Introduction to Fatigue Analysis with RISA

Fatigue failure remains one of the most common—and most dangerous—causes of structural collapse in engineering systems subjected to repeated loading. Bridges, cranes, offshore platforms, wind turbine towers, and industrial machinery all experience millions of cyclic stress events over their service lives. Conducting a thorough fatigue analysis is not just a regulatory requirement under codes such as AISC 360, Eurocode 3, or AWS D1.1; it is a fundamental practice for ensuring long-term safety and minimizing lifecycle costs.

RISA (Rapid Interactive Structural Analysis) software provides a comprehensive suite of fatigue analysis tools that integrate directly into the structural design workflow. Unlike standalone fatigue solvers, RISA allows engineers to build, analyze, and optimize models in a unified environment, reducing data transfer errors and accelerating design iterations. This guide covers the full workflow—from theoretical foundations to practical step-by-step procedures—enabling you to perform robust fatigue assessments using RISA.

Understanding Fatigue Analysis

Fatigue is the progressive, localized structural damage that occurs when a material is subjected to cyclic loading. Even if the applied stress is well below the material's yield strength, repeated cycles can initiate cracks and propagate them until catastrophic failure occurs. The total number of cycles a component can withstand before failure defines its fatigue life.

Three primary approaches exist for fatigue assessment:

  • Stress-Life (S-N) Method – Based on empirical S-N curves (stress amplitude versus cycles to failure). Suitable for high-cycle fatigue (typically >10e4 cycles) where stresses remain mostly elastic.
  • Strain-Life (ε-N) Method – Uses local strain amplitude versus cycles. Better for low-cycle fatigue (under 10e4 cycles) where plastic deformation occurs.
  • Fracture Mechanics Method – Assumes an initial crack size and calculates crack growth under cyclic loading. Used for damage tolerance assessments in critical components.

RISA primarily supports the Stress-Life approach, which aligns with most structural steel design codes. The software handles constant‑amplitude and variable‑amplitude loading, applies appropriate safety factors, and outputs fatigue life estimates for every member and connection in the model.

Preparing Your Model in RISA for Fatigue Analysis

The accuracy of any fatigue analysis depends on the quality of the structural model. Spending time on proper preparation will yield reliable results and reduce rework.

1. Define All Structural Elements

Create a complete representation of your structure in RISA-3D or RISA-2D, including beams, columns, braces, trusses, and connections. Use the correct section shapes and sizes—fatigue failure often initiates at stress concentrations such as welds, bolt holes, or sharp geometric transitions. For welded connections, model the weld group explicitly using RISA’s connection design tools.

2. Assign Appropriate Fatigue Properties

Navigate to the Material Manager and input fatigue‑specific material data:

  • S-N Curve Data – Import experimental S-N curves for the material grade (e.g., A36, A572 Gr. 50, or stainless steel). RISA includes default curves for common structural steels, but you can customize them based on your test data or code provisions.
  • Endurance Limit – For ferrous alloys, set the endurance limit (stress level below which fatigue life is theoretically infinite).
  • Stress Concentration Factors (Kf) – Assign fatigue notch factors for details such as cover plates, stiffeners, or bolted splices. RISA can automatically apply Kf based on connection type if defined in the design preferences.
  • Allowable Stress Range – For code‑based checks (e.g., AISC 360 Appendix 3), enter the allowable stress range for each detail category.

3. Apply Boundary Conditions and Cyclic Loads

Realistic load application is critical. For fatigue analysis, define the load cases and combinations that represent the expected cyclic spectrum:

  • Constant Amplitude Loading – Use a single load case with known maximum and minimum values (e.g., a crane rail load that cycles between zero and the rated capacity).
  • Variable Amplitude Loading – Define a sequence of load cases representing different operational conditions. Use the “Load History” feature to apply a repeating block of cycles.
  • Rainflow Counting – If you have measured strain histories from field data, preprocess them into load ranges and cycle counts using an external rainflow algorithm, then input the resulting histogram into RISA.

Ensure that boundary conditions (supports, releases, springs) reflect the actual structural behavior. Over‑constrained models can produce unrealistic stress ranges that distort fatigue life predictions.

Performing Fatigue Analysis in RISA

Once the model is ready, execute the fatigue analysis through a structured workflow.

Step 1: Navigate to Fatigue Settings

From the main menu, select Analysis → Fatigue Analysis. The Fatigue Analysis dialog appears, prompting you to configure parameters.

Step 2: Input Cyclic Load Parameters

  • Load Combinations – Select the load combinations that represent the fatigue loading envelope. Typically, you will use the service‑level combinations (factored for fatigue, not strength).
  • Number of Cycles – Enter the design number of cycles (e.g., 2×10e6 for a bridge designed for 75 years of truck traffic).
  • Loading Type – Choose constant amplitude, variable amplitude, or a stress history file.
  • Stress Ratio (R) – Define the ratio of minimum to maximum stress (R = σmin/σmax). For fully reversed loading, R = -1; for zero‑to‑tension, R = 0.

Step 3: Select Fatigue Criteria

RISA offers multiple failure criteria based on industry standards:

  • S-N Curve Approach – Select a predefined curve or import one. The software computes the damage at each member using Miner’s linear damage rule.
  • Goodman Diagram – For mean stress correction, enable the Goodman (or Gerber, Soderberg) relationship. This adjusts the stress amplitude based on the mean stress to account for its effect on fatigue life.
  • Code‑Based Checks – If using AISC 360‑16, AWS D1.1, or Eurocode 3 part 1-9, select the applicable code and detail category. RISA will automatically apply the allowable stress range and partial safety factors.

Step 4: Run the Analysis

Click Run Fatigue Analysis. RISA will compute stress ranges for each element, apply stress concentration factors, and calculate the cumulative damage ratio. The progress bar shows the solving stage; for large models with hundreds of load cases, analysis time may be several minutes.

Step 5: Review the Summary Report

Immediately after completion, RISA displays a summary table listing:

  • Member ID and location
  • Maximum stress range (MPa or ksi)
  • Design number of cycles
  • Calculated fatigue life (cycles)
  • Damage ratio (D = n/N, where n = design cycles, N = allowable cycles)
  • Pass/Fail status based on threshold (D ≤ 1.0 = acceptable)

Interpreting the Results in Detail

Raw numbers alone do not tell the whole story. Proper interpretation separates a superficial review from a comprehensive fatigue assessment.

Identify Critical Members

Sort the results table by damage ratio descending. Members with D > 1.0 require immediate attention. However, even members with ratios between 0.7 and 1.0 should be flagged for design review—small changes in loading or fabrication quality can push them over the limit.

Look for patterns: high damage often concentrates at:

  • Welded connections with low detail categories (e.g., E′ or F in AISC).
  • Regions with abrupt geometric changes (e.g., cope holes, beam seat supports).
  • Members subjected to high stress reversal (tension–compression cycles).

Visualize Stress Contours

Use RISA’s post‑processing viewer to display stress range contours across the structure. This helps spot local hot spots that the summary table might not highlight. Adjust the color scale to emphasize damage ratios between 0.5 and 1.5 for quick visual screening.

Check Mean Stress Effects

If the Goodman correction is enabled, review the mean stress for each member. High tensile mean stress can reduce fatigue life significantly. Members that show a mean stress above 30% of yield strength should be investigated for potential redesign, even if damage ratios are acceptable.

Best Practices and Advanced Tips

Following these guidelines will improve the reliability and defensibility of your fatigue analysis results.

Validate with Static Results

Before running the fatigue analysis, confirm that the static solution is accurate. Run a single-cycle load case and compare member forces and displacements with hand calculations or a simpler model. An error in the static baseline propagates directly into fatigue life estimates.

Use Conservative Load Parameters

Fatigue loading is inherently uncertain. Apply a reasonable overload factor (typically 1.5 to 2.0) on the number of design cycles to account for traffic growth, maintenance delays, or unforeseen dynamic events. Where field data is available, use the 95th percentile stress range rather than the mean.

Incorporate Field Measurements

For existing structures undergoing fatigue evaluation, RISA can accept strain gauge data. Weld a few strain gauges at critical locations, record a representative time history, and use a rainflow counting algorithm to produce a stress histogram. Import this histogram as a stress history file in RISA—this dramatically improves accuracy compared to assumed load distributions.

Regularly Update Materials and Codes

Check for RISA software updates that include new S-N curves or code provisions. Standards such as AWS D1.1 and Eurocode 3 are periodically revised with improved fatigue categories. Using outdated data can lead to unconservative designs.

Integrating Fatigue Analysis into the Design Workflow

Fatigue analysis should not be an afterthought. Integrate it early in the design process to avoid costly rework later.

  • During Conceptual Design – Run preliminary fatigue checks on candidate member sizes using conservative loads. This guides material selection and connection details before detailed modeling.
  • During Detail Design – Once the model is finalized, perform the full fatigue analysis. Adjust connection types (e.g., change from fillet‑welded to groove‑welded details) to improve fatigue performance without increasing member sizes.
  • During Fabrication Documentation – Export fatigue‑critical members and connections to the shop drawings. Specify weld profiles, grind marks, and inspection requirements accordingly.

Documenting and Reporting Results

A well‑documented fatigue analysis is essential for peer review, code compliance, and future inspections. RISA generates detailed reports that you can customize:

  • Input Summary – List all material properties, load cases, S-N curves, and code parameters used.
  • Detailed Results Table – For each member, show stress ranges, damage ratios, and reference to the applicable code clause.
  • Critical Member Identification – Highlight all members with D > 0.5 in a separate table for attention.
  • Visual Plots – Include screenshots of stress contour plots, damage ratio histograms, and load histograms.

Attach a narrative explaining the methodology, assumptions, and recommended design changes. This forms part of the structural calculations package required by most building authorities.

External Resources for Further Learning

These resources provide the theoretical background and code requirements that complement RISA’s capabilities:

Common Pitfalls and How to Avoid Them

Even experienced engineers can make mistakes in fatigue analysis. Watch out for these pitfalls:

  • Ignoring Stress Concentrations at Supports – Supports often introduce high stress gradients. Model the support region with a finer mesh or use stress concentration factors from Table A-1 of AISC Appendix 3.
  • Using Ultimate Load Combinations – Fatigue checks must use service‑level loads, not strength‑level factor combinations. Using factored loads overestimates stress ranges and predicts premature failure.
  • Neglecting Secondary Effects – P‑delta effects, thermal stresses, and vibration may contribute to cyclic stress ranges. Include these in the load cases if they are significant.
  • Assuming Infinite Life – If the endurance limit exists, some members may pass with D = 0. However, verify that the maximum stress range never exceeds the endurance limit under any service condition.

Conclusion

Fatigue analysis using RISA software transforms a complex, code‑intensive process into a manageable, repeatable workflow. By combining proper model preparation, accurate input data, and careful interpretation of results, you can design structures that safely withstand millions of cycles over their design life. The tools and best practices outlined in this guide provide a solid foundation for engineers seeking to integrate fatigue assessment into daily practice—ensuring both compliance with international standards and real‑world reliability.

Start with a well‑calibrated model, validate your assumptions, and use RISA’s visualization capabilities to communicate findings to stakeholders. As you accumulate experience, you will develop an intuitive sense for which details need extra attention and how to optimize designs for fatigue performance without over‑engineering. The result is a safer, more durable structure that stands the test of time.