20. No. 2 Zhang Yafang et al. Fractural Process and Toughening Mechanism of Composites acoustic emission step 40-0) acoustic emission step 46(0) (a)ceramic block structure (b)laminated composite ceramic Fig. 4. Numerical simulation of AE in laminated model IV. THE EFFECT OF STRENGTH OF SOFT LAYERS To investigate the effect of strength of soft layers using the same beam models presented in the previous section(Fig. 1), numerical simulation on a group of eight specimens with different strengths of the soft layer has been conducted. The strength ratios of soft layers to hard layers are set to be 0. 12, 0.16, 0.2, 0.24, 0.28, 0.32, 0.36 and 0.4 for each specimen, while other features remain unchange The results of the simulation are presented in Fig. 5, where the curves of fractural work K and th peak load versus strength ratio are plotted as Fig. 5(a) and 5(b), respectively. From these figures, it is clear that the fractural work K and the peak load P decrease while the strength of the soft layers increases In particular, when the strength ratio is in the range 0. 24-0.28, both K and P decrease rapidly This implies that if the strength of the soft layer is very high, the deflection and the softening of the crack tips can hardly happen. Meanwhile, if the strength of the soft layer is very low, it will not be advisable to improve the mechanical characteristics of the composite ceramic. In fact, a specimen with strength ratio of 0.08 is also tested, and it is found that all the fractures occur along the soft layer, so that no crack initiates or propagates vertical 3410 0080.120.160.200.240.280.320360.400.44 080.120.160200240.280.320.360.400.44 strength ratio of soft and hard layers strength ratio of soft and hard layers Fig. 5. Curves of fracture energy and peak load of the laminated models versus the strength of soft layer The explanation about this effect can be deduced from Fig. 6, where the fractural process is presented for each specimen. For the specimens No. 1 to 4 with relatively low strength soft layers, an obvious crack deflection can be observed. First the cracks initiate near the tip of the notch, and then propagate vertically along the load direction. Passivation on the crack tips happens when a soft layer is met and the crack deflects along the horizontal direction, i.e. along the soft layer. The lower the strength of the soft layer is, the longer distance the crack goes over horizontally and more energy is dissipated in the cracking process. On the other hand, for the specimens No 5 to 8 with relatively high strength soft layers, the cracks develop upwards quickly but over a shorter distance horizontally. Compared with the cases in previous specimens, less deflection happens and less energy is dissipated. So the toughening effect is not manifest and the laminated ceramic has nearly similar fractural work K as the ceramic block, i. e, no critical improvement has been made on the toughness of the ceramic. Based on the previous discussion, the strength of the soft layers is crucial to the attempt at improving the toughness of the laminated ceramic material. Only those soft layers of adequate strength can increaseVol. 20, No. 2 Zhang Yafang et al.: Fractural Process and Toughening Mechanism of Composites · 145 · Fig. 4. Numerical simulation of AE in laminated model. IV. THE EFFECT OF STRENGTH OF SOFT LAYERS To investigate the effect of strength of soft layers using the same beam models presented in the previous section (Fig.1), numerical simulation on a group of eight specimens with different strengths of the soft layer has been conducted. The strength ratios of soft layers to hard layers are set to be 0.12, 0.16, 0.2, 0.24, 0.28, 0.32, 0.36 and 0.4 for each specimen, while other features remain unchanged. The results of the simulation are presented in Fig.5, where the curves of fractural work K and the peak load versus strength ratio are plotted as Fig.5(a) and 5(b), respectively. From these figures, it is clear that the fractural work K and the peak load P decrease while the strength of the soft layers increases. In particular, when the strength ratio is in the range 0.24-0.28, both K and P decrease rapidly. This implies that if the strength of the soft layer is very high, the deflection and the softening of the crack tips can hardly happen. Meanwhile, if the strength of the soft layer is very low, it will not be advisable to improve the mechanical characteristics of the composite ceramic. In fact, a specimen with strength ratio of 0.08 is also tested, and it is found that all the fractures occur along the soft layer, so that no crack initiates or propagates vertically. Fig. 5. Curves of fracture energy and peak load of the laminated models versus the strength of soft layer. The explanation about this effect can be deduced from Fig.6, where the fractural process is presented for each specimen. For the specimens No.1 to 4 with relatively low strength soft layers, an obvious crack deflection can be observed. First the cracks initiate near the tip of the notch, and then propagate vertically along the load direction. Passivation on the crack tips happens when a soft layer is met and the crack deflects along the horizontal direction, i.e. along the soft layer. The lower the strength of the soft layer is, the longer distance the crack goes over horizontally and more energy is dissipated in the cracking process. On the other hand, for the specimens No.5 to 8 with relatively high strength soft layers, the cracks develop upwards quickly but over a shorter distance horizontally. Compared with the cases in previous specimens, less deflection happens and less energy is dissipated. So the toughening effect is not manifest and the laminated ceramic has nearly similar fractural work K as the ceramic block, i.e., no critical improvement has been made on the toughness of the ceramic. Based on the previous discussion, the strength of the soft layers is crucial to the attempt at improving the toughness of the laminated ceramic material. Only those soft layers of adequate strength can increase