4.3.3 Creep Testing by Nanoindentation During each test, data collected was used to calculate the creep compliance and the stress exponent defined in Eq. 4.1 [14–18]: ε tð Þ¼σ0J tð Þ ð4:1Þ where σ0 is the constant stress applied and J(t) is calculated using Eq. 4.2 J tð Þ¼A tð Þ= 1 ν ð ÞP0 tanθ ð4:2Þ *InEq. 4.2A(t) is the contact area, P0 constant applied load, θ is the effective cone angle which is 70.3 for a Berkovich indenter and the Poisson’s ratio ν is assumed to be 0.3. The strain versus time behavior during creep is characterized by a high strain rate _ε ¼dε=dt in the primary stage of creep and then in the secondary, steady state stage of creep, the strain rate is given in Eq. 4.3 can be written as _ε ¼Kσn ð4:3Þ where Kis a constant andnis the stress exponent. The strain rate is calculated in the software and in turnnis obtained from the log-log plot of train rate versus stress in the secondary stage of creep. The materials under consideration are heterogeneous in nature and the fact that the nanoindenter testing is carried out over a small area/volume, a large scatter in the data is observed. According to Fig. 4.7 it seems that the creep compliance values are not quite dependent to the iron oxide content of the compositions. It shows an alteration depending on the loading values. Therefore, effect of the iron oxide content to the creep compliance cannot be directly deducted from this type of curves. The average indentation modulus and stress exponent values were obtained from the 20 indentations performed under 20 mN and 50 mN constant loads and they are presented in Fig. 4.8a, b with error bars showing one standard deviation. From Fig. 4.6b it is seen that, increasing Fe3O4 amount results in a rise in the stress exponent. However, for the modulus it cannot be done the same straightforward inference. These results also demonstrate that these materials cannot conform to the idealized linear viscoelastic behaviour under the contact creep conditions used here. It means that there is no linear path during loading and unloading. 4.3.4 Wear Testing by Nanoindentation During each nano wear test, one cycle is defined as a pass and return of the indenter over the track, so the total distance covered for one wear test was 0.050 m. Speed of the tip during wear tests was 50 μm/s. Total of 10 wear tests for each sample were performed under the two normal loads. The wear in a track is characterized as the area between the initial profile and the residual profile of the wear track. In the same way, the averages of the wear track deformation are shown in Fig. 4.9. As expected the applied load has a significant effect on the wear track deformation values. However, wear track deformation values seem insensitive to the iron oxide content inside the composites. This situation can be explained by the anisotropy over the matrix. 4.3.5 Damage Analysis by Means of Scratch Test and 3D Optical Roughness Meter After realizing two different group of macro scratch tests, three dimensional damages were obtained by 3D optical surface scanner. The results are presented in the Figs. 4.10 and 4.11. Additionally, the filler particle size at a constant weight fraction has a significant influence on the scratch resistance of the composites. This is also confirmed with nano indentation tests. It means that certain fillers play an important role for the scratch resistance that they were used as reinforcement elements here. 4 Mechanical Characterization of Epoxy: Scrap Rubber Based Composites Reinforced with Nanoparticles 39
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