Softening effect, stress

Another important effect of fillers is stress softening, or the Mullins effect. If a filled sample is stretched for the first time to 100%, the stress-strain curve27 will follow that illustrated in Figure 6-11. Now the strain is removed, and the sample is restretched to 200%. The stress in the second cycle is lower than that in the first up to 100%, after which it continues in a manner following the first cycle. If we repeat the stress-strain in a third cycle, we again see a softening up to 200% due to the previous strain history. This stress-softening effect was first discovered by Mullins, after whom it is named. 

Other information of the composite structure can also be obtained by studying the recovery behavior fi-om the stress softening effect. This is especially interesting in the composites made by the freeze-drying method (28), where the recovery curves of the CB composite lie above the G of the first strain cycle (Figure 13(b)). 

A very significant observation is the stress softening effect, also called the Mullins effect (Mullins and Tobin, 1965 Mullins, 1969). In this experiment, a compound sample is stretched to ei and returned to zero strain, then stretched again. For strain below si, its stress-strain curve is significantly below the first one but rejoins it at si. Stress softening is dependent on the initial strain level it can be partially reduced by thermal treatment but not be totally effaced (Figure 8.7). 

FIGURE 8.7 Schematical illustration of stress softening effect. 

Obviously, the obtained homogenization directly depends on the maximum strain at which the compound has been accommodated. In other words, chain segment lengths are homogenized for any strain below the maximum strain at higher strains, a new homogenization should occur [148]. This naturally corresponds to the so-called stress softening effect previously described [146]. 

Rapid decrease of E was observed with strain increase. Pure NR and composites with a low CNT content (1 wt%) showed a moderate increase of stress up to about 75% strain, a sort of plateau up to 300% strain and a final more evident increase above 300%. Said increase could be due to rubber crystallization and to CNT alignment. The stress-softening effect, known as Mullins effect, was observed at large strain and attributed to detachment of rubber molecules from the surface of filler particles. The presence of CNT bundles (at 5, 7, 10 wt%) was commented to bring about a decrease of the stress." ... 

It is observed that the values do not reach its initial position within the relaxation time of the experiment, but a recovery of the E values have been attained (Fig. 18). This behaviour of a rubber can be explained by the stress softening effect during the dynamic strain. Nevertheless, a high extent of recovery in the reverse amplitude sweep indicates that a good filler-filler network has been re-established at a low loading of tubes in the S-SBR-BR matrix. So, at least it can be said that rather than damage or permanent break of the tubes, the amplitude sweep disrupted the filler-filler network in the rubber matrix. It is noted that the absolute values of E at small amplitudes are somewhat differed from each other as compared with the value obtained from the phr CNT-filled compound. The difference may be developed from ageing of the samples. 

Harwood JAC, Mullins L, Payne AR (1966) Stress softening in natural rubber vulcanizates. Part II. Stress softening effects in pure gum and filler loaded rubbers. Rubber Chem Technol 39 814-22... 

Dynamic Stress Softening Effect 5.1.10.1 Physical Considerations... 

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