Safety factor
Nductile ≥ max(N1, N2, N3) ; based on yield strength
Nbrittle ≥ max(N1, N2, N3) ; based on ultimate strength
N = N1 . N2
N1 = f(A, B, C), from the first table
N2 = f(D, E) , from the second table
Parameters:
A Quality of materials, workmanship, maintenance and inspection
B Control over applied loads
C Accuracy of stress analysis, experimental data, or experience with similar parts
D Danger to people
E Economic impact
The transition from engineering model to reality is usually facilitated by including a factor of safety in the design to accommodate uncertainly in material properties and the design process, the consequences of failure, risk to people and degree of characterization of and control over the service environment.
Safety factor is a simple ratio that is intended to be greater than 1.
Safety factors for ductile materials are based on yield strength. Safety factors for brittle materials are based on ultimate strength and are twice the recommended values for ductile materials. Safety factors for primarily cyclic loading are based on endurance limit. Impact loads require a safety factor of at least 2 multiplied by an impact factor from 1.1 to 2.
The factor safety is often specified in a design code or standard.
Factor safety is affected by:
- Material strength basis: Brittle materials use ultimate strength and ductile use yield strength.
- Manner of loading: Static applied slowly, repeated; fatigue failure may occur; impact; high initial stresses develop.
- Possible misuse: Designer must consider any reasonable foreseeable use of misuse of product.
- Complexity of stress analysis.
- Environment: Temperature, weather, radiation, quimical…
- Hazard of failure; What happens if the parts fail?
- Cost.
- Market segment: Different safety factor for different markets (aerospace, health…)