When selecting materials for an engineering application, critical mechanical properties of the material must be reviewed. Two such properties are yield strength and tensile strength. They are both measures of a material's resistance to failure, either by deformation or fracture. Despite this similarity, yield strength and tensile strength are two very different parameters.
In short, yield strength is the maximum stress a material can endure beyond which it begins to permanently deform, not able to return to its original dimensions. Whereas, tensile strength is the maximum tensile stress beyond which a material fails and breaks.
One property can be considered more important than the other depending on the type of material. If the material is ductile, yield strength is usually the more prominent property to consider, while tensile strength takes precedence in brittle materials.
In this article, you will learn about:
When subjected to stress, a material undergoes recoverable deformation. The yield strength of a material represents the stress beyond which its deformation is plastic. Any deformation that occurs as a result of stress higher than the yield strength is permanent.
Because of the linearity of elastic deformation, yield strength is also defined as the greatest stress achievable without any deviation from the proportionality of stress and strain. Beyond this point, large deformations can be observed with little or no increase in the applied load. Yield strength is measured in N/m² or pascals.
The yield strength of a material is determined using a tensile test. The results of the test are plotted on a stressstrain curve. The stress at the point where the stressstrain curve deviates from proportionality is the yield strength of the material. Some plastics’ deformation is linearly elastic, and once the maximum strength is reached, the material fractures.
The yield strength of a material can be increased by certain material processes. However, it is difficult to define an exact yield point for certain materials from the stressstrain curve. This is because these materials do not display an abrupt curve; rather the onset of yield occurs over a range. It is, therefore, practical to use proof stress as a representation of the yield strength.
Proof stress is the practical limit beyond which a material would permanently deform. With further applied stress, a material can ultimately undergo fracture.
Also known as offset stress, proof stress is simply how much stress a material can withstand until it exhibits a marginal amount of plastic deformation or strain. That amount of strain is arbitrarily chosen as 0.2%. This ruleofthumb approximation was originally based on the yield strength of steel and can be used for materials of which the yield point is difficult to define, such as aluminium, copper, and magnesium.
On the stressstrain curve, proof stress is measured by drawing a straight line starting at 0.2% of the plastic strain parallel to the initial tangent to the elastic curve. The stress at the point where this line intercepts the curve is the proof stress.
Figure 1. A graph showing how 0.2% proof stress is measured.
Often referred to as ultimate tensile strength (UTS), tensile strength is the maximum tensile load a material can withstand prior to fracture. It is a measure of a material's resistance to failure under tensile loading.
The tensile strength of a material is determined using a tensile test. It is the highest point on the stressstrain curve, which is plotted after the test. Tensile strength can also be determined using this formula:
Where P_{f} is the load at fracture, A_{o} is the original crosssectional area, and σ_{f} is the tensile strength, measured in N/m² or pascals.
It is important to note that the tensile strength of a material is a specific value under controlled standard test conditions. However, in practical applications, tensile strength varies with temperature. At 100°C, the tensile strength of copper falls from 220 MPa at room temperature to 209 MPa. These variations are compensated for by using a factor of safety, which is usually a fraction of the original tensile strength in design considerations.
Figure 2. The stressstrain curve showing the yield strength, tensile strength, fracture, and what happens in between.
The following are some of the major differences between yield strength and tensile strength:
Below are examples of the yield and tensile strength values of some engineering materials.
Material 
Yield strength (MPa) 
Tensile strength (MPa) 
Copper 
70 
220 
Aluminium 
95 
110 
Structural steel 
250 
400 
Cast iron 4.5% 
130 
200 
Stainless steel 
502 
860 
Titanium alloy 
750800 
900 
High strength alloy steel 
690 
760 
Chromiumvanadium steel 
620 
940 
941 
15001800 

Kevlar 
3620 
3757 
_{(Table Source: https://www.engineeringtoolbox.com/youngmodulusd_417.html)}
If you've enjoyed reading this article, you can also read more related content here:
Pelleg, J. (2012) Mechanical properties of solids. Illustrated edn. Berlin: Springer science and business media
Khurmi, R. S. (2008) Strength of materials. Revised edn. New Dehli: S. Chand publishers
Rajput, R.K. (2006) Strength of materials. 4th edn. New Dehli: S. Chand publishers
Smallman, R. E. and Bishop, R. J. (1999) Modern physical metallurgy and materials engineering. 6th edn. London: Butterworth Hieneman