What definition best describes an understress failure?

• A. The part failed due to excessive deformation
• B. The part failed after exceeding design tension
• C. The part failed because of external factors
• D. Part failed at or under a stress level supposedly within its design capability

Answer: (D)

Understress failure means a part fails while the applied stress is at or below the design capability. In other words, the load wasn’t high enough to explain a break if the material were performing exactly as designed—the actual strength of the component was reduced by factors like material defects, improper heat treatment, corrosion, or service-induced damage such as fatigue. So the failure occurs despite being within what should be a safe stress range, pointing investigators to hidden weaknesses rather than an overload.

This matches the idea that the part failed at or under a stress level supposedly within its design capability. The other descriptions describe failures caused by too much load or by external factors, which are not the same as an understress condition where the stress level is within design limits but failure still occurs due to compromised strength.