Which statement about endurance limit for materials is true?

Which statement about endurance limit for materials is true?

• A. For many steels, there is a stress below which fatigue failure does not occur; some materials do not have a true endurance limit.
• B. Endurance limit is the maximum stress at which a material will not fail indefinitely and is present for all metals.
• C. Endurance limit equals ultimate tensile strength for all materials.
• D. Endurance limit is only relevant for polymers.

Answer: (A)

Endurance limit describes the stress threshold below which a material can endure cyclic loading for an infinite number of cycles without fatigue failure. For many steels, there is such a limit: if the alternating stress stays below that value, the part can, in theory, last forever under reversed loading. But not all materials exhibit a true endurance limit; several metals—especially many nonferrous alloys like some aluminum and copper alloys—do not show a flat fatigue plateau, and their S-N curves continue to decline with the number of cycles, meaning there isn’t a finite stress that guarantees infinite life. The endurance limit is not equal to the ultimate tensile strength; it is typically a fraction of the UTS and depends on the loading condition. It is also not exclusive to polymers—fatigue behavior exists in polymers too, but the endurance-limit concept is most characteristic of certain steels and metals.

Endurance limit describes the stress threshold below which a material can endure cyclic loading for an infinite number of cycles without fatigue failure. For many steels, there is such a limit: if the alternating stress stays below that value, the part can, in theory, last forever under reversed loading. But not all materials exhibit a true endurance limit; several metals—especially many nonferrous alloys like some aluminum and copper alloys—do not show a flat fatigue plateau, and their S-N curves continue to decline with the number of cycles, meaning there isn’t a finite stress that guarantees infinite life. The endurance limit is not equal to the ultimate tensile strength; it is typically a fraction of the UTS and depends on the loading condition. It is also not exclusive to polymers—fatigue behavior exists in polymers too, but the endurance-limit concept is most characteristic of certain steels and metals.