The strength of a material, in general, is the value by which yielding, fracture or excessive deformation occurs in a loadcarrying member. Shear strength is a material property that describes a material’s resistance against a shear load before the component fails in shear. The shear action or sliding failure described by shear strength occurs parallel to the direction of the force acting on a plane.
In construction, automotive, aerospace and other engineering industries, knowing the shear strength of materials is vital for the design of mechanical and structural devices as well as the selection of materials to be used for an application. It is also a major consideration in sizing components or parts [1].
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Shear strength and shear stress are often used interchangeably, but there is a technical distinction between the two. Shear stress is relative and it changes in relation to the amount of shear load applied to a material per unit area. On the other hand, shear strength is a fixed and definite value in the general nature of a material [1, 2].
In order to understand shear strength, understanding different theories of failure are important.
Generally for brittle materials, the cause of failure is due to a force in tension.
For ductile materials, the cause of failure is often due to shearing forces.
The stressstrain curve for these two types of material is different. Typically, material strength parameters are identified by conducting uniaxial tests. But in reality, situations produce multidimensional and combined stresses that act at a point or throughout a member, and for which it would be impractical to measure each of the stresses and its effect.
Failure theories are necessary in order to predict a material’s performance in relation to its tensile strength and to establish a relationship of each stress source induced.
For brittle materials, the failure by shear is caused by the sliding of the material’s atoms brought on by shear stresses and distortion. Therefore, for design purposes, the yield strength in shear is the strength parameter to be used [2, 3, 4].
This theory states that material failure will take place when the maximum shear stress brought on by combined stresses will equal or be greater than the obtained shear stress value at yield in a uniaxial tensile test [24].
Suppose in a uniaxial test, the principal stresses are:
`sigma _{1} = S_{y}`
`sigma _{2} = 0`
`sigma _{3} = 0`
where σ_{1}, σ_{2}, σ_{3} = maximum normal stress a body can withstand at a certain point; S_{y} = yield strength.
The shear strength at yielding is:
`S_{sy}=\frac{(\sigma _{1}\sigma_{2})}{2}=\frac{S_{y}}{2}`
or
`S_{sy}=\frac{(\sigma _{1}\sigma_{3})}{2}=\frac{S_{y}}{2}`
For a homogenous, isotropic, ductile material with two or threedimensional static stress, to identify and compute for σ_{1}, σ_{2}, σ_{3} and the maximum shear stress, 𝜏_{max}:
`tau_{max}=\frac{(\sigma _{1}\sigma_{2})}{2}`
According to the maximum shear stress theory, we can then compare the maximum shear stress to the failure criterion:
`tau _{max}\leq S_{sy}`
And therefore, the factor of safety can be given by:
`N=\frac{S_{sy}}{\tau _{max}}`
Shear strength is one of the key considerations in designing many parts, including fasteners such as pins, bolts and screws. Shear tests can be conducted in two ways in order to determine the maximum shear stress.
Many materials are tested for their shear strength to be known, but they can be categorised into three groups: rigid substrates, layered composites and adhesives. Shear strength testing helps categorise an adhesive according to shear strength when pulled from the material to which it adheres. Metals, wood and plastics fall under the rigid materials category. Layered composites’ shear strength is determined by testing the limits of its laminates, layers and fibres [5].
Here are the shear strength values of some interesting materials with their relative applications.
Material 
Shear Strength (MPa) 
Applications 
620 
Flash butt welding, parts for electrical components 

538 
Aircraft landing gear bearings, bending dies, gear wheels 

448483 
Aircraft bearings, pump and marine shafts and wear rings 

414 
Die rings, inserts, forming rolls 

250430 
Springs, clips, gauzes, architectural applications 

185 
Fluid Systems, Fluid Storage, Automotive 

175 
Fluid Systems, pressure vessels, commercial vehicles 
[1] G.N. Frantziskonis, Essentials of the Mechanics of Materials, PA: DEStech Publications, Inc., 2013.
[2] "Theories of Failure", n.d. [Online] Available at: https://www.engr.mun.ca/~katna/5931/Theories%20of%20Failure2.pdf [Accessed on: 29.11.2019]
[3] "Theories of Failure", n.d. [Online] Available at: http://thegateacademy.com/files/wppdf/Theoriesoffailure.pdf [Accessed on: 29.11.2019]
[4] "The second graph is of only the plastic region beyond the strain hardening", n.d. [Online] Available at: https://www.coursehero.com/file/p60393o/ThesecondgraphisofonlytheplasticregionbeyondthestrainhardeningThis/ [Accessed on: 29.11.2019]
[5] "Shear Test", n.d. [Online] Available at: https://www.testresources.net/applications/testtypes/sheartest/ [Accessed on: 29.11.2019]