Metal fatigue is a process that occurs when a metal is subjected to repeated stress or strain over time. This can lead to the formation of small cracks within the material, which can grow larger with continued use. Eventually, these cracks can cause the metal to fail, even if the stress levels are below the material's ultimate strength. Factors such as temperature, environment, and the number of stress cycles can influence the rate of metal fatigue. Common examples of metal fatigue can be found in structures like bridges and aircraft, where materials are regularly subjected to varying loads and conditions, making understanding this phenomenon crucial for safety and durability.