Which two strength values are commonly cited for steel indicating the limits before plastic flow and fracture?

• A. Elastic Modulus and Shear Strength
• B. Hardness and Toughness
• C. Fatigue Limit and Creep Strength
• D. Yield Strength and Ultimate Tensile Strength

Answer: (D)

In steel, two strength values are routinely cited to describe the limits of how much load the material can carry before it behaves plastically or fails. The first is the yield strength, which marks the onset of plastic deformation. Up to this stress, the material responds elastically and will return to its original shape when the load is removed; beyond it, permanent (plastic) deformation starts.

The second is the ultimate tensile strength, the highest stress the material can withstand in a tensile test. It represents the maximum load-carrying capacity; after reaching this point, the steel typically necks and eventually fractures.

These two values are fundamental for design: ensure operating stresses stay below yield strength to avoid permanent deformation, and use the ultimate tensile strength as a measure of the material’s overall strength before failure.

Other options don’t fit because they describe different properties. Elastic modulus relates to stiffness, not the onset of plastic flow. Hardness and toughness measure surface resistance to indentation and energy to fracture, respectively, but not the two standard tensile thresholds. Fatigue limit and creep strength pertain to cyclic loading and long-term high-temperature behavior, not the basic monotonic yield and fracture limits.