13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- stresses based on the Virial theorem[16] were used to estimate global normal (yy) stresses applied to the system, which could be defined as the volume average of potential for all of the atoms in the system: 1 1 2 N ij ij ij j i i r f V σ ≠ = ⊗ ∑ (6) Zhou[15] has presented the specific formula for calculation: , 1 1 1 2 N ij i j rr r r V r r αβ α β φ σ ∂ = − = ∂ ∑ (7) The strain used here is the nominal strain actually. The results were used to map the stress-strain curves. To reduce thermal oscillation, values of stress and strain were averaged over 10 time steps (each time step is 0.001ps). A short averaging time of 0.01ps was used to retain the time dependence of the properties. While this averaging time is small, it still helps mitigate thermal noises as the averaging is performed over many atoms. The normal stress-normal strain curve obtained from the tensile mode simulation is shown in Fig. 6. It indicates that during the tensile test, the normal stress initially linearly increases as the normal strain is increased, corresponding to an elastic deformation of the system. To examine the effect of the presence of the crack, a similar normal stress-normal strain curve obtained from the tensile mode loading of with a crack sample is included in Fig.7, the maximum normal stress at same temperature is significantly reduced and only about half value of the no crack model. -4 -2 0 2 4 6 8 10 12 14 0 0.1 0.2 0.3 0.4 0.5 0.6 Normal Stress σyy Normal Strain εyy no crack, 300K no crack, 473K no crack, 673K -2 -1 0 1 2 3 4 5 6 7 0 0.05 0.1 0.15 0.2 0.25 0.3 Normal Stress σyy Normal Strain εyy with crack, 300K with crack, 473K with crack, 673K Figure 6. The normal stress-normal strain curve of models without crack at different temperatures Figure 7. The normal stress-normal strain curve of models with cracks at different temperatures The simulation results for the elastic modulus and maximum stress as a function of the simulated temperature for both with and without crack models are summarized in Table 1. They have the same changing regularity for both perfect interface and imperfect interface, which is found the elastic modulus and maximum stress are reduced by the increase of simulated temperature, and the maximum values of modulus and maximum stress for no crack model at roomtemperature are seen to be 332.67GPa and 12.17GPa, respectively. Presence of a crack reduces the maximum load carrying capacity and additionally reduces the stiffness of the composite system. Table 1. simulation results of model failure. Crack Simulated temperature (K) Modulus (GPa) Maximum stress (GPa) No 300 332.67 12.17 473 210.11 11.40 673 193.18 9.88 Yes 300 186.94 6.04
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