Fatigue failure is a phenomenon in which materials fracture due to repeated cyclic loading—even when the applied stress is lower than the material’s tensile strength or yield strength. Over time, microscopic cracks initiate and propagate until sudden fracture occurs.
Because fatigue failure often occurs without visible deformation before failure, it is difficult to detect through visual inspection. Therefore, properly evaluating fatigue risk during the design stage is essential for ensuring product reliability.
This article provides a comprehensive explanation of.
• The microscopic mechanism of fatigue failure
• How to interpret S-N curves for life prediction
• Practical prevention strategies for design and manufacturing
【What You’ll Learn】
- The three stages of fatigue failure: initiation, propagation, and final fracture
- Differences in fatigue limits between steel and aluminum
- Key factors affecting fatigue life such as stress concentration and mean stress
- Practical countermeasures including shot peening and geometric optimization
What Is Fatigue Failure?
Definition and Comparison with Other Failure Modes
Understanding fatigue failure begins by recognizing that it differs fundamentally from failures caused by a single large load.
Why Failure Occurs Below Tensile Strength
In mechanical design, strength is typically evaluated based on:
•Yield strength
•Tensile strength
However, fatigue failure can occur even within the elastic range, below these limits, due to repeated loading cycles.
Additionally:
•There is little to no visible plastic deformation before fracture
•Damage progression is difficult to detect externally
Comparison of Failure Modes
| Item | Fatigue Failure | Ductile Failure | Brittle Failure |
| Cause | Repeated loading | Excessive load | Impact or low temperature |
| Deformation before fracture | Minimal | Significant plastic deformation | Minimal |
| Fracture surface | Striations, beach marks | Dimples | Cleavage surface |
| Stress level | Below yield strength | Near tensile strength | Near tensile strength |
Mechanism of Fatigue Failure
Three Stages of Crack Development
Fatigue failure occurs progressively through three stages
Stage 1: Crack Initiation
•Begins at stress concentration points:
Thread roots
Holes
Surface defects
Inclusions
•Repeated shear stress causes microscopic slip at the atomic level
•Surface irregularities form (extrusions and intrusions)
•These evolve into microcracks
Stage 2: Crack Propagation
•Cracks grow gradually under cyclic loading
•Characteristic fracture patterns appear
Key Features:
- Striations：Fine lines formed per stress cycle (visible under a microscope)
- Beach Marks：Macroscopic concentric patterns indicating crack growth
These are critical clues in failure analysis.
Stage 3: Final Fracture
•Effective cross-sectional area decreases
•Remaining material can no longer support the load
•Sudden fracture occurs
Final fracture resembles static failure:
•Ductile materials show dimples
•Rapid crack propagation dominates
Predicting Fatigue Life
Understanding the S-N Curve
Fatigue life is determined by the relationship between:
•Stress amplitude (S)
•Number of cycles to failure (N)
This relationship is represented by the S-N curve.
How to Read the S-N Curve
•Vertical axis: Stress amplitude
•Horizontal axis: Number of cycles (log scale)
Trend:
• Higher stress → shorter life
• Lower stress → longer life
Fatigue Limit vs Finite Life Design
| Material Type | Fatigue Limit | Design Approach |
| Steel / Titanium | Exists | Infinite life below limit |
| Aluminum / Copper | Does not exist | Design based on finite cycles |
Key Point
• Steel materials: Exhibit a fatigue limit where the curve becomes horizontal
• Non-ferrous metals: Continue degrading with increasing cycles
Engineers must choose:
• Infinite-life design (steel)
• Finite-life design (aluminum)
5 Factors Affecting Fatigue Life
1. Stress Concentrati
• Occurs at geometric discontinuities
• Evaluated using stress concentration factor (Kt)
2. Mean Stress
Fatigue behavior depends on both:
• Stress amplitude (σa)
• Mean stress (σm)
Effects:
• Tensile mean stress → reduces fatigue life
• Compressive mean stress → improves fatigue life
Used in techniques like shot peening.
3. Surface Condition
• Rough surfaces reduce fatigue strength
• Smooth or polished surfaces improve performance
4. Size Effect
• Larger components have lower fatigue strength
• More likely to contain internal defects
5.Environmental Factors
• Corrosion accelerates crack growth
• High temperature and vibration also impact life
How to Prevent Fatigue Failure
Effective prevention requires both:
• Design optimization
• Manufacturing improvements
Design Strategies
Design Strategies
• Add fillets (R) to reduce stress concentration
• Avoid abrupt geometry changes
• Optimize load distribution
• Use damage-tolerant design when necessary
【Manufacturing and Surface Treatments】
| Method | Description | Effect |
| Shot Peening | Bombarding surface with steel shots | Introduces compressive stress |
| Surface Hardening | Carburizing, nitriding。 | Improves crack resistance |
| Surface Finishing | Polishing | Reduces stress concentration |
| CAE Analysis | Simulation (FEM) | Identifies weak points |
Important Insight
Shot peening is particularly effective because:
• It counteracts tensile stress
• Suppresses crack initiation and growth
Conclusion
Fatigue failure is a complex phenomenon involving:
• Material properties
• Geometry
• Manufacturing processes
To ensure product reliability:
• Do not rely only on tensile strength
• Consider fatigue behavior comprehensively
Key Takeaways
• Fatigue failure occurs under repeated stress below strength limits
• Crack initiation and propagation are gradual but lead to sudden failure
• S-N curves are essential for life prediction
• Multiple factors must be considered in design
• Preventive strategies must combine design and processing
Nichidai’s Engineering Support
Nichidai offers:
• Advanced precision forging technologies
• High-quality tooling solutions
• Design support to reduce fatigue failure risks
From design to manufacturing, integrated solutions are provided to improve reliability.
FAQ
疲労破壊の設計や不具合調査において、現場の技術者からよく寄せられる質問をまとめました。実務での判断や対策の検討に役立ててください。
How can fatigue failure be detected early?
Use non-destructive testing (NDT):
- Magnetic particle testing (MT)
- Dye penetrant testing (PT)
- Ultrasonic testing (UT)
Difference between high-cycle and low-cycle fatigue?
| Item | HCF | LCF |
| Cycles | 10⁴～10⁵ | Up to thousands |
| Stress | Elastic range | Plastic range |
Does higher hardness improve fatigue strength?
Generally yes, but:
• Too much hardness increases sensitivity to defects
• Balance between strength and toughness is important.
Why are bolts prone to fatigue failure?
• High stress concentration at thread roots
• Improper tightening causes additional stress
