tension,
  • the sample temperature is accurately measured,
  • a very large stiffness range can be covered in one measurement (up to 8 decades).

 

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