Fatigue testing — types of stress-time variations covered: Fatigue tests are carried out under which of the following patterns of fluctuating stress?

Introduction / Context:
Fatigue is failure under repeated or fluctuating stresses. Materials may fail at stress levels far below static strength when subjected to many cycles. Fatigue testing therefore uses different stress-time histories to represent service conditions.

Given Data / Assumptions:

• Low-cycle and high-cycle fatigue regimes are possible.
• Mean stress and stress amplitude both influence life (e.g., Goodman, Soderberg diagrams).
• Common loading patterns: fully reversed, fluctuating with mean tensile/compressive stress, and pulsating (one sign only).

Concept / Approach:
Fatigue life depends on the combination of stress amplitude and mean stress. Hence testing protocols include multiple patterns: fully reversed (σmax = −σmin), alternating about a nonzero mean (unequal opposite limits), and pulsating (σmin ≥ 0 or σmax ≤ 0).

Step-by-Step Solution:

Verification / Alternative check:
S–N curves are often generated for fully reversed loading; modified approaches introduce mean stress corrections to cover unequal limits and pulsating loads.

Why Other Options Are Wrong:

• Restricting to only one loading pattern ignores the real variety of service stresses.

Common Pitfalls:
Assuming fatigue is synonymous only with fully reversed bending; in practice, many components operate with substantial mean stress.

Final Answer:
All of the above