Quick Answer
Use the Fatigue Life (S-N Curve) Estimator to estimate fatigue life cycles from an S-N curve power-law relationship. In plain terms, enter Fatigue Coefficient (MPa), Stress Amplitude (MPa), S-N Slope Exponent (dimensionless) and the calculator returns Estimated cycles to failure with supporting values where the formula produces them.
This page is built for mechanical engineers, designers, maintenance teams, students, and manufacturing reviewers. It is most useful for screening checks for bearings, springs, bolts, welds, fatigue, vibration, braking, cams, fits, rivets, and acoustics. The calculator keeps every input unit visible, shows the governing equation, and separates formula math from design approval so humans, search engines, and AI agents can understand exactly what is being computed.
Formula
The formula block above is the calculation used by the tool. The variable list below the equation defines the symbols in the same context as the calculator fields, so you can audit the math before relying on the result.
How to Use This Calculator
- Enter each known value using the unit printed beside the field. For this calculator, common starting inputs include Fatigue Coefficient (MPa), Stress Amplitude (MPa), S-N Slope Exponent (dimensionless).
- Confirm that coefficients, material properties, pressure basis, and geometry match the real system you are checking.
- Read the primary output first, then review any secondary values for intermediate checks or interpretation.
- Change one input at a time when comparing alternatives. This makes sensitivity checks easier and helps identify which assumption controls the result.
- Save or share the calculator URL after entering non-default values if you need a repeatable calculation record.
Inputs and Units
| Input | Unit | Default | Why it matters |
|---|---|---|---|
| Fatigue Coefficient | MPa | 900 | Represents a material property, coefficient, or empirical factor that should come from reliable data. |
| Stress Amplitude | MPa | 250 | Feeds the displayed formula directly, so the value should match the label and unit exactly. |
| S-N Slope Exponent | dimensionless | 0.12 | Represents a material property, coefficient, or empirical factor that should come from reliable data. |
Example Workflow
A practical workflow is to start with the default values, replace Fatigue Coefficient with your project value in MPa, then update the remaining inputs from drawings, field measurements, lab data, supplier tables, or project specifications. After the result updates, compare it with an independent hand check and with any project limits that apply to the same load case or operating condition.
For AI agents and spreadsheet workflows, use the exact input IDs from the public manifest or API payload contract rather than guessing from the visible labels. This prevents unit mix-ups and keeps the calculation reproducible.
Result Interpretation
The primary result is Estimated cycles to failure. In machine design, machine-design results should be compared against material limits, fatigue life, manufacturing tolerance, duty cycle, and supplier data. A result that looks unexpectedly high, low, or sensitive to a small input change is usually a signal to check units, assumptions, boundary conditions, and the valid range of the equation before moving on.
Use this output as a transparent engineering calculation, not as a hidden design decision. For safety-critical or regulated work, document the input source, the formula assumption, the applicable standard, and the review path.
Assumptions and Limits
- Loads, geometry, material properties, coefficients, and duty cycle match the simplified formula.
- The calculation does not replace finite element analysis, fatigue testing, manufacturer catalogs, tolerance stacks, or product safety review.
- Wear, lubrication, temperature, impact, corrosion, preload scatter, and dynamic loading may control the final design.
- The calculator does not add hidden safety factors, resistance factors, load combinations, code allowances, inspection requirements, or permit rules.
Common Mistakes
- Using static formulas for fatigue, impact, or variable-amplitude duty without adding the proper design method.
- Ignoring tolerance, preload, surface finish, lubrication, or temperature effects.
- Treating handbook screening equations as acceptance criteria for a production machine.
- Entering values with the right number but the wrong unit, such as using mm where m is expected or using a nominal dimension where an internal dimension is required.
References and Further Checks
These references are useful for context and validation, but the calculator itself remains a simplified formula tool:
- NIST Engineering Laboratory - measurement and engineering context for mechanical systems.
- ASME Codes and Standards - standards references for mechanical engineering practice.
For final engineering decisions, compare the result with governing codes, manufacturer data, site-specific measurements, and professional judgment.