Device Development for the Analysis of Non-equiliblium Fracture of Ceramic Composites under Ultra Severe Condition

采用自行研制的高温抗弯强度测试装置，对多晶Al_2O_3和SiC晶须增强Al_2O_3复合材料的抗弯强度和断裂功进行了测试。对尺寸为3 * 4 * 36 mm 3的梁进行了内跨10 mm、外跨30 mm的四点弯曲试验，以估计弯曲强度。为了估计断裂功，对中心带有人字形切口的梁进行跨度为30 mm的3点弯曲。从室温到1200 ℃，没有观察到明显的塑性变形。多晶Al_2 O_3的断裂功几乎没有变化，为20 J/m ~ 2。相反，复合材料的断裂功随着温度的增加而增加，从室温下的约50 J/m2增加到1100 ℃下的最大值120 J/m2。听下面一段材料，回答第10至15题。 ...更多信息 由于Al 2 O3和SiC之间的热膨胀不匹配，导致了残余热应力的减小，并导致了桥接晶须的数量和拔出长度的增加。模型计算结果表明，多晶Al_2 O_3的抗弯强度在1000 ℃以下基本保持不变（约450 MPa），并考虑了晶须的摩擦桥接和拔出桥接。从1200 ℃开始，随着温度的升高而逐渐降低。相反，复合材料的弯曲强度从室温到900 ℃没有变化。在1000 ℃时，压力从400 MPa急剧下降到300 MPa。弯曲强度的变化对应于裂纹扩展阻力的变化，因为实验证实，在低于1200 ℃的温度下，两种材料都没有裂纹愈合和宏观塑性变形。C.复合材料在1000 ℃下的弯曲强度显著降低。C是由于晶须周围的压缩残余热应力在室温下松弛而降低了裂纹闭合应力。少

The bending strength and the work-of-fracture of polycrystalline Al2O3 and one reinforced with SiC whiskers were measured at elevated temperatures using an equipment developed in this study. Four-point bend test with an inner span of 10mm and an outer span of 30mm was carried out for a beam with a dimension of 3 * 4 * 36mm3 in order to estimate the bending strength. For the estimation of the work-of-fracture, 3-point bending with a span of 30mm was done against the beam with a chevron notch at its center. Remarkable plastic deformation was not observed in each material from room temperature to 1200。C, so that linear fracture mechanics could be applied to analyze the mechanical properties measured.There was almost no change in the work-of-fracture of the polycrystalline Al2O3, which was 20J/m2. On the contrary, the work-of-fracture of the composite increased with increasing temperature from about 50J/m2 at room temperature up to the maximum of 120J/m2 at 1100。C.This was caused by the re … More laxation of residual thermal stresses which was due to thermal expansion mismatch between Al2O3 and SiC, and was leaded the increase in the number as well as the pull-out length of bridging whiskers. The rationality of the argument was supported by the result of model calculation in which both frictional and pull-out bridging of whiskers were considered, and by the observation of pulled-out whiskers on a fractured surface.The bending strength of polycrystalline Al2O3 was constant (about 450MPa) below 1000。C, and gradually decreased with increasing temperature from 1200。C.On the contrary, the bending strength of the composite did not change from room temperature to 900。C, and steeply decreased from about 400MPa to about 300MPa at 1000。C.The bending strengths changed with correspond to the change in crack extension resistance, because it was experimentally confirmed that there was no crack healing and macroscopic plastic deformation in both materials below 1200。C.The significant decrease in the bending strength of the composite at 1000。C was caused by the decrease of crack closure stress owing to the relaxation of compressive residual thermal stress around whiskers which bridged natural flaws at room temperature. Less