Inelastic Analysis of Stress-Strain Response in Engineering Ceramics at Elevated Temperatures

对烧结氮化硅陶瓷进行了高温拉伸试验和拉伸蠕变试验。在试件的标尺长度内加工四个凸起，并利用激光束式引伸仪测量试件一侧两个凸起之间的位移。此外，还开发了用于评估陶瓷高温疲劳性能的新测试系统。然后在室温到1200ﾟC的温度范围内对烧结氮化硅陶瓷进行了推拉疲劳试验，结果表明：1.拉伸应力应变关系受试验温度和位移速率的影响较大。对由实测载荷(P)-位移(Delta)关系计算的名义应力-应变关系进行了修正，使弹塑性有限元计算的P-Delta曲线与实测值吻合。修正后的应力-应变关系似乎给出了烧结氮化硅陶瓷在高温下的本征关系。烧结氮化硅陶瓷的蠕变曲线先是短暂蠕变阶段，然后是稳定蠕变阶段。根据最小蠕变应变速率与名义应力在双对数尺度上的关系，可以用两条直线进行拟合，其斜率在高应力区大于低应力区。应力幅值与破坏循环次数的关系呈S-N曲线。S-N曲线的斜率随试验温度的升高有减小的趋势。

The tensile tests and tensile creep tests were performed for the sintered silicon nitride ceramics at elevated temperatures. The four projections were machined in the gauge length of the specimen, and the displacement was measured between the two projections on one side of the specimen by means of the laser-beam-type extensometer. Furthermore, the new test system was developed for evaluating the fatigue properties of ceramics at elevated temperatures. Then push-pull fatigue tests were performed for the sintered silicon nitride ceramics in the temperature range from room temperature to 1200ﾟC. The results obtained are summarized as follows :1. The tensile stress-strain relationship was markedly influenced by test temperature and displacement rate.2. The nominal stress-strain relationships calculated from the measured load (P)-displacement(delta) relationships were corrected so that the p-delta curve calculated using the elastic-plastic F.E.M agreed with the measured one. The corrected stress-strain relationship seems to give the intrinsic relationship of sintered silicon nitride ceramics at elevated temperatures.3. The creep curves of sintered silicon nitride ceramics exhibited the transient creep period, being followed by the steady creep period.4. According to the relationships between the minimum creep strain rate and nominal stress plotted in double logarithmic scales, the data could be fitted by two straight lines, and their slopes are larger in the higher stress region than in the lower stress region.5. The relationships between stress amplitude and number of cycles to failure showed the shape of a so called S-N curve. The slope of S-N curve, however, had a tendency to decrease with increase in test temperature.