ity facto at constant ap stress kAppa fracture Kappa=om Fig. 5. Condition of unstable crack growth in the internal stress field It follows from Eq(26) and Fig. 5 [9 that unstable crack growth occurs if the slope of straight line corresponding to the stress intensity factor at constant applied stress is no less than the slope of tangent line to the fracture resistance curve at the same point igure 6 shows dependence of the apparent fracture toughness on crack length parameter a in laminate Si3N4/Si3N4-20 wt TIN, specimen 1(solid curve). The areas corresponding to compressive and tensile layers are shown in gray and white, respectively. The fracture toughness of layer material is shown as horizontal straight line The dependence of apparent fracture toughness on a is non-monotonous. The apparent fracture toughness increases in the compressive layers and decreases in the tensile layers. The peak values of K app correspond to interfaces between layers. The apparent fracture toughness of the layered composite varies from 2 to 10 MPa m" depending on the crack length. The initial notch tip is in tenth layer that is under residual tension. Measured value of the apparent fracture toughness corresponding to the initial notch is 5.57 MPa m"that is in accord with the calculated value Unloading was made after small advance of crack from the initial notch. Crack arrest occurred in the 12th layer of specimen. The length of arrested crack was measured. Then the next loading resulted in the total failure of specimen. Measured value of the apparent fracture toughness corresponding to arrested crack is 7.42 MPa m"that is also in accord with the calculated value Figure 7 shows dependence of the apparent fracture toughness on crack length parameter a in specimen 2 Designations are the same as in Fig. 6. The dependence of the apparent fracture toughness on crack length parameter is non-monotonous as well. The fracture toughness behavior in compressive and tensile layers in specimen 2 is qualitatively similar to that in specimen 1. However, difference of specimen geometry results in some difference of the apparent fracture toughness range. Specifically, the apparent fracture toughness of specimen 2 varies from 3 to 11 MPa- m". The initial notch tip in the specimen is also in tenth layer that is under residual tension. In this case, measured value of the apparent fracture toughness corresponding to the initial notch is 6.39 MPa.m". That is in accord with the calculated value too. After unloading crack was arrested in 12th layer like specimen 1. Specimen 2 with arrested crack demonstrates the apparent fracture toughness value of 6.27 MPa m". This is in good accord with the Additionally to Si3 N//Si3 N4-20 wt TiN layered specimens, the mechanical behavior of Si3N4/Si3N4- 70 wt. TIN laminates was studied by the compliance technique. Figure 8 shows cyclic load -displacement diagram of layered specimen Si3N4/Si3N4-70 wt TiN with crack. An interesting feature of this diagram is a number of hysteresis loops recorded during specimen unloading and its further loading. It can be connected with some energy dissipation during unloading-loading cycle. A similar effect was also observed, e.g., in the studies of R-curves for graphite [19]. It was connected with the amount of energy dissipated by plastic strains. The microscopic analysis of fractured specimens demonstrated that tensile-stressed layers containing 70% Tin display multiple channel cracks 299It follows from Eq. (26) and Fig. 5 [9] that unstable crack growth occurs if the slope of straight line corresponding to the stress intensity factor at constant applied stress is no less than the slope of tangent line to the fracture resistance curve at the same point. Figure 6 shows dependence of the apparent fracture toughness on crack length parameter ~a in laminate Si3N4/Si3N4–20 wt.% TiN, specimen 1 (solid curve). The areas corresponding to compressive and tensile layers are shown in gray and white, respectively. The fracture toughness of layer material is shown as horizontal straight line. The dependence of apparent fracture toughness on ~a is non-monotonous. The apparent fracture toughness increases in the compressive layers and decreases in the tensile layers. The peak values of Kapp correspond to interfaces between layers. The apparent fracture toughness of the layered composite varies from 2 to 10 MPa m⋅ 1 2/ depending on the crack length. The initial notch tip is in tenth layer that is under residual tension. Measured value of the apparent fracture toughness corresponding to the initial notch is 5.57 MPa m⋅ 1 2/ that is in accord with the calculated value. Unloading was made after small advance of crack from the initial notch. Crack arrest occurred in the 12th layer of specimen. The length of arrested crack was measured. Then the next loading resulted in the total failure of specimen. Measured value of the apparent fracture toughness corresponding to arrested crack is 7.42 MPa m⋅ 1 2/ that is also in accord with the calculated value. Figure 7 shows dependence of the apparent fracture toughness on crack length parameter ~a in specimen 2. Designations are the same as in Fig. 6. The dependence of the apparent fracture toughness on crack length parameter is non-monotonous as well. The fracture toughness behavior in compressive and tensile layers in specimen 2 is qualitatively similar to that in specimen 1. However, difference of specimen geometry results in some difference of the apparent fracture toughness range. Specifically, the apparent fracture toughness of specimen 2 varies from 3 to 11 MPa m⋅ 1 2/ . The initial notch tip in the specimen is also in tenth layer that is under residual tension. In this case, measured value of the apparent fracture toughness corresponding to the initial notch is 6.39 MPa m⋅ 1 2/ . That is in accord with the calculated value too. After unloading crack was arrested in 12th layer like specimen 1. Specimen 2 with arrested crack demonstrates the apparent fracture toughness value of 6.27 MPa m⋅ 1 2/ . This is in good accord with the calculated value. Additionally to Si3N4/Si3N4–20 wt.% TiN layered specimens, the mechanical behavior of Si3N4/Si3N4– 70 wt.% TiN laminates was studied by the compliance technique. Figure 8 shows cyclic load – displacement diagram of layered specimen Si3N4/Si3N4–70 wt.% TiN with crack. An interesting feature of this diagram is a number of hysteresis loops recorded during specimen unloading and its further loading. It can be connected with some energy dissipation during unloading-loading cycle. A similar effect was also observed, e.g., in the studies of R-curves for graphite [19]. It was connected with the amount of energy dissipated by plastic strains. The microscopic analysis of fractured specimens demonstrated that tensile-stressed layers containing 70% TiN display multiple channel cracks 299 Fig. 5. Condition of unstable crack growth in the internal stress field