Physics-based machine learning for fatigue lifetime prediction under non-uniform loading scenarios

Accurate lifetime prediction of structures and structural components subjected to cyclic loading is vital, especially in scenarios involving non-uniform loading histories where load sequencing critically influences structural durability. Addressing this complexity requires advanced modeling approaches capable of capturing the intricate relationship between loading sequences and fatigue lifetime. Traditional high-cycle fatigue simulations are computationally prohibitive, necessitating more efficient methods. This work highlights the potential of physics-based machine learning ($\varphi$ML) to predict the fatigue lifetime of materials under various loading conditions. Specifically, a feedforward neural network is designed to embed physical constraints from experimental evidence, including initial and boundary conditions, directly into its architecture to enhance prediction accuracy. It is trained using numerical simulations generated by a physically based anisotropic continuum damage fatigue model. The model is calibrated and validated against experimental fatigue data of concrete cylinder specimens tested in uniaxial compression. The simulations used for training quantify the effects of load sequences considering scenarios under two different loading ranges. The proposed approach demonstrates superior accuracy compared to purely data-driven neural networks, particularly in situations with limited training data, achieving realistic predictions of damage accumulation. To this end, a general algorithm is developed and successfully applied to predict fatigue lifetimes under complex loading scenarios with multiple loading ranges. In this approach, the $\varphi$ML model serves as a surrogate to capture damage evolution across load transitions. The $\varphi$ML based algorithm is subsequently employed to investigate the influence of multiple loading transitions on accumulated fatigue life, and its predictions align with trends observed in recent experimental studies. The presented contribution demonstrates physics-based machine learning as a promising technique for efficient and reliable fatigue life prediction in engineering structures, with possible integration into digital twin models for real-time assessment.