Research On Fatigue Life Prediction Method Under Multiaxial Out-of-frequency Loading

At present, the research on multiaxial fatigue theory is focused on two aspects: proportional and non-proportional. Many researchers use different phases as parameters of non-proportional to control the experiment of multiaxial fatigue. In fact the load have many non-proportional forms, all of these state for example out-of-phase, out-of-frequency, different waves, and the existence of mean stress will lead to non-proportional loads. There is few research on the out-of-frequency multiaxial fatigue in the existing literatures. Actually, under out-of-frequency loading, the stress forces of the components are more complex, and many components of the engineering often subjected to the multiaxial out-of-frequency load, such as offshore oil drilling platform, aircraft landing gear and so on. Therefore, to research the multiaxial out-of-frequency fatigue is better understanding the fatigue damage behavior and failure process of the components which subjected to complex load even random load, and is also essential to solve the problems of engineering effectively.In this paper, the characteristic of strain and stress vary with time under multiaxial out-of-frequency loading and which difference of out-of-phase loading was researched. A method to determinate the critical plane was proposed. A new damage parameter was proposed and a new life prediction model was advanced.First, using the method of coordinate rotation to analysis the strain and stress behavior on the plane of the materials, then compare these with the out-of-phase loading. The results showed that the strain and stress is variable amplitude and cyclical on the plane of material under out-of-frequency loading.Second, to determinate the critical plane, the concept of weight-added function is defined in low and high cycle fatigue field respectively. This concept is based on the characteristic of the shear strain and normal stress change over time at any plane of the material.Third, proposed new damage parameters in low and high cycle fatigue field respectively by non-linear combination of shear stress and normal stress range, and normal stress and shear strain range. Advanced new fatigue life prediction model use the new parameters. Then compared the predicted results which computation by the new model with the experimental results from the relevant literature, the results showed that the model in this paper have a good prediction ability.