Determination of fragments of multiaxial service loading strongly influencing the fatigue of machine components

The paper presents a method for determination of multiaxial load segments from original service histories, where the loaded machine part is meaningfully subjected to fatigue damage. These load segments are directly separated from a service-loading history, which can be of a random character. Typical procedures used for fatigue life assessment under multiaxial random loading are implemented to perform this task. While reduction from the multiaxial stress state to the equivalent uniaxial one, application of the linear multiaxial fatigue failure criteria was proposed. The equivalent stress history is subjected to the rainflow cycle-counting method, which allows to determine the amplitudes and mean values of counted cycles, their occurrence moment and time of duration. Influence of the stress mean value was taken into consideration with the Morrow's model. On the assumption of the linear Palmgren–Miner hypothesis of damage accumulation and the stress–life fatigue characteristics of the material, the damage-time function was determined. The load segments, where the influence on the material fatigue was significant, were determined on the basis of the fixed damage-intensity level and the proposed damage-intensity function. The presented method was studied on a hook loaded with two independent forces. FEM program which has the possibility to perform fatigue analysis was used during the computation for determining the expected place of crack initiation. The service-loading course was compressed to shorter one, so that only a small decrease of the fatigue damage in comparison with full length of the original service loading was observed. The proposed method seems to be right for preparing multiaxial loading histories in order to cut down the fatigue tests, especially in the case when the correlation between particular loading channels is very important.