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为进一步弄清缺口持久断裂的机理,作者采用持久试验,金相分析、有限元计算相结合的方法进行研究,修正了传统的概念。1.各国标准都以棒状缺口试样持久试验的结果来衡量材料的高温性能,传统上认为是反映材料的“缺口敏感性”。研究表明,标准缺口持久试验的实质是反映材料在多向应力作用下的抗主裂纹萌生与扩展的能力。2.各国标准都以理论应力集中系数K_t作为确定试样几何参数的主要依据。对K_t的物理意义,传统上都按缺口处轴向应力的最高值与平均值之比来理解。研究表明,实际的应力集中系数远较K_t值小,K_t实质上是反映了缺口拘束度和应力三向性的高低。3.各国标准都只用棒状缺口试样进行试验,而美国标准ASTME 292-69除棒状外还允许采用板状缺口试样。研究表明,棒状和板状缺口试样的断裂形式及应力应变再分布都不同:K_t值相同或相近的两种试样,持久寿命可能相差达数十倍之多。因此,应特别注意这两种试样各自适用的场合。故传统上一律采用棒状缺口试样的做法是不妥的。
In order to further understand the mechanism of the long-term fracture of the gap, the author uses the method of lasting test, metallographic analysis and finite element calculation to study and amend the traditional concept. 1. The standards of all countries use the results of long-term tests on stick-shaped notched specimens to measure the high-temperature performance of the material, which is traditionally considered to reflect the “notch sensitivity” of the material. The research shows that the essence of the standard notch durable test is the ability to reflect the initiation and propagation of the main crack under the multi-directional stress. 2. National standards are the theoretical stress concentration factor K_t as the main basis for determining the geometric parameters of the sample. The physical meaning of K_t is traditionally understood by the ratio of the highest value of the axial stress in the notch to the average value. The study shows that the actual stress concentration factor is far smaller than the K_t value, K_t essentially reflects the degree of gap restraint and stress three-way. 3. The national standards are only used to test the stick-shaped notched specimen, while the American Standard ASTME 292-69 in addition to the stick-shaped plate also allows the use of notched specimens. The results show that the fractured forms and stress-strain redistribution of rod-shaped and plate-shaped notched specimens are different: the two specimens with the same or similar K_t values may have a difference of several ten times in the longevity of life. Therefore, special attention should be paid to the application of the two samples. Therefore, it is not proper to adopt the stick-shaped notched specimen conventionally.