23.03.2023 by Martin Rosenschon

Why You Need High- and Low-force DMAs

Dynamic mechanical analysis (DMA) is a method that provides information on the elastic and viscous behavior of a material as a function of temperature and load frequency. A test sample is subjected to a defined, oscillating load and the resulting deformation is measured. 

Dynamic Mechanical Analyzers (DMAs) can be classified into low-force devices, which typically generate dynamic forces in the single to mid double-digit newton range, and high-force systems capable of applying up to several kilonewtons of dynamic load. In addition to the dynamic force, DMAs can generally produce a static load on the sample.            

The maximum force of a system determines the testing mode ‒ for example, tension, bending, or shear ‒and the strains at which a specific material can be characterized. The storage modulus E' is the limiting material property in this regard. It defines the stresses in the material that are realized during a measurement at a given strain. The resulting force is determined by the geometry of the test specimen.

Figure 1 shows a comparison between the 3-point bending, tension and compression testing modes with selected geometries and different storage modulus values regarding the respective load requirements. A dynamic strain of 0.1% was assumed (with the exception of 3-point bending with a bending length of 50 mm). The maximum strain reached is based on a force factor of 1.1, which describes the ratio of static to dynamic load. All testing modes shown require a static force in addition to the dynamic force. This helps keep the upper tool in contact with the sample (bending and compression) and prevent the sample from buckling (tension).


Figure 1: Comparison between different measuring modes with exemplary geometries and modulus ranges in terms of the resulting load requirement.

It should be noted that the figure only shows a section of the possibilities. By reducing the sample geometry or decreasing the strain amplitude, the measurable modulus spectrum can usually be expanded. However, manufacturable and representative test specimens should always be taken into account.


Almost all Materials Characterizable!

Using suitable test parameters, such as sample geometry and sample holder, almost all materials can be characterized in low-force systems. Even materials like aluminum, steel, or ceramics, which have storage modulus values of about 70 GPa, 210 GPa and more, can be tested with dynamic forces up to 10 N in 3-point bending (see Figure 1: l: 50 mm, b: 6 mm, h: 1 mm, dyn. strain: 0.05%). High-load systems (500 N and more) are required to analyze such materials in compression or tension, of course with a correct clamping of the specimen being ensured.              

The choice of a system and a measuring setup is also connected to the temperature range to be investigated and the related development of visco-elastic properties. Thus, characterization of materials in a defined measurement setup is often possible at a certain temperature. However, if the temperature range changes and the mechanical properties shift outside the detected range of the selected setup, analysis can no longer be performed.        

Figure 2 shows a DMA measurement on a WPC material (Wood Polymer Compound) in 3-point bending with a free bending length of 50 mm. WPC materials consist partly of plastic (in this case PVC) and partly of the renewable resource wood. A typical application of WPC is decking boards.


Figure 2: DMA measurement on WPC in 3-point bending


At a temperature of 15°C, the material has a storage modulus E' of 8.1 GPa, which is relatively stiff. As the temperature increases, the value decreases almost linearly to approximately 6.2 GPa at 65°C. In the Température de Transition VitreuseThe glass transition is one of the most important properties of amorphous and semi-crystalline materials, e.g., inorganic glasses, amorphous metals, polymers, pharmaceuticals and food ingredients, etc., and describes the temperature region where the mechanical properties of the materials change from hard and brittle to more soft, deformable or rubbery.glass transition at around 78°C, the polymer chains of the amorphous regions of the polymer can move against each other, and the material rapidly loses stiffness. After the glass transition, the storage modulus E' is only 302 MPa at 120°C.

Supposed that, due to a testing specification or a realistic ContrainteLa Contrainte est définie par un niveau de force appliquée sur un échantillon d’une section bien définie. (Contrainte = force/surface). Les échantillons qui possèdent une section rectangulaire ou circulaire peuvent être comprimés ou étirés. Les matériaux élastiques comme les élastomères peuvent être étirés jusqu’à 5 à 10 fois leur longueur initiale.stress situation, the material must be measured in tension mode with a strain amplitude of 0.1% (maximum total deformation: 0.21%). For a storage modulus of approximately 8.1 GPa at 15 °C, a cross-section of a maximum of 1.23 mm² would be required to characterize the material in the load range up to 10 N. Besides the nearly impossible preparation of such a sample, homogeneity of the material cannot be ensured, which is particularly important for representative measurement results in filled materials.

According to Figure 1, the material can be measured using a specimen with a cross-sectional area of 3 mm² in a device with 25N dynamic force without any problems. Samples with an even larger cross-section, such as 10 mm², would require a device with approximately 80 N.

NETZSCH DMA Instruments for Your Special Requirement

Often, a standardized characterization of a material is necessary, which ensures consistent testing conditions and thus comparability of results between different institutions. For example, elastomer and rubber materials are commonly tested in compression mode with a specimen of 10 mm in height and 10 mm in diameter according to DIN 53513[1]. Below the glass transition temperature, these material groups have a storage modulus of up to 4 GPa in the unfilled state, often over 8 GPa when filled. Material testing accordingly requires high-force systems (see also Figure 1).

Selection of a DMA device and its force range also depends on the effect to be characterized. For typical rubber phenomena such as the Payne or Mullins effect certain strain levels are required that can only be achieved in devices with sufficiently maximum force.

Whether you want to measure soft elastomers, unfilled or filled thermoplastics and thermosets up to metals and ceramics in bending, tension, shear, or compression, NETZSCH Analyzing & Testing offers DMA instruments that are specifically tailored to your requirements. Our products are designed for the loads that correspond to your specific application. 

[1] DIN 53513:1990-03: Prüfung von Kautschuk und Elastomeren; Bestimmung der visko-elastischen Eigenschaften von Elastomeren bei erzwungenen Schwingungen außerhalb der Resonanz. Berlin: Beuth-Verlag 1990

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