Prediction of non-proportionality in 3D-6D strain history using the 3D Area Swept by First Principal Strain method and development of a new multiaxial fatigue life prediction model

This paper studies the area swept by the first principal strain in three-dimensional (3D) principal strain space and to establish a method, termed the 3D Area Swept by First Principal Strain (ASFPS) method, for predicting the non-proportionality of 3D-6D loading history. Unlike traditional methods that maintain a consistent strain level by controlling the same equivalent strain to investigate non-proportionality, the 3D ASFPS method achieves the uniform strain level by controlling the same maximum first principal strain to study non-proportionality. This method provides a more intuitive description of the variations in both magnitude and direction of the first principal strain in the 3D principal strain space, offering a clearer explanation of the effect of non-proportional additional hardening resulting from changes in the principal axes of strain. Subsequently, we successfully completed the development of the 3D ASFPS method to the 2D ASFPS method that can effectively predict the non-proportionality of the 2D strain history. And a new multiaxial low cycle fatigue life prediction model based on the maximum first principal strain was proposed and verified. The new model, compared to other models, not only demonstrates better performance in predicting materials but also has significant advantages of a concise form and a straightforward computation. Furthermore, our method and model can predict non-proportionality and fatigue life within the fatigue failure zone of engineering structures solely by analyzing variations in strain states, even when loading path is uncertain. This indicates significant potential for practical applications in engineering.