The 0.2% proof stress is used to define the yield stress for some metals. a) True b) False

a) True

The 0.2% proof stress, also known as the 0.2% offset yield point, is indeed used to define the yield stress for some metals, especially those which do not exhibit a clear yield point. The yield stress is an important material property, particularly in engineering and materials science, as it describes the stress at which a material begins to deform plastically, meaning it will not return to its original shape when the stress is removed.

Extra: To understand this concept, it's important to know about the stress-strain curve that is obtained from a tensile test. When a material is subjected to a tensile test, the resulting stress-strain curve shows the behavior of the material under load. For some materials, such as mild steel, there is a clear yield point where the curve shows a distinct transition from elastic to plastic deformation. However, for many other materials, such as aluminum or stainless steel, there isn’t a clear yield point on the curve.

This is where the 0.2% proof stress comes into play. It is defined as the amount of stress required to produce a permanent strain of 0.2% in the material. To determine this, a line is constructed parallel to the initial linear elastic portion of the stress-strain curve, but offset by 0.2% (0.002 times the original length) of strain. The intersection of this line with the stress-strain curve gives the 0.2% proof stress, which is used as a proxy for the yield stress.

Engineers use the concept of yield stress to design components that can withstand applied stresses without permanently deforming. The yield stress represents the limit of elastic behavior and the beginning of plastic behavior. Components are generally designed to operate below the yield stress to ensure that they return to their original shape after loading. This concept is fundamental to ensuring that structures and mechanical parts perform their functions safely and reliably.