Question

The ratio of design stress to the estimated or expected maximum stress is called

• A. Reliability
• B. Factor of safety
• C. Efficiency of design
• D. Overload factor

Hint:

FOS is always $> 1.0$. Higher FOS is chosen for cyclic loading, corrosive environments, or critical human safety applications.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
In machine design and structural mechanics, the Factor of Safety (FOS) is a design margin used to ensure components operate safely under unexpected overloads, material variations, and wear.
Key Formula or Approach:
\[ \text{Factor of Safety (FOS)} = \frac{\text{Design / Yield / Ultimate Stress}}{\text{Allowable / Working / Expected Maximum Stress}} \]

Step 2: Detailed Explanation:
Engineers specify a design or failure stress (such as yield strength for ductile materials or ultimate tensile strength for brittle materials).
The maximum expected stress under operational loading is the working stress.
The ratio of allowable material design strength to the actual maximum working stress is termed the Factor of Safety:
\[ \text{FOS} = \frac{\sigma_{\text{design}}}{\sigma_{\text{working}}} \]
A factor of safety greater than 1 ensures that the structural member will not fail under normal service loads.

Step 3: Final Answer:
Thus, the ratio is called the Factor of safety, corresponding to option (B).