High-temperature K_R-curve behavior of low ductility ceramic materials

Kr increases with the increase of crack length in ceramic materials under temperature. With the increase of temperature, the toughness index m of the ceramic material decreases, the crack resistance decreases, and the Kr curve becomes flatter. The increased Kr curve behavior of ceramic materials at high temperatures is closely related to the interaction between the crack's turning direction and crack surface in the tail zone. The load-unload technology combined with the softness calibration test method is an effective and simple method for measuring high temperature Kr curve of ceramic materials. Ceramic materials often have low fracture toughness or fracture resistance due to their inherent brittleness. Materials at lower temperatures often exhibit linear elastic mechanical behavior, but with increasing temperature, the material will have a certain degree of plastic deformation. This deformation behavior is mainly attributed to the viscous behavior of intercrystalline glass at high temperatures. The induced slow crack growth (SCG), even at room temperature, can sometimes observe significant nonlinear stress-strain behavior, which often occurs in coarse-grained materials or materials with high microcrack density under slow loading conditions. The crack growth resistance of a material is often characterized by measuring the crack resistance curve (ie Kr-curve). Crack growth resistance curves have a variety of measurement techniques. These measurement techniques are mostly complicated to operate and time-consuming. For the study of the Kr curve behavior of ceramic materials, many studies have reported that the crack growth resistance increases with the growth of cracks. For non-phase-change ceramics, this increased Kr curve behavior is attributed to the crack tail region. Bridging and crack tip microcracking.

Obtained, but relatively few studies on this aspect at high temperature. This work uses loading-unloading technology to study the Kr curve behavior of SiC ceramics at high temperatures, and discusses the influence of microstructure on Kr-curve curves. This method assumes that the energy release rate Kr is the same as the linear elasticity state in the quasi-static crack propagation. Therefore, Kr can be calculated as m: where ea is the applied stress; a is the crack length w is the width of the sample; Aa is the crack Increment; Y(a/w) is the shape factor.

The calculation of the crack length a is more complicated. According to the calculation method recommended by M. Sakai and RC Bradtl1l, the flexibility of the specimens with different notch depths is calibrated at room temperature, and regression processing is performed on the computer. We can obtain: (relation coefficient r =0. The experimental method of combining the unloading technique with the soft compliance calibration of the specimen by closely related loading is an ideal method for measuring the high temperature Kr curve behavior of the ceramic material, ie, saving time and material.

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