There are the following differences between tensile strength and yield strength:

Tensile strength in tensile testing machine

When the steel yields to a certain extent, its resistance to deformation increases again due to the rearrangement of internal grains. At this time, although deformation develops rapidly, it can only increase with the increase of stress until the stress reaches its peak value. Afterwards, the ability of the steel to resist deformation significantly decreased, and significant plastic deformation occurred at weak points. At this point, the cross-section of the specimen rapidly shrank, leading to necking until fracture and failure occurred. The stress value of steel before tensile fracture is called tensile strength.

Yield strength in tensile testing machine

When the stress exceeds the elastic boundary, the deformation increases rapidly, resulting in not only elastic deformation but also partial plastic deformation. When the stress reaches point B, the plastic stress increases sharply, and a small platform of fluctuation appears on the curve, which is called yield. The maximum and minimum stresses in this stage are called the upper yield point and the lower yield point, respectively. Due to the relatively stable value of the lower yield point, it is used as an indicator of material resistance, known as yield point or yield strength.