This documentation explains the core engineering concepts behind fatigue analysis,
including stress cycles, S–N curves, damage models, rainflow counting, and how FatigueLab’s calculators process your data. Whether you’re an engineer, student, or researcher, this guide helps you understand how fatigue is calculated, what inputs are required, and how lifetime predictions are generated.
1. Introduction to Fatigue Analysis
Fatigue is the progressive failure of a material caused by repeated or fluctuating stresses over time.
Even when the stress is below the yield strength, components can fail after many cycles.
Fatigue analysis helps estimate the total life of parts such as shafts, beams, gears, rotating machinery,
aircraft components, automotive suspensions, and structural members.
2. Why Fatigue Matters
More than 80% of real engineering failures occur due to fatigue. Without fatigue analysis,
a design may appear safe in static loading but still fail prematurely under cyclic loads.
Understanding fatigue allows engineers to estimate safe life, optimize material usage, reduce unexpected failures,
and design safer, more durable components.
3. Fundamentals of Stress Cycles
Fatigue depends on how stress varies over time. A stress cycle is characterized by the maximum stress,
minimum stress, mean stress, and stress amplitude.
• Stress Amplitude
σₐ = (σmax − σmin) / 2
• Mean Stress
σₘ = (σmax + σmin) / 2
• Common Cycle Types
Fully-reversed, zero-to-tension, tension-only, and combined compression–tension cycles.
Different cycle types produce different levels of fatigue damage.
4. S–N Curve (Stress–Life Curve)
The S–N curve describes the relationship between stress amplitude (S) and the number of cycles to failure (N).
At high stress, failure occurs quickly; at low stress, a component can survive millions of cycles.
Steels often show an endurance limit, while aluminum has no true endurance limit.
S–N curves follow Basquin’s law:
σₐ = A · (N)−b
Where A and b are material constants.
5. Rainflow Cycle Counting
Real stress data contains irregular peaks and valleys. Rainflow counting extracts individual stress cycles
from complex signals, allowing damage to be calculated accurately. FatigueLab automatically applies rainflow
counting to convert your CSV stress data into usable cycles.
6. Miner’s Rule – Damage Accumulation
After cycles are identified, damage is calculated using Miner’s Linear Damage Rule:
D = Σ (nᵢ / Nᵢ)
Failure is predicted when D ≥ 1.0.
Here nᵢ is the number of applied cycles, and Nᵢ is the number of cycles to failure for that stress level.
7. Inputs Required for Fatigue Analysis
• Stress CSV File
A simple one-column file containing stress values in MPa. No header. Even spacing recommended.
• Material S–N Parameters
Provide Basquin constants, fatigue strength coefficient, endurance limit (if applicable),
and other S–N curve data.
• Mean Stress Correction
Goodman, Gerber, or Soderberg corrections adjust fatigue life for non-zero mean stresses.
8. FatigueLab Calculators Overview
• Instant Fatigue Calculator
The Instant Fatigue Calculator is a quick tool that gives a unit-less fatigue score based on applied load versus allowable limit. It is useful for rapid screening and early design checks.
• Detailed Fatigue Damage Calculator
The Fatigue Damage Calculator performs full engineering fatigue analysis including rainflow counting, S–N curve fatigue life, damage accumulation, stress history charts, bar charts, and complete lifetime prediction.
9. Stages of Fatigue Failure
Fatigue failure progresses through crack initiation, crack propagation, and final fracture.
Surface defects, stress concentrations, and poor finish accelerate crack initiation.
Once a crack forms, it grows with each cycle until the remaining cross-section can no longer withstand the load.
10. Factors Affecting Fatigue Life
Fatigue life depends on material type, stress level, surface finish, geometry, temperature,
residual stresses, loading frequency, corrosion, and manufacturing quality.
Controlling these factors improves fatigue resistance.
11. Engineering Best Practices
• Remove noise from stress data before analysis.
• Avoid sharp corners and notches (major stress risers).
• Validate S–N curves from reliable data sources.
• Use proper mean stress correction.
• Apply conservative limits for safety-critical components.
• Ensure consistent units (MPa for stress).
12. Frequently Asked Questions
Q: Can this tool analyze any material?
A: Yes, as long as S–N curve parameters are provided.
Q: Is fatigue life exact or estimated?
A: Fatigue life is always probabilistic. Miner’s rule gives an accepted engineering estimate.
Q: Does it detect overloads?
A: Yes, stress spikes appear clearly in cycle distribution and outputs.
Q: Do I need equal time intervals in the CSV?
A: Yes, evenly spaced data gives accurate cycle extraction.