Research on Fatigue Testing Method of Bolts for Quality Assurance

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In 1977 the ISO standard on fatigue testing method of threaded fasteners was published. Now the revision of this standard is in progress. The draft of revised standard includes "Quality control test", but the description on this item is very simple. In order to apply this item to the practical situation, many problems should be investigated.The fatigue strength is one of the important quality characteristics of bolts, and how to carry out the quality assurance for this characteristic is one of the problems to be solved for users and suppliers of bolts. From the above standpoint, the fatigue testing method of bolts for quality assurance is investigated, and the obtained results are as follows:(1) As the specimens, the bolts of M10-8.8 and the nuts of M10-8 (8.8 and 8 are strength property classes) are prepared, and the 14 S-N testing method specified in the draft of revised ISO standard is applied. In this case the required number of test specimens in a series of test is 14.(2) As the testing method for quality assurance, two methods are proposed after several methods have been discussed.(3) The proposed method 1 is obtained by the simplification of 14 S-N testing method, and the required number of test specimens is 8.(4) The proposed method 2 is based on the estimated standard values of fatigue limits of bolts, and the required number of test specimens is only 4.(5) Concerning these two methods, the experiments in the wider range are carried out, and their merits and demerits are clarified. From the viewpoint of quality assurance, it is found that the method 2 is better than the method 1.