What stages of damage can occur during material fatigue?

In the context of materials testing, the process of fatigue failure—the progressive fracture of a solid under cyclic loading—occurs in three distinct stages:1

• Crack Initiation: Repeated stress cycles cause microscopic damage to accumulate within the material, eventually forming a minuscule crack. During this initial phase, the damage is typically minimal and not visible to the naked eye.1
• Crack Growth: As cyclic loading continues, the crack gradually propagates. This phase involves the steady degradation of the material's structure as the crack increases in size.1
• Fracture: Once the crack reaches a critical size, the remaining unaffected portion of the material becomes too weak to support the applied load. This leads to a sudden and abrupt final rupture.1

Understanding these stages is vital in engineering to design structures capable of withstanding cyclic forces, such as those found in bridges, aircraft, or offshore oil platforms.12