0=168 MP Lo=176 M Fig. 4. Optical photograph of layered specimen with a typical notch tip The specimens for mechanical tests were prepared from hot-pressed plates. SEVNB specimens were used for testing. The test data have confirmed that the SEVNB method can be easier applied in practice and used for the majority of advanced ceramics and ceramic particulate composites [18]. The V-notches with tip radii of an order of 10-15 um were made in the specimens by a two-stage technique. In the first stage, the specimens were notched by a diamond saw. Then sharp tip of notch was obtained using stainless steel blade and diamond abrasive. Optical photograph of a typical notch tip along with average values of calculated residual stresses acting in the layers is presented in Fig. 4. The depth of the notches was about 60-80% of the specimen thickness being 3.37-4.0 mm Specimens of larger thickness were ground down to such sizes A stiff load cell ensuring the rigid loading of specimens under three-point bending with a 16 mm span was used for mechanical tests [18]. This"autonomous"cell is equipped with specific rigid dynamometer providing an ultimate load of 2000 N with a specimen deflection measuring system using a deflectometer suspended on the specimen. The testing machine used is designed only for the displacement of a loading crosshead and control of its speed. To study R-curve effect the compliance technique was used. Notched specimen was placed into the hard load cell. Then loading of the specimen was performed up to crack growth onset followed by unloading. In addition to recording of the load-deflection diagrams, after each unloading of specimen, its polished lateral surface was examine by an optical microscope(x1000) to measure crack length. After measurement of crack length the next loading unloading cycle was made. The operations were repeated up to the total failure of specimen. Apparent fracture toughness was calculated by using expressions(12),(23), and (24) Results and Discussion. Asymmetric structure of layered specimens under study results in linear variation of residual stresses within each layer. The critical issue to analyze fracture behavior of laminates is a choice of the coordinate system. Calculated values of apparent fracture toughness Kapp in layered specimens under study are analyzed depending on crack length parameter a, where a=Y(a)aV2. The crack length parameter a is the most appropriate to demonstrate critical conditions of crack growth. One of advantages of this parameter is that the stre intensity factor of an edge crack for fixed value of the applied stress om is depicted in the coordinate system Kapp -a as a straight line from the coordinate origin. Indeed, it follows from(22) that K1=oma, therefore, the slope of straight line equals to the applied stress om. The conditions for unstable crack growth in the internal stress field are as follows [9]: K,(om, a)=kapp(a), dK,(om, a)/da 2 dK app(a)/da. Using parameter a, these conditions become:oma=Kapp(a),om>dAnn(a ) /da. The last two conditions can be reduced to v(a)a≥dK app(a)/da (26)The specimens for mechanical tests were prepared from hot-pressed plates. SEVNB specimens were used for testing. The test data have confirmed that the SEVNB method can be easier applied in practice and used for the majority of advanced ceramics and ceramic particulate composites [18]. The V-notches with tip radii of an order of 10–15 µm were made in the specimens by a two-stage technique. In the first stage, the specimens were notched by a diamond saw. Then sharp tip of notch was obtained using stainless steel blade and diamond abrasive. Optical photograph of a typical notch tip along with average values of calculated residual stresses acting in the layers is presented in Fig. 4. The depth of the notches was about 60–80% of the specimen thickness being 3.37–4.0 mm. Specimens of larger thickness were ground down to such sizes. A stiff load cell ensuring the rigid loading of specimens under three-point bending with a 16 mm span was used for mechanical tests [18]. This “autonomous” cell is equipped with specific rigid dynamometer providing an ultimate load of 2000 N with a specimen deflection measuring system using a deflectometer suspended on the specimen. The testing machine used is designed only for the displacement of a loading crosshead and control of its speed. To study R-curve effect the compliance technique was used. Notched specimen was placed into the hard load cell. Then loading of the specimen was performed up to crack growth onset followed by unloading. In addition to recording of the load-deflection diagrams, after each unloading of specimen, its polished lateral surface was examined by an optical microscope (×1000) to measure crack length. After measurement of crack length the next loading– unloading cycle was made. The operations were repeated up to the total failure of specimen. Apparent fracture toughness was calculated by using expressions (12), (23), and (24). Results and Discussion. Asymmetric structure of layered specimens under study results in linear variation of residual stresses within each layer. The critical issue to analyze fracture behavior of laminates is a choice of the coordinate system. Calculated values of apparent fracture toughness Kapp in layered specimens under study are analyzed depending on crack length parameter ~a, where ~aY a = α( ) 1 2 . The crack length parameter ~a is the most appropriate to demonstrate critical conditions of crack growth. One of advantages of this parameter is that the stress intensity factor of an edge crack for fixed value of the applied stress σ m is depicted in the coordinate system K a app − ~ as a straight line from the coordinate origin. Indeed, it follows from (22) that K a 1 = σ m ~, therefore, the slope of straight line equals to the applied stress σ m . The conditions for unstable crack growth in the internal stress field are as follows [9]: K aK a 1 m app ( , ) ( ), σ = dK a da dK a da 1 m app ( , ) () σ ≥ . Using parameter ~a, these conditions become: σ m app aK a ~ ( ~ = ), σ m app ≥ dK a da ( ~) ~. The last two conditions can be reduced to: K a a dK a da app app ( ~) ~ ( ~) ~ ≥ . (26) 298 Fig. 4. Optical photograph of layered specimen with a typical notch tip