Goodman fatigue screening
A preliminary way to combine mean and alternating stress before a detailed fatigue assessment.
ENGINEERING PRINCIPLE
The modified Goodman relation represents a linear interaction between alternating stress capacity and mean tensile stress.
1/n = σa/Se + σm/SutVariables and units
- n = preliminary safety factor
- σa = alternating stress (MPa)
- σm = mean stress (MPa)
- Se = corrected endurance limit (MPa)
- Sut = tensile strength (MPa)
Worked example
With Sut = 550 MPa, assumed Se = 275 MPa, σa = 100 MPa and σm = 80 MPa, n = 1/(100/275 + 80/550) = 1.96.
Calculation method
- Separate the stress history into mean and alternating parts.
- Use an endurance limit appropriate to the component condition.
- Evaluate the selected fatigue criterion and compare it with the required safety policy.
Practical applications
- Rotating shafts
- Cyclic brackets
- Repeated-load machine parts
Review boundary and reference
- Surface, size, reliability, temperature, notch and loading factors cannot be omitted in a design release.
- Inspect local stress concentration and service spectrum rather than nominal stress alone.
Formula and displayed units independently checked for the stated conditions. This is not a code-compliance design method.
Source: J. E. Shigley et al., Mechanical Engineering Design, modified Goodman fatigue relation.