In enclosure design, the maximum factor used for low-strength enclosures is two-thirds of the ultimate strength.

• A. 1/2
• B. 2/3
• C. 3/4
• D. 5/6

Answer: (B)

In enclosure design, you don’t use the full ultimate strength of the material; you apply a safety margin by limiting the working stress to a fraction of that ultimate strength. For low-strength enclosures, that fraction is two-thirds, meaning the maximum allowable stress is 0.666... times the material’s ultimate strength. This provides a buffer against uncertainties such as manufacturing variances, aging, temperature effects, and unexpected loads, helping prevent failure.

That’s why two-thirds is the best answer: it establishes the required safety margin for low-strength enclosures. Using a smaller fraction (like one-half) would be more conservative, while larger fractions (three-quarters or five-sixths) would reduce the margin and risk approaching failure under real-world conditions.