Safety factors and allowable stress

Explore the relationship

These are learning examples, not certified strengths or recommended factors.

n = R / σ　　σ_allow = R / n_req　　MS = n / n_req − 1

Target and calculated factors are different

Calculated n is the chosen reference strength divided by actual stress. The target n_req is specified from the application, code and uncertainty. n ≥ n_req meets that particular comparison.

How allowable stress is derived

For a simple static yield comparison, σ_allow = Sy / n_req. Match Sy to grade, thickness, product form, treatment and temperature. Enter an existing code allowable directly rather than applying the same factor twice.

Choose the relevant failure criterion

Yield, ultimate failure, fatigue, buckling, welds and contact fatigue require different criteria. Sy / √3 applies to static pure shear under the ductile von Mises yield criterion. Base-metal yield is not a long-term plastic or weld allowable.

How should the target be selected?

There is no universal factor that proves safety. Consider applicable requirements, load variability, impact and cycling, manufacturing variation, corrosion, temperature and failure consequences. The value 2 here is an example, not a recommendation.

What to check after reading the result

Meeting a target is conditional on the selected inputs and criterion. Check deflection, fatigue, local buckling, joints, supports and actual loads separately. Euler Pcr / P is theoretical and requires an applicable elastic slender-column model.