Monday, February 12, 2024

Shear Tests

 There are two main types of test which may be used for the determination of the properties of a material in shear.

SHEAR TESTS

There are two main types of test which may be used for the determination of the properties of a material in shear. They are : 

1. Test performed using a pure shear force, and

2. Test performed using the application of a torque to a cylindrical specimen i.e., torsion test.

† Shear force simply means a tangential force.


1. Shear Test Using a Pure Shear Force

Fig.5.17 illustrates the shear test, performed using a pure shear force. In this type, the force is imposed parallel to the upper and lower faces of the body.


Shear stress: The shear stress or shear strength is the value of load applied tangentially to shear it off across the resisting section. Mathematically,

Shear stress,τ = F / Ao ….. (5.13)

where

F = Shear force applied, and

Ao = Area of shear (as shown in Fig.5.17)

Shear strain: The shear strain is defined as the tangent of the strain angle 0, as indicated in Fig.5.17.

Mathematically, shear strain, γ = tan θ

1. Testing Methods

Three main systems used for shear tests are:

(i) Double shear system for round bar test-pieces using a fork and eye device;

(ii) Double knife shear system for specimens with a rectangular section; and

(iii) The shearing of a disc from sheet materials using a punch and die.

The test equipment generally takes the form of attachments which can be used in conjunction with a universal testing machine.

The disadvantages of these methods are:

■ Always there will be bending of the test-piece, therefore the stresses within the material will not be of the pure shear type, but they include bending stresses.

■ Very difficult to obtain accurate quantitative data from them.


2. Torsion Test

Torsion is a variation of pure shear, wherein a structural member is twisted in the manner of Fig.5.18.

Torsional forces produce a rotational motion about the longitudinal axis of one end of the member relative to the other end.


To determine the shear strength t, the following general torsion equation can be applied within the elastic range :


where

τ = Shear stress

r = Radius of the cylinder,

T = Applied torque,

J = Second polar moment of area,

G = Modulus of rigidity of the material,

θ = Angle of twist in radians, and

L = Gauge length of the test-piece.

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