16 A. B. Irez et al. Table 2.6 Drop weight tests results on CJ specimens Composition name Maximum force (N) Absorbed energy (J) CJ I 2232.25 ±34.68 1.27 ±0.04 CJ II 2647.45 ±29.94 1.49 ±0.17 CJ III 3188.8 ±348.74 1.66 ±0.21 Table 2.7 Scratch test results for CJ compositions Composition name 10N 15N Worn surface (mm2) Worn volume (mm3) Worn surface (mm2) Worn volume (mm3) CJ I 1.32 0.0886 1.59 0.135 CJ II 1.16 0.0697 1.33 0.0972 CJ III 1.23 0.0811 1.73 0.136 From Table 2.6 it is clearly seen that GnPs improve the impact resistance of the composites. The maximum force during the impact is proportional to the absorbed energy [6, 7].Layered structure of GnPs is possibly enables to improve the damping characteristics of the composites. Shortly, it is noticed that the absorbed energy for each specimen during the impact tests is related to the increment of the plastic zone in the structure due to the debonding of the nano particles. Higher dispersion of the values is also related to the test specimens prepared under laboratory conditions; hot compaction, cutting notch effect, etc. 2.3.4 Damage Analysis by Means of Micro Scratch Test and 3D Optical Surface Roughness Meter After completing mechanical tests, tribological characterization of the composites were done by micro scratch tests. Three dimensional damages were observed three dimensionally by an optical surface scanner. The 3D damage results are presented in the Fig. 2.4. Quantitative results related to scratch tests were given in Table 2.7. It seems that the increase of the reinforcement content, cause in the composites high resistance to wearing. In reality, because of the high shear stress at the interfaces the interfacial shear stress should probably be the main reason for damage of the matrix and reinforced filler interfaces. When the indenter is slipping, tangential tensile stress is caused on the surface behind the indenter, while in front of the indenter the tangential stress is compressive. As expected higher force values resulted in higher damage traces. Damaged volume is significantly proportional with the force level. However, increasing force level did not manifest in the same manner for the damaged surface. In addition, increasing GnPs content generally improves the wear resistance for the composites in consideration. 2.3.5 Numerical Approach for These Composites After realizing experimental procedures, mechanical test results were compared with finite element modelling (FEM). In FEM procedure, simplified approaches were utilized. In this regard, after generating 3D solid model of the tested specimens in 3 PB tests, these models were meshed with quadratic tetrahedral elements (C3D10). After that, boundary conditions were applied according to two selected approach. In the first approach, maximum load obtained from the 3 PB tests was imposed to the material and maximum stress and the displacement in the critical region (z-direction) were obtained. In the second approach, maximum deflection in the middle of the specimen which was obtained from 3 PB tests was imposed to the material and maximum stress in the middle of the specimen was determined. FE models were solved in linear elastic mode by using Abaqus FEM software. First approach was executed and results were illustrated in Fig. 2.5. The comparison of experimental and FEM results according to first approach was given in Table 2.8.
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