Results of uniaxial tensile tests

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addition, the average, minimum and maximum curves for unloading are observed in this figure.

Figure 4-17: The stress-strain behavior of test specimen No. 1 with 3 Shore A hardness. It shows the minimum and the maximum deviation of stress values in addition to average stress for uniaxial tensile test cycles of Figure 4-14. The abbreviation letters were described in Table 4-4.

So, it shows the minimum and the maximum deviation of stress values in addition to average stress for three repetitions of cycles in each strain step. The deviation in the repetitions is very small. So, the stress-strain behavior does not change significantly when the material is used multiple times. The hysteresis effect is ignored, because the loading-unloading curves are nearly located on each other. There is not any Mullins effect because the curves have the same behavior in different strain steps.

Figure 4-17 is only related to one test specimen (No. 1) of 3 shore A hardness. Figure 4-18 shows the test results for specimens No. 2 and No. 3 with the same process as described in Figure 4-17. The strain-stress curves in these figures are not single lines because of the execution of loading-unloading tests. There is similarity in the test processes and there is a small deviation between the minimum, maximum and average curves. So, the curves are shown in smaller scale in the following to increase the efficiency of this dissertation.

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Figure 4-18 shows the stress-strain behavior of silicone rubber with hardness of 3 Shore A. This figure relates to measured test data for two different test specimens with names of “Test specimen No. 2” and “Test specimen No. 3” which are shown in Figure 4-18 (a) and Figure 4-18 (b). The dashed line shows the maximum and minimum values of stress-strain and the solid line shows its average value. They are related to iteration of the same test process in each strain step for three times. There is a small deviation between the average curve and curves of minimum and maximum in both test specimens after repeating the test in each strain step for three times. It shows a high accuracy in measurement method because of similarity in test results after three times repetitions.

(a) (b)

Figure 4-18: The stress-strain behavior of test specimen No. 2 (a) and No. 3 (b) with 3 Shore A hardness. It shows the minimum and the maximum deviation of stress values in addition to average stress for uniaxial tensile test cycles of Figure 4-14. The abbreviation letters were described in Table 4-4.

Afterwards, an average value of the stress-strain behavior of all three test specimens (Figure 4-17, Figure 4-18 (a) and Figure 4-18 (b)) is calculated and is shown in Figure 4-19. It describes the stress-strain behavior of silicone rubber with hardness of 3 shore A. This figure gives an average value and the deviations of the experimental test results of three different test specimens which twelve test cycles (loading-unloading) in four different strain steps according to Figure 4-14 was performed on each of them.

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Figure 4-19: The uniaxial tensile stress-strain behavior of silicone rubber with hardnesses of 3 Shore A. The curves show the minimum and the maximum deviation of stress values in addition to the average stress for hardness of 3 Shore A. It is achieved with three repetitions of cycles in each strain step for three different test specimens. The abbreviation letters were described in Table 4-4.

This method is performed for other hardnesses, too. Figure 4-20 (a), Figure 4-20 (b) and Figure 4-20 (c) display the stress-strain behavior of silicone rubber with 6, 12 and 18 Shore A hardnesses. Here, the stress-strain behavior during a uniaxial tensile test, shows a nonlinear behavior. This issue confirms the nonlinear behavior of silicone rubber as a hyperelastic material for different hardnesses.

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(a)

(b) (c)

Figure 4-20: The uniaxial tensile stress-strain behavior of silicone rubber with hardnesses of 6 (a), 12 (b) and 18 (c) Shore A. In each segment of the figure, curves show the minimum and the maximum deviation of stress values in addition to the average stress for related hardness. In each hardness, it is achieved with three repetitions of cycles in each strain step for three different test specimens. The abbreviation letters were described in Table 4-4.

Figure 4-21 shows comparative diagrams of stress-strain curves with different hardnesses. The average of the measurements clarifies some material characterizes. The hardness increase of silicone rubber leads to an increase of stress in different steps of strain in a uniaxial tensile test. It means, in the same strain value, the required stress to tension a silicone rubber specimen with more Shore A hardness is more than for one with less Shore A. The harder pad has less tension during the tension of silicone rubber in the printing process with the use of same printing force per unit of element area. The pad deformation is usually less than 50% during the printing process. So, the strain working area for printing process can be considered till 70% and other data are more related to find the behavior of silicone rubber. In Figure 4-21, specially till 70% strain step, there is not a perceptible hysteresis effect, because there is not a sensible

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difference between the stress of loading and unloading in the same strain value. In this range, when passing through the same loading and unloading path in different strain steps, the Mullins effect does not appear, too.

Figure 4-21: The uniaxial tensile stress-strain behavior of silicone rubber for hardnesses of 3, 6, 12 and 18 Shore A. The figure consists of the averaged data of different test specimens for each hardness. The abbreviation letters were described in Table 4-4.

The results of silicone rubber uniaxial tensile test with hardnesses of 3, 6, 12 and 18 Shore A were explained above. The compression test results are described in the next part.