Engineering design basics – factor of safety:\nState whether the following statement is correct: “The factor of safety (FOS) used in design is always greater than unity.”

Introduction / Context:
The factor of safety (FOS) is central to engineering design. It accounts for uncertainties in loads, material properties, dimensional tolerances, environmental effects, and modeling assumptions. Designers select FOS values to ensure that the working (allowable) stress or load is comfortably below the ultimate capacity.

Given Data / Assumptions:

• Working stress design: allowable stress = ultimate or yield stress / FOS.
• Limit state design: partial safety factors play an analogous role; overall reliability still implies reserve capacity.
• Normal civil/mechanical design scenarios, excluding special proof testing or certification edge cases.

Concept / Approach:
By definition in working stress methods, FOS = strength / working load (or allowable stress = strength / FOS). To reduce the chance of failure, the allowable value must be lower than the capacity; hence FOS > 1. Even in reliability-based design, separate partial factors on actions and resistances create an implicit FOS above unity for typical permanent structures and machines.

Step-by-Step Solution:

Verification / Alternative check:
Code specifications (e.g., for pressure vessels, cranes, structural steel) prescribe allowable stresses below yield/ultimate or apply partial factors that ensure capacity exceeds demand, maintaining FOS above 1.

Why Other Options Are Wrong:

• Incorrect/conditional answers: Suggest FOS ≤ 1, which leaves no safety margin and contradicts standard practice.
• “Depends solely on manufacturing”: FOS selection depends on multiple uncertainties, not just manufacturing.

Common Pitfalls:
Confusing test conditions (proof load tests) with design service conditions. A proof test may temporarily approach capacity, but design FOS for service remains above unity.

Final Answer:
Correct