Basic Study of Bolt Re-fastening under Employment

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