An S-N curve, also known as a Wöhler curve, is a graphical representation that shows the relationship between the number of cycles to failure (N) and the applied stress (S) on a material. It is commonly used in materials science and engineering to predict how materials will behave under repeated loading conditions, such as fatigue. The curve typically slopes downward, indicating that higher stress levels lead to fewer cycles before failure. The S-N curve is essential for understanding the fatigue life of materials, helping engineers design components that can withstand cyclic loads. By analyzing these curves, engineers can determine safe stress limits and improve the reliability of structures and products, such as bridges and automobiles.