Quick questions on Stress, strain and Young's modulus: HSC Engineering Studies Civil Structures

Ductile materials (mild steel, structural steel grades like 250 and 350) show a linear elastic region up to the yield point, a yield plateau, then strain hardening up to the ultimate tensile stress, then necking and fracture. Typical yield stress for grade 250 structural steel is 250 MPa; ultimate tensile strength is around 410 MPa.

Reading a tensile-test curve is a common HSC task. The features you must label, in order of increasing strain, are:

Civil engineers use allowable stress well below the yield stress, dividing by a factor of safety of 1.5 to 3. The allowable (working) stress is $\sigma_{\text{allow}} = f_y / FoS$. A member is acceptable when its working stress stays below this value; the actual factor of safety is the yield stress divided by the working stress. Concrete is strong in compression (typical 32 MPa) but weak in tension (about 3 MPa), which is why it is reinforced with steel.