Evaluating fatigue onset in metallic materials: Problem, current focus and future perspectives

The impact of mechanical fatigue on load-bearing metallic components and structures is highly significant, encompassing economy, environment and safety aspects. For nearly 200 years, engineers and scientists have been relentlessly trying to avoid fatigue failures and to understand their causes. The last few decades have seen the prominent advent of a wide range of experimental and computational techniques that allowed us to make once-unthinkable advances in this field. Despite this progress, a significant number of problems remain unsolved. This short note pinpoints the most critical aspect of fatigue failure: the conditions that initiate or allow propagating cracks to form. Specifically, the fundamentals of mechanical fatigue are examined, while acknowledging the crucial role of multiphysics aspects that often are present in real-life engineering applications. The first part frames the problem by introducing essential concepts and exploring mechanistic aspects across different length scales and loading regimes. Subsequently, a brief but wide-ranging review highlights current research trends. This foundation sets the stage for identifying outstanding challenges and exploring potential future research directions in the conclusive part of the article.