What is shear stress? Understanding its causes

Introduction

An object’s stress consists of how much deforming force is applied per unit area of the object. Shear and tensile stress are different kinds of tension applied to an object differently. On one hand, tensile stress is the result of applying deforming forces at right angles to a surface, whereas shear stress is caused by parallel applications of deforming forces. Parallel layers slide past each other when they are sheared. 

Natural processes are characterized by the resultant shear, which is linked to downward movements of earth materials and earthquakes. Solids and liquids may undergo shear stress; in the latter case, it depends on fluid viscosity. In this article, we examine what shear is and how it is calculated, as well as examples from everyday life. 

Shear 

Rather than merely elastic strain, shear often refers to a process that causes plastic strain within a material. In the presence of an external force, an object will deform. Thus, any stress experienced by an object resulting from a force directed parallel to the object’s plane will be shear or tangential stress. During this event, the material’s cross-sectional area is parallel to the force vector components. In summary, shearing stress is a type of stress that acts parallel to a cross-section of material. Shear forces cause this stress to occur. It is the force acting in the opposite direction and of the same magnitude on the opposite sides of the body. Vectorial quantities, such as shear stress, have both a direction and a magnitude. Taking the ratio of force to the unit area, we can determine the average shear stress:

τ= F/A

τ is the shear stress;

“F” is the applied force and;

“A” is an area parallel to a vector of force in a cross-section.

Several quantities characterizing the stiffness of materials can be measured using the shear modulus, which arises from the generalized Hooke’s law:

• Young’s modulus E indicates how the material will behave when subjected to uniaxial stresses in its direction.
• As a result of this uniaxial stress, Poisson’s ratio v describes the response in orthogonal directions.
• When subjected to hydrostatic pressure, the material’s bulk modulus K describes how it responds.
• Material response to shear stress is described by the shear modulus G.

Shear stress can also be observed in fluids. The fluid and boundary point of contact produces shear stress as it flows within solid boundaries. Each layer in a fluid has a different speed, but a layer at the same height from the boundary will travel at the same speed. Shear stress is primarily responsible for this difference in speed between layers. Moving on to fluids, stress and strain are connected in different ways. Shearing stress is dependent on strain rate, using viscosity as a proportionality constant.

There are imaginary points on sections, known as shear centers, where shear forces can be applied without causing torsion. It is not the centroid that is the shear center. The shear center is located on the axis of symmetry for cross-sectional areas that have one axis of symmetry. Shear centers lie on the centroid of cross-sections with two axes of symmetry. Shear motions in some materials like metal, plastic, sand rapidly localize into narrow bands, called shear bands. If that occurs, the sliding occurs inside the band and the material blocks on either side of the band glide by one another without deforming. If the fracture occurs along with a narrow band, further shearing will take place inside this fracture. This is a special case in brittle materials.

It will come as a surprise to you to learn that every single moment of your life is filled with shearing stress from the moment that you wake up to the moment you go right back to sleep. This stress plays a role in every aspect of our daily lives, so we can better understand it by looking at examples in our daily lives. The following are its examples: 

• Feed that is chewed between your teeth.
• A river bed is subjected to shear stress when water is flowing on it.
• When you slide on the screen of your smartphone is a very common example in today’s society.
• Writing with chalk on a blackboard.

Shear modulus formula

Shear forces cause objects to deform laterally when applied. This is known as the shear modulus of rigidity. As a ratio of the shear stress to the shear strain, the shear modulus is defined. Shear modulus, whose SI unit is the Pascal, is normally expressed in gigapascals instead. Following is the shear modulus formula: 

G=FlAΔx

Notations used in shear modulus formula are: 

• G refers to the shear modulus
• Force F is what acts upon the body
• The initial length is l 
• The area is represented by A
• ∆x is the change length

Shear modulus unit and dimension

The shear modulus SI unit is Pascal (pa). While the dimensional formula is M1L-1T-2.

Conclusion:

A shear force is defined as the attempt to cut off one part of a rigid body from the other when it is applied over the surface area of the body. Deformation of the body leads to strain as a result. Due to its rigidity, the body is resistant to deformations, resulting in reshaping forces along its surface. It is more likely that this restoring force will counter the shearing force that is being applied. Shear stress is therefore a direct consequence of shear strain. There are numerous instances, big and small, where we experience shearing stress. So, listing all the instances would be impossible, as it is a very common occurrence in our lives.