Issue 24

S. Psakhie et alii, Frattura ed Integrità Strutturale, 24 (2013) 26-59; DOI: 10.3221/IGF-ESIS.24.04

Figure 16 : The load-displacement diagrams for three-point bending test of model samples of metal-ceramic composite with “wide” (curve 1) and “narrow” (curves 2 and 3) interphase boundaries ( V load = 0.4 m/s). Curves 2 and 3 correspond to composites with different values of strength ( bound t  ) of “narrow” interfaces: 1.0 NiCr t  (2); 2.0 NiCr t  (3). As follows from analysis of stress distribution in model composites, high effective strength of wide interphase boundaries is a result of a significant expansion of the area of internal stress decreasing from high values in the reinforcing particles (which are stress concentrators in the composite [55]) to significantly lower values in the plastic binder (Fig. 17). This leads to two-three-fold decrease in the value of stress gradients for wide interphase boundaries as compared to "narrow" interfaces. As a consequence of a smoother distribution of internal stresses, the integral (“macroscopical”) deformation and strength characteristics of metal-ceramic composite significantly increase, and fracture mechanism changes from damages and crack formation at the interfaces to the crack propagation in the plastic binder.

(a) (b) Figure 17 : Stress intensity distribution in the central part of the samples of metal-ceramic composite with “narrow” (a) and “wide” (b) interphase boundaries. Three-point bending test. Stress distributions correspond to the bending angle  = 3.5  Analysis of simulation results indicate that increase of the width of transition zone at the interphase boundaries in metal- ceramic composites can provide an increase in the integral parameters of mechanical response of materials (strength, ultimate strain and fracture energy). Fig. 18 shows dependencies of the sample strength (Fig. 18a) and fracture energy (Fig. 18b) in model metal-ceramic composite depending on width of transition zone. It is seen that presence of even narrow mesoscale transition zone (~0.3  m) between the TiC particles and (Ni-Cr)-alloy increases specimen strength and the fracture energy in 1.5 and 5 times respectively. A further widening of the interface leads to decrease of the rate of increase of integral parameters of the composite response. And when the value of H if becomes comparable to the average distance between TiC particles ( H if >1  m) changing of strength and fracture energy of the composite becomes insignificant.

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