The Basics of Fatigue 

Fatigue involves the initiation and propagation of cracks due to cyclic loading. When a structure is repeatedly loaded, cracks eventually form and grow, potentially leading to catastrophic failure. In practice, fatigue calculations primarily focus on when cracks begin rather than how quickly they propagate to complete failure, as the period from crack initiation to failure (in high-cycle fatigue) is typically short.

Key Factors in Fatigue Calculations

• Stress and Strain: Fatigue calculations generally use a stress-life (high-cycle) or strain-life (low-cycle) approach. These methods differ slightly but fundamentally relate stress or strain amplitude to the number of cycles to failure.

• Mean Stress: The average load level significantly impacts fatigue life. Correcting for mean stress, such as through Goodman corrections, can be crucial.

• Load Variations: Real-world loads vary dramatically and are not simply repetitive cycles. Methods like the Palmgren-Miner rule allow combining damages from different load cycles into a cumulative damage metric.

Practical Examples of Fatigue

Real-life scenarios vividly illustrate the complexity of fatigue:

• Vehicle Loads: Driving conditions such as smooth roads, potholes, gravel paths, or cobblestones drastically affect fatigue life. The type and frequency of these conditions create varied stress histories that directly impact component durability.

• Engine Components (e.g., Connecting Rods): Engine parts experience varying stress based on engine speed and load. Higher RPM significantly increases stress levels, highlighting the importance of accurately defining operating conditions.

Defining Events and Mission Profiles

To manage complexity, engineers define "events," such as driving on specific road types, and aggregate these into a "mission profile" representing the intended lifecycle usage scenario (e.g., kilometres driven on different road conditions). This approach simplifies calculating cumulative fatigue damage.

Essential Inputs for Fatigue Life Determination

Accurate fatigue analysis requires three critical inputs:

1. Stress or Strain History: Precise knowledge of how stress varies over time under operational conditions.

1. Material Properties: Fatigue curves (S-N curves) indicate how materials perform under cyclic loads. If unavailable, properties can sometimes be estimated.

1. Fatigue Calculation Settings: Includes mean stress correction methods and stress measures, such as signed Von Mises or critical-plane approaches.

Measurement vs. Simulation

Determining stress history involves either measurement (e.g., strain gauges, load cells, accelerometers) or simulation (e.g., finite element analysis - FEA). Measurements require existing designs, whereas simulations can predict stresses across complex models before physical testing.

Influence on Fatigue Life

Fatigue life is influenced by:

• Load Characteristics: Magnitude, direction, frequency, and variability.

• Structural Response: Stiffness, load paths, dimensions, and natural frequencies.

• Damping and Dynamic Response: Particularly critical near resonance frequencies.

Practical Fatigue Analysis Approaches

• Measurements: Strain gauges provide direct strain/stress data at specific locations, whereas load cells and accelerometers help determine input loads.

• FEA Simulations: Static analyses combined with measured or calculated load histories, modal transient methods, and frequency response analyses using Power Spectral Density (PSD) inputs.

Why the Load History is Essential

Of all the factors required for fatigue calculations, accurately determining the load history is paramount. Without it, fatigue analysis cannot even begin. The most common challenge engineers face is identifying reliable load histories—whether from direct measurements, simulations, industry standards, or similar existing designs.

Conclusion 

Fatigue analysis starts with clearly understanding and accurately defining the load history, supported by precise material properties and appropriate fatigue settings. Only then can engineers confidently predict fatigue life and ensure safe, reliable structural performance.