This lecture covers the concept of stress in fatigue of materials, including the fatigue resistance defined by response to repeated stress variations. It discusses the endurance limit, deformation during fatigue, and the propagation mechanisms of fatigue cracks. The instructor explains how to adapt the Wöhler curve for different stress scenarios and the implications of cracks on material fatigue. Various empirical laws and factors affecting fatigue strength are also explored, along with real-world applications of fatigue in materials like ceramics, polymers, and metals.