Using Nanoindentation DMA for Viscoelastic Analysis of Rubber

The property of materials to exhibit both viscous and elastic characteristics when undergoing deformation is referred to as viscoelasticity.

Unlike an elastic material which tends to strain immediately when put under stress and returns to original state once the stress is removed, a viscous material resists shear ­flow and strains linearly with time when a stress is applied. Thus, a viscoelastic material displays elements of both characteristic, and therefore has a complex modulus.

Importance of Nanoindentation DMA for Rubber

When vehicles are on the road, tires are subject to cyclical high deformations. Many factors tend to jeopardize the lifetime of tires upon being exposed to harsh road conditions. These factors include: the wear of the thread, the heat generated by friction, rubber aging, among others.

Consequently, tires usually come with composite layer structures composed of carbon-fi­lled rubber, nylon cords, and steel wires, etc. What’s more, in an effort to provide varying functional properties, the composition of rubber at different areas of the tire systems is optimized. Such properties include wear resistant thread, cushion rubber layer and hard rubber base layer, but are not limited to just these.

To evaluate quality control and R&D of tires, as well as evaluate the life span of old tires, a reliable and repeatable test of the viscoelastic behavior of rubber is absolutely vital. One of the techniques for characterizing the viscoelasticity is Dynamic Mechanical Analysis (DMA) during Nanoindentation. Upon the application of controlled oscillatory stress, the resulting strain is measured, thus enabling the determination of the complex modulus of the tested materials.

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Measurement Objectives

Here the Nanovea Mechanical Tester was used, in Nanoindentation mode with DMA in an effort to analyze and record the comparative viscoelastic properties of rubber at varying thicknesses of a tire sample.

Tire sample on Nanovea Mechanical Tester

Tire sample on Nanovea Mechanical Tester

Testing Conditions

Figure 3 shows the distribution of the indentations when a series (40 points) of nanoindentation DMA was performed along the thickness of the tire sample. Table 1 summarizes the test conditions.

Table 1. Test conditions of the nanoindentation

Test Parameter Value
Maximum Force (mN) 75
Loading Rate (mN/min) 150
Creep (s) 20
Amplitude 15
Frequency (Hz) 10
Indenter Type 100 μm spherical diamond

Distribution of the indentation on the cross section of the tire sample

Figure 1. Distribution of the indentation on the cross section of the tire sample.

Results and Discussion

Thus, as shown in Figure 3, 40 indentations along the tire thickness were performed spaced 0.38 mm apart for a total distance of ~15.5 mm. The corresponding Tan (δ), Storage Modulus, Loss Modulus and Hardness measured from the DMA are depicted in Figures 4 through 7.

Distribution of Tan (δ) at different locations of the tire.

Figure 2. Distribution of Tan (δ) at different locations of the tire.

Distribution of Storage Modulus at different locations of the tire.

Figure 3. Distribution of Storage Modulus at different locations of the tire.

Distribution of Loss Modulus at different locations of the tire.

Figure 4. Distribution of Loss Modulus at different locations of the tire.

Distribution of Hardness at different locations of the tire

Figure 5. Distribution of Hardness at different locations of the tire.

Thus, at different layers of tires, the distribution of the hardness and complex modulus satisfies the functionality requirements of the rubber. At positions from 0.38 to 2.66 mm and from 8.22 and 11.64 mm, the rubber exhibits a relatively low hardness below 2 MPa and low complex modulus. It is vital to have this, for a relative soft feature like this enables the rubber in these regions to serve as a cushion layer, thus absorbing the shocks and vibrations.

In contrast, at the positions from 3.04 to 7.84 mm, the rubber displays an enhanced hardness and complex modulus. This is due to the composite structure that consists of reinforcing fabric or high tensile-strength steel wires encased in the rubber compound. Further, it is because of the enhanced mechanical properties of this layer, that the tire structure possesses strength and toughness.

When compared with the “cushion region” the positions from 12.02 to 15.82 mm reside in the tire tread and exhibit higher Hardness and complex modulus. The addition of the high carbon black concentration in this area provides reinforced abrasion resistance, cut resistance, as well as traction.

Conclusion

This study demonstrated how the Nanovea Mechanical Tester in Nanoindentation DMA mode analyzes the viscoelastic properties of a rubber sample (in this study, a used tire). The profi­le seen across the depth of the tire demonstrates how the different layers are used to create zones with very different behaviors. This is essential in tire design to decrease the vibration. Further, zones of high loss modulus tend to absorb differently than zones of low loss modulus.

Such studies are extremely essential today, to improve tires for smoother, safer rides that endure different weather conditions. The tests described above can be adapted to suit various temperatures and even under liquids. The test can also be conducted at different frequencies, in an effort to mimic the behavior of tires as speed increases.

Here, frequencies of 10 Hz were used, to correspond to a speed of about 67 Km per hour. It bears mentioning that the Nanovea DMA can go up to 100 Hz and is also possible to scan across various frequencies to obtain a sweep.

The Nanovea Mechanical Tester can display true close feedback control on load applied. Using a separate high sensitivity strain gage, the load application with the fast piezo is independent from the load measurement. Thus, this produces a distinct advantage during DMA – because the phase between depth and load is measured directly from the data collected from the sensor. Further, this calculation of phase is direct, without the need for mathematical modeling, which could add inaccuracy to the resulting loss and storage modulus. For coil systems, this is not the case.

To conclude, DMA measures hardness, loss and storage modulus, complex modulus and Tan (δ) as a function of contact depth, time, and frequency. During DMA, the optional heating stage allows the determination of materials phase transition temperature. Thus, the Nanovea Mechanical Testers is equipped to provide unmatched multifunction Nano and Micro/Macro modules, all on a single platform.

Further, both the Nano and Micro/Macro modules include modes such as scratch tester, hardness tester, and wear tester, providing the widest and most user-friendly range of testing available in one single module.

This information has been sourced, reviewed and adapted from materials provided by Nanovea.

For more information on this source, please visit Nanovea.

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