鋼の引張圧縮塑性疲労試験

Abstract

Fatigue failure of materials in the plastic range is considered to have an increasingly important significance nowadays, since the results obtained from these investigations provide the fundamental data not only for the design of some pressure vessels, but also for the study of thermal fatigue failure of materials. Several experimental results have been reported concerning the fatigue strength of some materials in the plastic range at room and elevated temperatures.We also planned these tests, and in the first place constructed a new device that made possible to carry out the completely reversed axial fatigue test with a conventional static testing machine. This device is necessary in most cases of this type of experiments, because the conventional static testing machine can not provide the tensile and compressive load to a specimen cyclically.Secondly, we made two types of experiments with low carbon steel using this device at room temperature-constant strain range test and contstant stress range test.The stress and the strain characteristics in the respective tests were compared in detail, and discussions were made concerning some hypotheses in order to interpret the relationship between these tests.The main results are as follows:(1) In the constant strain test the maximum tensile and compressive stress increase with the number of cycles due to the strain hardening effect. But in the constant stress test the mean tensile strain increases gradually with the number of cycles regardless of the initial loading directions'-tension or compression.(2) The true stress based on the actual cross sectional area must be considered when a discussion is made about the shape of stress-strain curve especially in the case of high stress or high strain conditions.(3) The total strain given to the specimen during the period up to the failure is found to be the most important factor affecting the fatigue failure in the plastic range. With this total strain we can interpret the respective test results of constant strain and constant stress together.