In order for one to understand the difference between tensile strength and yield strength, we must first define each of these properties in regard to steel materials. Tensile strength refers to the amount of load or stress that the steel can handle until it stretches or breaks; it is measured by testing the steel's resistance to tension caused by applying mechanical loads to it. Yield strength refers to the amount of stress a material, in this case steel, can withstand before it undergoes plastic deformation (the permanent alteration of shape, form or texture of a material due to the action of stress). It is worth noting that the tests done on materials to determine these measurements are similar.
As stated above, tensile strength is used to identify the point at which steel goes from elastic (temporary) to plastic (permanent) deformation. At the beginning stages of failure, the steel will undergo what is called a ductile failure. This type of failure refers to the point at which the steel surpasses its yield point and results in permeant deformation of the material. The final stage of failure is referred to as brittle failure, and this is also the point at which the tensile strength measurement is taken. In summary, tensile strength is the maximum point at which steel can be stretched or pulled apart before it breaks.
Like tensile strength, yield strength is also measured by applying a given amount of stress on a material. Yield strength can be described as the point at which the material reaches its limit of elasticity. If the amount of stress applied to the steel remains under the yield point, then the steel will return to its original shape once that stress is removed. Yield strength represents upper load limit that is safe to apply to a given material, so this is an important parameter for a wide variety of applications.
Each of these properties deal with the amount of stress a steel material can withstand. The main difference is that yield strength is measured at the point of plastic deformation, whereas tensile strength is measured at the point which the steel fractures. Tensile strength is used primarily for brittle materials, so this means that this measurement is rarely used in applications such as building structures made from ductile materials due to the amount of deformation they sustain. The tensile strength of steel can be calculated at 100% accuracy, as compared to yield strength which is estimated for most materials, including steel. It is important to know both properties for your steel materials, but it is equally important to be able to distinguish the difference between the two.