While Fracture Mechanics focuses on the behavior of materials already containing cracks, Fatigue Analysis is the engineering discipline of predicting when and how a material will fail under cyclic loading - repeated application and removal of stress - even when those stresses are well below the material's ultimate tensile strength. Most mechanical failures in the real world (estimated at 80-90%) are caused by fatigue. Because these failures often occur without visible warning, understanding the fatigue life of a component is critical for safety and reliability.
Fatigue failure is a progressive process that typically unfolds in three distinct phases:
- Crack Initiation: Microscopic cracks form at points of high stress concentration, such as surface scratches, sharp corners (notches), or internal material defects.
- Crack Propagation: With each subsequent load cycle, the crack grows incrementally. This stage is where Fracture Mechanics often overlaps with fatigue analysis to track the rate of growth (da/dN).
- Final Fracture: Once the crack reaches a critical size, the remaining material can no longer support the load, leading to a sudden and often catastrophic failure.
Depending on the application and the expected “life” of the part, engineers use different analytical approaches:
- Stress-Life Method: The oldest and most common approach, it relates the nominal stress amplitude to the number of cycles to failure. It's best for High-Cycle Fatigue (HCF), where the stresses remain within the elastic range.
- Strain-Life Method: This method accounts for local plastic deformation at the “notch root” or stress concentration point. It's best for Low-Cycle Fatigue (LCF), where high loads cause the material to deform plastically in each cycle. It combines elastic and plastic strain components to provide a more accurate prediction of the initiation phase.
- Fatigue Crack Growth: Instead of predicting when a crack starts, FCG uses Linear Elastic Fracture Mechanics (LEFM) to predict how long an existing crack will take to grow to a critical size. It's best for Damage-Tolerant design, where engineers assume cracks exist and schedule inspections to find them before failure.
Key factors influencing fatigue life are: mean stress, surface finish, size, and environment.