Calculate cycles to crack initiation from stress ranges and S–N data easily. Use Miner’s rule for spectra, then download clean CSV and PDF summaries.
S–N (power law): N = Nref × (Δσref / Δσeff)^m
Miner’s rule (spectrum): D = Σ(ni / Ni), and failure near D ≈ 1.
Equivalent life for the entered spectrum is Neq = (Σni) / D.
| Case | Mode | Δσref | Nref | m | Input Δσ | Output (approx.) |
|---|---|---|---|---|---|---|
| A | Constant | 100 MPa | 2,000,000 | 3 | 80 MPa | ~3,906,250 cycles |
| B | Constant | 120 MPa | 2,000,000 | 3 | 150 MPa | ~1,024,000 cycles |
| C | Miner | 100 MPa | 2,000,000 | 3 | Blocks: 120/50k, 90/200k | Damage-based Neq reported |
Fatigue in construction members usually initiates at welded toes, bolt holes, cutouts, and attachment points where stress concentrates. Repeated stress ranges, not peak static stress, drive crack growth. This calculator focuses on stress range Δσ and converts it to cycles-to-failure using S–N data suited to your detail category.
Most structural fatigue curves are anchored at a reference life such as Nref = 2,000,000 cycles. You enter Δσref at that point and the slope m. For many welded steel details, m near 3 is commonly used for preliminary checks. Increasing Δσref or decreasing m raises predicted life significantly.
The accuracy of fatigue life depends on how Δσ is obtained. Nominal stress from simple beam theory may be conservative or unconservative depending on attachments. Hot-spot or refined finite-element stresses better represent toe regions. Always keep units consistent (MPa or ksi) and document the stress extraction method in your report.
When a nonzero mean stress σm exists, the alternating component can be effectively larger than it appears. The optional Goodman correction uses σu to adjust the stress range, reducing life as σm approaches σu. For many welded details, mean-stress sensitivity may be limited, but base material checks can benefit from this option.
Real structures rarely see one constant stress range. Traffic, wind, waves, and crane operations produce spectra. Miner’s rule sums damage fractions D = Σ(ni/Ni). If D approaches 1, failure is expected near the accumulated history. This calculator reports block-by-block Ni and n/N to highlight the dominant contributors.
Design decisions are often time-based, not cycle-based. If you provide a cycle rate (cycles per day or per year), the tool estimates service life in years from the computed cycles-to-failure. For example, 4,000,000 cycles at 200,000 cycles/year corresponds to about 20 years, supporting inspection planning and retrofit timing.
When predicted life is low, reduce Δσ by stiffening, lowering live-load effects, smoothing geometry, or relocating attachments away from high-moment zones. Improve detail category through better weld profiles, toe grinding, peening, or higher-quality fabrication. Avoid abrupt thickness transitions and maintain corrosion protection because pitting accelerates crack initiation.
Fatigue checks are easier to defend when inputs are transparent. Use the built-in CSV and PDF exports to capture Δσ values, chosen S–N parameters, any fatigue limit, and Miner totals. Record the member ID, load source, and analysis basis so future reviewers can replicate results and update them after inspections.
Stress range Δσ is the peak-to-peak variation in a cycle. Stress amplitude is half of that value. This tool uses Δσ directly and internally converts to amplitude only for optional Goodman correction.
Use parameters from your governing standard, test data, or project specifications for the specific detail type. If unsure, start with conservative values and refine after confirming the detail category and stress definition.
Enable it when your method or standard defines a constant-amplitude fatigue threshold ΔσD below which damage is treated as negligible or capped. If your standard does not use a limit, leave it off.
Miner’s rule is a practical approximation. It ignores load sequence effects and some interaction behaviors. It is widely used for screening and design, but critical structures may require more detailed fracture mechanics assessments.
Tensile mean stress effectively raises crack-driving force for a given range. Goodman correction increases the equivalent alternating stress as σm grows relative to σu, resulting in fewer cycles to failure.
Very small D means your entered spectrum is mild relative to the S–N curve. It may indicate long life, or it may signal underestimated stress ranges or missing blocks. Review stress extraction and loading history.
The workflow is generic, but the S–N parameters must match the material and detail. For reinforced or prestressed concrete, use fatigue curves from the relevant concrete code and compatible stress definitions.
Design smarter by quantifying fatigue risk before failures occur.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.