It has often occured that bolts have broken at the re-fastening under employment. In this paper, the basic aspect of this phenomenon is studied by following experimental procedure using the notched specimen with circumferential half circular groove, and a criterion on the combined damage of fatigue pre-strain is proposed. The notch specimen is pulled to logarithmic plastic strain, εp=1.3%, then is applied by direct stress σ=11.0kg/mm2 for several ratios of repetitions, n1/N1, in which N1 is fatigue 1ife and n1 is number of repetitions at σ=11.0 kg/mm2. After this procedure, this specimen is pulled again to εp=1.3% and is applied by σ=11.0 kg/mm2. In the case of the second experiment, stress level is different the first stage from the second stage of above mentioned procedure, that is, the first applied stress is σ1=9.5 kg/mm2 and the second is σ2=11.0 kg/mm2. According to the results of these experiments, in the case of the small ratio of repetitions the specimen is strengthened, but in other cases are weakened, compared with the fatigue life of the pre-strained materials of εp= 2.6%, which is the same total plastic strain of the specimen, 1.3% plus 1.3%. This phenomenon is complicated by multiple plastic strains and stress repetitions and by characteristics of materials on work hardening and fatigue. Therefore, the author wil1 propose that we use the two S-N curves, εp=1.3% and εp=2.6%, and apply the Miner's damage rule. By this criterion, the strength of bolt at the re-fastening shall be clear.
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机译：经常发生螺栓在使用中重新紧固时折断的情况。本文采用带有圆周半圆形凹槽的缺口试样，通过以下实验程序研究了该现象的基本方面，并提出了疲劳预应变综合损伤的判据。将缺口试样拉至对数塑性应变εp= 1.3％，然后通过直接应力σ= 11.0kg / mm2施加多个重复率n1 / N1，其中N1为疲劳1ife，n1为重复次数σ＝ 11.0kg / mm 2。在此过程之后，将该样品再次拉至εp= 1.3％，并施加σ= 11.0 kg / mm2。在第二实验的情况下，第一阶段的应力水平不同于上述过程的第二阶段，即，第一施加应力为σ1= 9.5 kg / mm2，第二施加应力为σ2= 11.0 kg / mm2。根据这些实验的结果，在重复率较小的情况下，与εp= 2.6％的预应变材料的疲劳寿命相比，试样被增强，而在其他情况下被削弱。样品的总塑性应变为1.3％加1.3％。这种现象由于多种塑性应变和应力重复以及材料在工作硬化和疲劳方面的特性而变得复杂。因此，作者将建议我们使用两条S-N曲线εp= 1.3％和εp= 2.6％，并应用Miner损伤定律。按照此标准，螺栓在重新紧固时的强度应清楚。
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