Issue 24

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

(a) (b) Figure 13 : (a) The load-displacement diagrams for the model samples of metal-ceramic composite with different strength ( bound t  ) of “narrow” interphase boundaries under three-point bending test (diagrams are expressed in terms of normalized stress NiCr t   and bending angle  ): 0.5 NiCr t  (1), 1.0 NiCr t  (2), 1.5 NiCr t  (3), 2.0 NiCr t  (4); (b) scheme for definition of bending angle  . Results presented hereinafter correspond to the loading velocity of V load = 0.4 m/s. The change of integral parameters of the composite response with increase in strength of interphase boundaries ( bound t  ) has a pronounced non-linear character. Fig. 14a shows dependence of the sample strength (maximal value of  ) on the value of bound t  . This dependence can be divided into three main sections. The first one (section I in the Fig. 14a) corresponds to low values of interface strength ( bound NiCr t t    ) and is characterized by small values of integral strength of the composite. In the second (transition) section ( 2.0 NiCr bound NiCr t t t      ) there is a considerable increase in the strength of the composite. In the third section corresponding to composites with high strength of interfaces ( 2.0 bound NiCr t t    ), the value of composite strength reaches the saturation. The described stages are pronounced in the curve of interface strength depending on fracture energy (Fig. 14b). It can be seen that the fracture energy of composites with high-strength interphase boundaries increases by an order of magnitude in comparison with composites containing low-strength interfacial boundaries.

(a) (b) Figure 14 : Strength (a) and fracture energy (b) in model metal-ceramic composite depending on interphase boundary strength bound t  . Values of interphase boundary strength and “macroscopical” composite strength are normalized by tensile strength of the binder NiCr t  . Fracture energy was calculated as an area under the loading diagram expressed in the units “normalized stress – bending angle” (Fig. 13a). The analysis of simulation results showed that staging of the change in integral parameters of the composite response with increase in strength of interphase boundaries ( bound t  ) is associated with a change in the nature of fracture. Fig. 15 shows

48