Fatigue Life Estimation Calculator - S-N Curve Analysis Tool
Calculate fatigue life using Basquin's equation for S-N curve analysis and material durability assessment
Fatigue Life Calculator
Results
Note: Results based on Basquin's equation for S-N curve analysis.
What is Fatigue Life Estimation?
A Fatigue Life Estimation Calculator is a free engineering tool that predicts the number of stress cycles a material can withstand before failure using S-N curve analysis. Fatigue life (Nf) is determined using Basquin's equation, which relates stress amplitude to the number of cycles to failure.
This calculator is essential for:
- Component Design - Designing parts subjected to cyclic loading for adequate service life
- Failure Prevention - Predicting when fatigue failure may occur to schedule maintenance
- Material Selection - Comparing fatigue resistance of different materials
- Safety Analysis - Ensuring structures meet fatigue life requirements per design codes
For stress analysis, try our Beam Bending Stress Calculator.
For spring mechanics, use our Spring Constant & Deflection Calculator.
How Fatigue Life is Calculated
The calculation uses Basquin's equation for S-N curves:
To find fatigue life, we rearrange to:
Where:
- σa = Stress amplitude (MPa)
- σ'f = Fatigue strength coefficient (MPa)
- Nf = Number of cycles to failure
- b = Fatigue strength exponent (typically -0.085 to -0.15)
The stress range is calculated as:
For steel, the endurance limit is approximately 0.5 × σultimate.
Key Fatigue Analysis Concepts
S-N Curve
Stress-life (S-N) curve plots stress amplitude vs. cycles to failure on log-log scale. Fundamental tool for fatigue analysis.
Basquin's Equation
Power-law relationship describing high-cycle fatigue behavior in the elastic region of S-N curve.
Endurance Limit
Stress level below which infinite life is expected. Exists for steel (~0.5σult) but not for aluminum/titanium.
Stress Amplitude
Half of the stress range (σa = Δσ/2). Critical parameter determining fatigue life in cyclic loading.
How to Use This Calculator
- Select Material Preset (Optional): Choose Steel, Aluminum, or Titanium for typical values, or use Custom
- Enter Stress Amplitude (σa): Input the alternating stress amplitude in MPa
- Enter Fatigue Strength Coefficient (σ'f): Input the fatigue strength coefficient in MPa (auto-filled by preset)
- Enter Fatigue Strength Exponent (b): Input the exponent value (typically -0.085 to -0.15, auto-filled by preset)
- Enter Ultimate Tensile Strength (Optional): Input for endurance limit check (steel only)
- Calculate: Click "Calculate Fatigue Life" to compute cycles to failure and safety factors
- Review Results: Check fatigue life, fatigue type, stress range, and warnings
Example:
Steel with σa = 200 MPa, σ'f = 900 MPa, b = -0.085
Nf = [(200/900)(1/-0.085)] / 2 ≈ 7.89 × 105 cycles
Benefits of Using This Calculator
- Instant Results: Calculate fatigue life in seconds using Basquin's equation
- Material Presets: Quick selection for common materials (Steel, Aluminum, Titanium)
- Engineering Accuracy: Uses standard S-N curve methodology from fatigue mechanics
- Endurance Limit Check: Automatic verification against endurance limit for steel
- Fatigue Type Identification: Distinguishes between low-cycle and high-cycle fatigue
- Safety Factor Calculation: Computes safety factor based on endurance limit
- Educational Tool: Perfect for learning fatigue analysis and S-N curve concepts
- Professional Use: Suitable for mechanical engineers, designers, and students
Factors Affecting Fatigue Life
- Stress Amplitude: Higher stress reduces fatigue life exponentially per S-N curve
- Material Properties: Different materials have different fatigue strength coefficients and exponents
- Mean Stress: Tensile mean stress reduces fatigue life (not directly in Basquin's equation)
- Surface Finish: Rough surfaces initiate cracks earlier, reducing fatigue life
- Stress Concentration: Notches, holes, and geometric discontinuities reduce life significantly
- Temperature: Elevated temperatures generally reduce fatigue resistance
- Corrosion: Corrosive environments dramatically accelerate fatigue crack growth
- Loading Frequency: Higher frequencies may affect life in certain materials and environments
Frequently Asked Questions (FAQ)
What is fatigue life in materials?
Fatigue life is the number of stress cycles a material can withstand before failure occurs under cyclic loading. It's typically represented by the S-N curve (stress-life curve) which plots stress amplitude versus the number of cycles to failure.
What is Basquin's equation for fatigue life?
Basquin's equation describes the relationship between stress amplitude and fatigue life: σa = σ'f × (2Nf)b, where σa is stress amplitude, σ'f is the fatigue strength coefficient, Nf is cycles to failure, and b is the fatigue strength exponent (typically -0.085 to -0.15).
What is the difference between low-cycle and high-cycle fatigue?
Low-cycle fatigue (LCF) occurs at high stress levels with relatively few cycles to failure (N < 10,000 cycles), typically involving plastic deformation. High-cycle fatigue (HCF) occurs at lower stress levels with many cycles to failure (N > 10,000 cycles), remaining in the elastic region.
What is the endurance limit?
The endurance limit (fatigue limit) is the stress level below which a material can withstand an infinite number of loading cycles without failure. For steel, it's typically around 50% of the ultimate tensile strength. Note that aluminum and titanium do not have a true endurance limit.