by Dr. Sahbi Aloui, Applications Laboratory
What are Technical Elastomers?
Technical elastomers feature excellent elastic behavior. They can be repeatedly deformed and return to nearly their original length after mechanical relief. Depending on the type, technical elastomers can effectively store or dissipate, i.e., transform, mechanical energy. That is why they are used in many vibration control applications, such as in tires, vibration absorbers in motor and rail vehicles, conveyer belts, seals, hoses, etc.
Technical elastomers can be loaded either statically or dynamically or both at the same time. In the case of a static load, the load is constant over time and often commensurate with its own weight. The dynamic load, however, is a function of time and is either externally imposed (passive) or defined by a drive (active). Dynamic loads are caused, for example, by external influences such as earthquakes, sea waves or strong winds. They also occur in a large number of technical systems as a result of periodically moving masses.
The visco-elastic properties of the elastomer composites at different temperatures and frequencies are determined by means of dynamicmechanical analysis (DMA). DMA systems are designed for quality control, material as well as product release and material development. For static-dynamic loads, the static loads are first set and then the dynamic load is varied for each static load. Thereby, the sample is subjected to a sinusoidally changing mechanical load of constant frequency and constant amplitude.
DMA GABO EPLEXOR® – 2 Independent Drives
The main feature of the DMA GABO EPLEXOR® systems is the independent generation/setting of static and dynamic loads. The static pre-load is generated by a servo motor and introduced into the sample via the force transducer and the sample holder. The dynamic load is generated by an electrodynamic oscillator and also transferred to the sample. Although using two independent drives requires greater technical effort, it also results in significantly higher flexibility in use.
Static and Dynamic Load
In contrast with shear experiments, it is absolutely mandatory in tension, compression and bending load tests for the static pre-load to be higher than the dynamic load. This restriction is due to the fact that a tensile sample can buckle under alternating tensile loads if the dynamic load amplitude exceeds the static load component. Alternating pressure loads result in a temporary loss of contact between the sample and the sample holder. Correct testing free of artefacts is not possible in this case.
“Allowing Alternating Load”
For some applications such as rubber conveyor belts, drive belts or rubber-metal bearings, deviations from the above rule – that the static pre-load must be higher than the actual dynamic load – may occur in practice if buckling or lifting is prevented by other technical measures. By means of the “Allow Alternating Load“ parameter, the restriction that a dynamic amplitude should be smaller than the static load is removed, if required. In this mode, it is therefore also possible to exactly simulate the load situation of the respective application (see figure 1). For such load conditions, samples that are short and thick are generally recommended as they do not tend to “bulge” as long, thin specimens do.
Figure 1: Independent adjustment of the static and dynamic load with the “Allow Alternating Load“ function. At a static deformation of 0%, the dynamic deformation may increase from 0.05% to 10%
Payne Effect of Carbon Black Filled SBR Vulcanisates
Figure 2 shows the example of a dynamic load sweep under tensile stress for a carbon black-filled SBR sample. The measurement was carried out at room temperature and a frequency of 10 Hz. In the first test, the dynamic deformation amplitude was increased stepwise from 0.05% to 10% (blue curve); for the second test, this was carried out in reverse and the dynamic amplitude was reduced stepwise from 10% back to the initial amplitude of 0.05% (red curve). A static pre-strain was not applied here. The modulus of elasticity |E*| decreases with increasing deformation amplitude (figure 2, blue curve). The dependence of the storage modulus on the deformation amplitude for filled elastomers is also known as the Payne effect.
Figure 2: Dependence of the elasticity modulus of the strain amplitude for SBR with 70 phr N 234 at room temperature and a frequency of 10 Hz. The static deformation amounts to 0% while the dynamic deformation increases from 0.05% to 10%
The Mullins Effect
With a decreasing deformation amplitude (figure 2, red curve), |E*| increases, but does not reach the slope of the “virgin” curve (blue curve). This effect of tension softening is known as the Mullins effect. Reversible and irreversible changes in the polymer matrix, the crosslinking structure and the filler network during load are responsible for this behavior. Some causes include desorption of adsorbed chain sections from the filler surface, breaking of the crosslinking points and/or collapse of the filler agglomeration under the influence of mechanical stress.
The flexibility of the DMA GABO EPLEXOR® through its independent drives allows for the realization of a great variety of test conditions from practical applications in a laboratory setting, as shown by the above example of dynamic deformation variation.
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