Strain dynamic

Since ozone attack on rubber is essentially a surface phenomenon, the test methods involve exposure of the rubber samples under static and/or dynamic strain, in a closed chamber at a constant temperature, to an atmosphere containing a given concentration of ozone. Cured test pieces are examined periodically for cracking. 

The length and amount of cracks is assessed according to the Bayer method [72,73]. The ISO standard ozone test conditions involve a test temperature of 40°C zE 1°C and an ozone level of 50 5 pphm, with a test duration of 72 h. Testing is done under static [72] and/or dynamic strain [73]. These are accelerated tests and should be used for the relative comparison of compounds, rather than for the prediction of long-term service life. The method is rather complicated and demands a long duration of ozone exposure. Therefore, in some cases the rate constants of the antiozonants reaction with ozone in solution are used instead to evaluate the efficiency of different antiozonants [74]. 

Like any dynamic strain instrument, the RPA readily measures a complex torque, S (see Figure 30.1) that gives the complex (shear) modulus G when multiplied by a shape factor B = iTrR / ia, where R is the radius of the cavity and a the angle between the two conical dies. The error imparted by the closure of the test cavity (i.e., the sample s periphery is neither free nor spherical) is negligible for Newtonian fluids and of the order of maximum 10% in the case of viscoelastic systems, as demonstrated through numerical simulation of the actual test cavity." ... 

Both rolling resistance and heat buildup are related to hysteresis that is the amount of energy that is converted to heat during cychc deformation. It is well known that hysteresis of tread compounds, characterized by the loss factor, tan 8, at high temperature, is a key parameter. It not only governs heat buildup of the compounds under dynamic strain but also shows a good correlation with the... 

ISO 1431-1 2004 Rubber, vulcanized or thermoplastic - Resistance to ozone cracking -Part I Static and dynamic strain testing... 

Fig. 43 Effect of dynamic strain amplitude on storage modulus (a). Stress-strain behavior of CR/ EPDM blend in the absence and presence of nanoclay (b). For this experiment, tension mode was selected for the variation of the dynamic strain from 0.01 to 40% at 10 Hz frequency...

D.X. Du, R.L. Axelbaum, and C.K. Law. Experiments on the Sooting Limits of Aero-dynamically Strained Diffusion Flames. Proc. Combust. Inst., 22 387-394,1988. 

G dynamic stress modulus, storage modulus G dynamic strain modulus, loss modulus. 

The compatibility, mechanical properties, and segmental orientation characteristics of poly-e-caprolactone (PCL) blended with poly (vinyl chloride) (PVC) and nitrocellulose (NC) are described in this study. In PVC blends, the amorphous components were compatible from 0-100% PCL concentration, while in the NC system compatibility teas achieved in the range 50-100% PCL. Above 50% PCL concentration, PCL crystallinity was present in both blend systems. Differential IR dichroism was used to follow the dynamic strain-induced orientation of the constituent chains in the blends. It was found for amorphous compatible blends that the PCL oriented in essentially the same manner as NC and the isotactic segments of PVC. Syndio-tactic PVC segments showed higher orientation functions, implying a microcrystalline PVC phase. 

Differential IR dichroism was used to follow the dynamic strain-induced orientation of the constituent chains in PVC/PCL and NC/PCL blends. It was found for amorphous compatible blends that PCL oriented in essentially the same manner as NC and the isotactic segments of PVC. Syndiotactic PVC segments showed much higher orientation functions, which implied the existence of a microcrystalline PVC phase. 

When a sinusoidal strain is imposed on a linear viscoelastic material, e.g., unfilled rubbers, a sinusoidal stress response will result and the dynamic mechanical properties depend only upon temperature and frequency, independent of the type of deformation (constant strain, constant stress, or constant energy). However, the situation changes in the case of filled rubbers. In the following, we mainly discuss carbon black filled rubbers because carbon black is the most widespread filler in rubber products, as for example, automotive tires and vibration mounts. The presence of carbon black filler introduces, in addition, a dependence of the dynamic mechanical properties upon dynamic strain amplitude. This is the reason why carbon black filled rubbers are considered as nonlinear viscoelastic materials. The term non-linear viscoelasticity will be discussed later in more detail. 

Another important point is the question whether static offsets have an influence on strain amplitude sweeps. Shearing data show that this seems not to be the case as detailed studied in [26] where shear rates do not exceed 100 %.However, different tests with low dynamic amplitudes and for different carbon black filled rubbers show pronounced effects of tensile or compressive pre-strain [ 14,28,29]. Unfortunately, no analysis of the presence of harmonics has been performed. The tests indicate that the storage (low dynamic amplitude) modulus E of all filled vulcanizates decreases with increasing static deformation up to a certain value of stretch ratio A, say A, above which E increases rapidly with further increase of A. The amount of filler in the sample has a marked effect on the rate of initial decrease and on the steady increase in E at higher strain. The initial decrease in E with progressive increase in static strain can be attributed to the disruption of the filler network, whereas the steady increase in E at higher extensions (A 1.2. .. 2.0 depending on temperature, frequency, dynamic strain amplitude) has been explained from the limited extensibility of the elastomer chain [30]. 

Little information has been published on the question of how filler network structure actually affects the energy dissipation process during dynamic strain cycles. The NJ-model focuses on modeling of carbon black network structure and examination of the energy dissipation process in junction points between filler aggregates. This model was further developed to describe the strain amplification phenomenon to provide a filler network interpretation for modulus increase with increasing filler content. 

According to the definition of strain rate sensitivity maitioned above, the values of strain rate sensitivity in both low and dynamic strain rate regions are calculated and listed in Table 2. It is found that dynamic strain rate sensitivity decreases with plastic strain, but low strain rate sensitivity is independent of plastic strain. Furthermore, the dynamic strain rate sensitivity is generally higher than low strain rate sensitivity for eaeh true strain level. 

Table 2 Comparison of the static and dynamic strain rate seaisitivity obtained at different plastic strain levels.

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