CORE PRINCIPLES
What to establish first
- Stress range and mean stress both influence fatigue risk.
- Notches and surface condition can reduce endurance strength.
- A safety factor should use a material condition and duty cycle that are documented.
Fatigue & Failure
Fatigue criteria, stress concentration and failure theories.
MODULE OVERVIEW
Fatigue & Failure in practical engineering work.
Fatigue is damage caused by repeated stress, often at levels below static yield. Mean stress, alternating stress, surface condition, size, notches and environment must be established before using a fatigue criterion.
DESIGN CONCEPTS
Terms worth checking
- S–N data
- Mean-stress correction
- Stress concentration
Common applications: Rotating shafts · Welded structures · Springs and cyclic fixtures
KEY RELATIONSHIP
Modified Goodman relation
1/n = σa/Se + σm/Sut
- σa — alternating stress
- σm — mean stress
- Se — corrected endurance limit
- Sut — ultimate strength
It is a linear screening criterion; actual endurance data and modification factors are required for design.
WORKED EXAMPLE
Known values → substitution → interpretation
- Known values
- Sut = 550 MPa; assumed Se = 275 MPa; σa = 100 MPa; σm = 80 MPa
- Method
- Substitute in the Goodman relation
- Interpretation
- The preliminary safety factor is about 2.02 before surface, notch and reliability corrections.
KEY DESIGN CHECKS
Confirm the conditions behind the number
- Define the governing load case, duty cycle and required design life before using a simplified relation.
- Use compatible units and material data for the actual condition, temperature and manufacturing state.
- Compare the result with strength, stiffness, fatigue, safety, serviceability and applicable-code requirements.
COMMON ENGINEERING MISTAKES
Keep the model within its scope
- Applying modified goodman relation outside the assumptions shown on this page.
- Using a nominal condition while a peak, alternating, transient or environmental case governs the design.
- Treating a calculated value as a final component selection without checking interfaces, tolerances and the current governing standard.
AVAILABLE CALCULATORS
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RELATED RESOURCE
Open Fatigue Safety Factor Calculator
Visualize mean and alternating stress in a preliminary Goodman check.
Open related resource →REFERENCES & LIMITATIONS
Use this content as a transparent starting point
ReferenceJ. E. Shigley et al., Mechanical Engineering Design, fatigue chapters; use test data and governing codes for critical equipment.ScopeRelationships are presented for the assumptions stated on this page and linked tool.Design reviewCheck material condition, loads, environment and the applicable current code.SafetyResults are educational/preliminary and do not certify safety or compliance.
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