| ZrB2-based ultra high temperature ceramics have become the best of ceramic matrix composites, due to many advantages such as high melting point, high thermal conductivity and strength. Meanwhile, they have been condinates for applications in critical parts of vehicles. According to the inherent brittleness and application surroundings of high temperature and velocity, characterizations on high temperature mechanical behavior and dynamic properties of ultra high temperature ceramics need to develop deeply. Subsequently, investigations on high temperature fracture mechanisms are important foundations for their applications.This paper investigated the effects of temperature and strain rate on mechanical properties and constitutive relationship of Zr B2-Si C-G ultra high temperature ceramics composites. The studies were carried out from high temperature mechanical experiments, dynamic compression experiment, fracture mechanism analysis, constitutive model establishment and thermal shock resistance. These results can lay the foundations for characterizing mechanical performances and failure analysis for ultra high temperature ceramics under high temperature and veloci ty surroundings.Firstly, the high temperature tensile benhavior of Zr B2-Si C-G composite was studied through tensile experiments at room and high temperatures. The effects of temperature on elastic modulus, strength and the nonlinear constitutive relastionship were discussed. The thermal damage equation was provided based on the degradation on tensile modulus with temperature, and the mechanical damage expression was obtained according to the statistical distribution of tensile strength dispersibility. Thus, the high temperature damage constitutive model of Zr B2-based ultra high temperature ceramic was established, which can predict the brittle-ductile transition temperature and clearly reveal the effect of temperature on fracture mechanism. Furthermore, the subroutine of this high temperature damage constitutive model was compiled to calculate deformation, stress and mechanical damage distributions of tensile specimen, and its correction was verified by the experimental results.Secondly, fracture toughness of Zr B2-Si C-G composite was measured at room and high temperatures. The effect of temperature on fracture toughness was obtained and explained by combining with the macro- and microstructures. Results show that temperature has an obvious influence on fracture behavior of Zr B2-Si C-G composite. Below the brittle-ductile transition temperature, Zr B2-Si C-G composite exhibits brittle fracture mode, whereas it expresses the ductile fracture mode above the brittle-ductile transition temperature. Specifically, the relaxation of residual stress leads to the reduction in fracture toughness from room temperature to 1300 °C. The plastic flow at the crack tip results in the slight increase of fracture toughness from 1300 to 1600°C. However, the increase in grain size of Zr B2 particle induces the generation of crack and cavity at 1800°C, which is the main cause of the reduction in fracture toughness. Moreover, the preoxidation effect can obviously enhance fracture toughness of Zr B2-Si C-G composite during 800 and 1300°C in the oxygen environment.Furthermore, we presented finite element models including no notched beam(NNB) and single edge notched beam(SENB). The high temperature damage constitutive model was employed to obtain the stress and mechanical damage distributions of Zr B2-Si C-G composite under the bending condition. Meanwhile, the effect of temperature on defect sensitivity was investigated. Results show that the mechanical damage of SENB is obvious lower than NNB below the brittle-ductile transition temperature. Differently, the mechanical damage of SENB approaches to the value of NNB above 1400°C. This can reveal that the defect sensitivity decreases above the brittle-ductile transition temperature, which has a good accordance with the experimental result that strength has low dispersity at high temperaures.The static and dynamic compressive experiments were carried out at room temperature and 800°C. The effect of strain rate on compressive mechanical performance was investigated by comparing static and dynamic compressions. Results suggest that Zr B2-Si C-G composites exhibit obvious strain rate effect at room temperature and 800°C. As strain rate increases, compressive strengths increase, while the fragment sizes obviously decrease. Based on the observations of microstructures at fracture surfaces, the effect of strain rate on the fracture mode for Zr B2-Si C-G composite was discussed. Meanwhile, the theoretical models were used for predicting dynamic compressive strengths and fragment sizes, which also well agree with the experimental results.Finally, thermal shock behavior of Zr B2-Si C-G composite was calculated by numerical methods, which considered the effects of inertia and coulpling items. The finite element model with temperature dependent material properties was established according to experimental specimen and conditions to compute thermal stress under heating and cooling processes. Results declare that the inertia item has more significant influence on thermal shock behavior than the coupling item. As the surface transfer coefficient increases, thermal stress increases and the inertia effect enhances. The maximum ratio between dynamic and static thermal stress is 1.33 under the water quenching of 400°C. On the basis of dynamic compressive experimental results, thermal fracture criteria can be employed to evaluate thermal shock resistance of Zr B2-Si C-G composite. |
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