Application of combined loading to determine the fatigue limit

Abstract

The article considers the problem related to the accuracy of experimental determination of the average probable value of the fatigue limit using two methods based on combined loading. This type of loading involves a combination of either stationary and non-stationary modes, or two non-stationary modes with increasing and decreasing loads. The accuracy of estimating the fatigue limit using the methods of this group largely depends on the dispersion of the strength properties of the test objects, so the problem under consideration is relevant. An analysis of the accuracy of two methods is carried out, the first of which is based on testing objects with increasing and decreasing loads, and the second – on a combination of stationary and non-stationary loads. At the same time, a specific example shows that the first method is characterized by a systematic error, which is embedded in the very scheme of the experiment. This error occurs when a less durable object is tested with a decreasing load from the initial stresses corresponding to the destruction of a stronger object. The condition for testing according to the second method is the equality of the destructive stress of the first object to the stress level of the second object tested under a steady load. In this case, a ratio was obtained between the total durability before the destruction of the first object and the durability of the second. An analysis of this ratio was carried out for two main equations of the fatigue curve. It showed that the fluctuation of this ratio depends on the choice of the fatigue curve model, but at the same time it slightly affects the accuracy of the method.