Direct Shear Test and Apparatus



The geotechnical direct shear test is a mechanical test used to study the shear strength and deformation characteristics of soil or geosynthetics.

Principle of direct shear test

The principle of the direct shear test is to place the sample in a shear test frame, apply two parallel horizontal forces above and below, so that the sample undergoes shear failure in a direction perpendicular to the horizontal plane, and calculate the shear strength of the sample based on the stress-strain curve obtained in the experiment.

Direct shear formula

Now that we understand the principles of direct shear, how do we calculate a shear test?
A shear test typically involves applying a force to a specimen and measuring the amount of shear stress the specimen can withstand before failing or undergoing plastic deformation. The calculations in a shear test depend greatly on the geometry of the specimen and the type of test being performed, but a typical calculation generally involves the following steps:
Measuring Applied Force: Testing machines measure the amount of force applied to a specimen. This value is usually recorded in units of force, such as Newtons (N) or pounds-force (lbf).
Calculating Shear Stress: Shear stress (τ) is calculated by dividing the applied force (F) by the cross-sectional area (A) of the specimen to which the shear force is applied. Shear stress is calculated as:
t = F / A
Shear stress is usually measured in Pascals (Pa) in the metric system or pounds per square inch (psi) in the imperial system.
Measuring Shear Strain: Shear strain (γ) is a measure of deformation and represents the displacement of the specimen in the direction of the force. It is calculated as the change in length (ΔL) divided by the original length of the specimen (L0):
c = ΔL / L0
Shear strain is a dimensionless quantity.
Calculating Shear Modulus: Shear modulus (G), also known as the modulus of rigidity, represents the resistance of a material to shear stress. It is calculated as the ratio of shear stress (τ) to shear strain (γ). The formula is:
G = t / c
Shear modulus is typically given in units of Pa or psi.
These calculations provide a basic understanding of the shear properties of a material. It is important to remember that while this method is generally applicable to many shear tests, some variations may require different calculations depending on the test setup and material type. Always ensure that your testing and calculations are performed or supervised by a materials science expert or a certified testing laboratory.

Direct shear instrument test steps

Place the specimen flat on a rigid horizontal base at the bottom of the shear box, with the front end clamped in front of the shear zone. The specimen is glued to the base. After bonding, the specimen should be flat, without wrinkles or folds. During the test, there is no relative slip between the specimen and the base.
Search for more details about direct shear apparatus in FYI Tester

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