The S-N curve, also known as the Wöhler curve, is a graphical representation that illustrates the relationship between the number of cycles a material can endure before failure (N) and the applied stress level (S). It is commonly used in materials science and engineering to predict the fatigue life of materials under cyclic loading conditions. The curve typically has an S-shape, where the initial steep slope indicates a high number of cycles at low stress levels, while the flatter region shows that materials can withstand fewer cycles at higher stress levels. This information is crucial for designing components in various industries, including automotive and aerospace, to ensure safety and reliability.