Fatigue Failure & Crack Initiation

What Is Fatigue Failure?

Fatigue failure is the progressive structural damage caused by repeated cyclic loading. A single load application may produce no visible damage, but after thousands or millions of repetitions, a crack initiates, grows and ultimately causes fracture. Fatigue is the dominant failure mechanism in many engineering structures — aircraft, vehicles, machinery and equipment subjected to vibration or repeated loading. Unlike overload, fatigue operates at stress levels that may be well below the static strength of the material.

FATIGUE IS DRIVEN BY CYCLIC DAMAGE — NOT SIMPLY BY THE HIGHEST LOAD THE COMPONENT HAS EVER SEEN. A structure that survives its maximum load once may fail after that load, or smaller loads, are repeated many times.

The Three Stages of Fatigue

Fatigue develops in three broad stages. Understanding these stages helps the investigator interpret fracture surface evidence and distinguish fatigue from other mechanisms.

1. Crack initiation — microscopic damage accumulates at a highly stressed location, typically a surface or stress concentration. Persistent cyclic slip creates intrusions and extrusions at the grain level.
2. Crack growth — the initiated crack propagates perpendicular to the principal tensile stress. In the early stage, growth may be crystallographic; later it becomes macroscopically planar.
3. Final fracture — the remaining cross-section can no longer sustain the applied load. The final overload area may be ductile or brittle depending on material, geometry and load magnitude.

Key Fatigue Parameters

Fatigue is governed by the stress cycle — the variation of stress between maximum and minimum values. Several parameters define the cycle, and each has physical significance. Fatigue damage depends on the full cycle, not just the maximum stress.

σ_max  =  maximum stress in the cycle
σ_min  =  minimum stress in the cycle

Δσ  =  σ_max − σ_min           [stress range]
σ_a  =  (σ_max − σ_min) / 2   [stress amplitude]
σ_m  =  (σ_max + σ_min) / 2   [mean stress]
R    =  σ_min / σ_max          [stress ratio]

S–N Concept

The S-N curve relates stress amplitude (or range) to the number of cycles to failure. It is generated by testing multiple specimens at different stress levels. At high stress, lives are short. At lower stress, lives increase. For some materials — notably certain steels — the curve may approach a horizontal asymptote called the endurance limit, below which failure does not occur. Many non-ferrous alloys, including most aluminium alloys, show no true endurance limit — the curve continues to decrease. S-N curves used for design must have a stated statistical basis — mean curves predict 50% failure and are not appropriate for design.

Where Fatigue Cracks Initiate

Fatigue cracks initiate at locations where local stress is elevated or where the surface condition is imperfect. Understanding where to look is as important as understanding the mechanism. The initiation site is the starting point for the investigation — it reveals the local condition that allowed the crack to form.

• Holes — fastener holes, access holes, drainage holes, oilways
• Threads — screw threads, bolt threads, tapped holes
• Fillets and radii — sharp internal corners, inadequate fillet radii
• Surface defects — machining marks, scratches, nicks, tool damage
• Welds — weld toes, weld ripples, undercut, slag inclusions
• Fretting contacts — clamped interfaces with micro-slip
• Machining marks — feed marks, chatter marks, grinding burns
• Corrosion pits — localised surface damage concentrating stress
• Inclusions — subsurface material defects that initiate at the surface

Factors Affecting Fatigue Life

Fatigue life is not a material constant — it depends on many factors beyond the nominal stress. A fatigue analysis that considers only the nominal stress and the material S-N curve is incomplete. Each factor can change the fatigue life by factors of two to ten.

Fracture Surface Evidence

A fatigue fracture surface often — but not always — displays characteristic features that reveal the crack history. The presence or absence of these features depends on the material, the load level, the environment and the crack size. The investigator must be cautious: not all fatigue fractures display classic beach marks or striations, and not all smooth fracture surfaces are fatigue.

• Initiation site — typically a small, smooth region at the origin, often at a stress concentration
• Beach marks — curved markings on the fracture surface indicating successive crack positions under variable loading
• Striations — microscopic ridges visible under SEM, each representing one load cycle (in some materials)
• Final fracture area — the remaining cross-section that failed by overload, often rougher and differently coloured
• Directionality — the crack propagation direction can be traced from the origin outward through the beach marks

FORENSIC CHECK: Not all fatigue fractures display beach marks or striations. Absence of these features does not rule out fatigue, and their presence should be confirmed by laboratory examination — not visual inspection alone.

High-Cycle vs Low-Cycle Fatigue

Fatigue is broadly divided into two regimes. In high-cycle fatigue (HCF), stresses are nominally elastic and lives are long — the S-N method is typically used. In low-cycle fatigue (LCF), local plastic strain occurs on each cycle and lives are short — the strain-life method is more appropriate. The distinction matters because the analysis method and the evidence may differ between the two regimes.

Distinguishing Fatigue from Overload

Fatigue and overload produce different evidence on the fracture surface and in the surrounding structure. In clear cases, the distinction is straightforward. In mixed-mode or marginal cases, laboratory examination is required. The investigator should not diagnose fatigue solely from the appearance of a smooth fracture surface — other mechanisms can produce similar appearances.

COMMON MISTAKE: Diagnosing fatigue solely from a smooth fracture surface. Laboratory examination — optical microscopy, SEM — is needed to confirm fatigue features. Some brittle fractures and stress corrosion cracks also produce smooth surfaces.

Cross-Link to Fatigue & Damage Tolerance

This article introduces fatigue failure from a failure-investigation perspective. For detailed treatment of fatigue analysis methods, S-N curves, mean stress effects, cumulative damage, variable-amplitude loading, crack growth and damage tolerance assessment, see the Fatigue & Damage Tolerance category.

• S–N Fatigue Analysis — stress-life method for high-cycle fatigue
• Mean Stress Effects — how mean stress modifies fatigue capability
• Fatigue Crack Growth — Paris law and crack growth prediction
• Damage Tolerance Philosophy — designing for inspectable damage
• Residual Strength — remaining strength with a crack present
• Inspection Intervals & Detectable Flaw Size — linking crack growth to inspection

Key Takeaways

• Fatigue is progressive damage from cyclic loading, often at stresses well below static strength
• Cracks initiate at stress concentrations and surface defects, then grow until the remaining section fractures
• The full stress cycle — range, mean and ratio — governs fatigue, not just the maximum stress
• Surface finish, residual stress, size, environment and processing all affect fatigue life significantly
• Beach marks and striations are characteristic but not universal — laboratory examination is needed for confirmation
• Fatigue and overload produce different evidence — do not diagnose from appearance alone in marginal cases