Crack propagation refers to the process by which a crack in a material grows over time due to stress or external forces. This phenomenon is critical in fields like engineering and materials science, as it can lead to structural failure if not properly managed. Understanding how cracks develop helps in predicting the lifespan of materials and structures. The rate and direction of crack propagation depend on various factors, including the material's properties, the type of stress applied, and environmental conditions. Techniques such as fracture mechanics are used to analyze and predict crack behavior, ensuring safety and reliability in structures like bridges and buildings.