fatigue-analysis

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Specialized skill for fatigue life assessment and durability prediction under cyclic loading conditions

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Fatigue Life Prediction Skill

Purpose

The Fatigue Life Prediction skill provides specialized capabilities for assessing fatigue life and durability under cyclic loading conditions, enabling systematic evaluation of component life using stress-life, strain-life, and fracture mechanics approaches.

Capabilities

Stress-life (S-N) curve application and analysis
Strain-life (epsilon-N) methodology implementation
Fracture mechanics crack growth prediction (NASGRO, AFGROW)
Load spectrum development and cycle counting (rainflow)
Damage accumulation using Miner's rule
Mean stress correction methods (Goodman, Gerber, Soderberg)
Multiaxial fatigue assessment
Fatigue report generation with life predictions

Usage Guidelines

Fatigue Analysis Approaches

Stress-Life Method (S-N)

Application
- High-cycle fatigue (N > 10^4 cycles)
- Elastic stress conditions
- Rotating machinery, vibration loading

S-N Curve Development

S = A * N^b

Where:
S = stress amplitude
N = cycles to failure
A, b = material constants

Endurance Limit Modifiers

Se = Se' * ka * kb * kc * kd * ke * kf

Where:
ka = surface factor
kb = size factor
kc = load factor
kd = temperature factor
ke = reliability factor
kf = miscellaneous effects

Strain-Life Method (epsilon-N)

Application
- Low-cycle fatigue (N < 10^4 cycles)
- Plastic strain present
- Notched components

Coffin-Manson Equation

epsilon_a = (sigma_f'/E) * (2Nf)^b + epsilon_f' * (2Nf)^c

Where:
epsilon_a = strain amplitude
sigma_f' = fatigue strength coefficient
b = fatigue strength exponent
epsilon_f' = fatigue ductility coefficient
c = fatigue ductility exponent

Neuber's Rule for Notches
```
(Kt * S)^2 / E = sigma * epsilon
```

Fracture Mechanics

Application
- Damage tolerance analysis
- Crack growth life prediction
- Inspection interval determination

Paris Law

da/dN = C * (delta_K)^m

Where:
da/dN = crack growth rate
delta_K = stress intensity factor range
C, m = material constants

Stress Intensity Factor

K = beta * S * sqrt(pi * a)

Where:
beta = geometry factor
S = remote stress
a = crack length

Load Spectrum Development

Rainflow Cycle Counting
- Extract cycles from complex load history
- Identify cycle ranges and means
- Generate cycle count matrix
Damage Summation
```
D = sum(ni/Ni)

Failure when D >= 1.0
```
Load Sequence Effects
- Consider overload retardation
- Evaluate block loading effects
- Apply appropriate interaction models

Mean Stress Corrections

Method	Equation	Application
Goodman	Sa/Se + Sm/Su = 1	Conservative, most common
Gerber	Sa/Se + (Sm/Su)^2 = 1	Less conservative
Soderberg	Sa/Se + Sm/Sy = 1	Very conservative
Morrow	Sa/Se + Sm/sigma_f' = 1	Strain-life approach

Process Integration

ME-008: Fatigue Life Prediction

Input Schema

{
  "component": "string",
  "material": {
    "name": "string",
    "Su": "number (Pa)",
    "Sy": "number (Pa)",
    "Se_prime": "number (Pa)",
    "sigma_f_prime": "number (Pa)",
    "epsilon_f_prime": "number",
    "b": "number",
    "c": "number"
  },
  "loading": {
    "type": "constant_amplitude|spectrum",
    "stress_amplitude": "number (Pa)",
    "mean_stress": "number (Pa)",
    "spectrum_file": "string (if spectrum)"
  },
  "geometry": {
    "Kt": "number (stress concentration)",
    "surface_finish": "string",
    "size": "number (mm)"
  },
  "target_life": "number (cycles)",
  "reliability": "number (0-1)"
}

Output Schema

{
  "fatigue_life": {
    "predicted_cycles": "number",
    "safety_factor": "number",
    "critical_location": "string"
  },
  "damage_summary": {
    "total_damage": "number",
    "damage_by_range": "array"
  },
  "analysis_details": {
    "method_used": "string",
    "mean_stress_correction": "string",
    "modifying_factors": "object"
  },
  "recommendations": {
    "design_changes": "array",
    "inspection_interval": "number (if applicable)"
  }
}

Best Practices

Use appropriate method for expected life regime (HCF vs LCF)
Include all relevant modifying factors
Account for mean stress effects
Validate material properties from tested data
Consider multiaxial stress states for complex loading
Apply appropriate safety factors per industry standards

Integration Points

Connects with FEA Structural for stress inputs
Feeds into Test Planning for validation requirements
Supports Material Selection for fatigue-resistant materials
Integrates with Design Review for life certification