Introduction
When it comes to measuring the hardness of a material, you can perform several different tests. The Leeb Hardness Test is one of these methods used to test a material’s hardness.
The Leeb Hardness Test is a material-specific test. The Leeb Hardness Test tests determine if a material is suitable for specific applications and when a material is required to meet specification or performance requirements. This method was invented in 1975 by two Swiss engineers, Leeb and Brandestini.
In this article, you will learn about how to perform a Leeb hardness test.
I. What is the Leeb Hardness Test?
The Leeb Hardness Test measures the force required to impact the test material with a carbide ball. The carbide ball generates a rebound shock when it hits the test material at a constant speed. The hardness of the test sample is given by the impact and rebound velocity ratio. The rebound velocity is calculated by measuring the time required for the carbide ball to bounce back up after hitting the material.
This test is not an absolute test, which means it will not give identical values every time it’s performed.
II. Principle of Leeb Hardness Test
The principle of the Leeb Test depends on a combination of friction, elastic and plastic deformation. When a material is subjected to a tensile force, the deformation element (i.e., the carbide ball) glides against the material’s surface. At the same time, it tries to follow its original direction of motion. The latter is attributable to friction between the carbide ball and the material’s surface; this force prevents further gliding, concentrating all sliding on a small portion in contact with the collision with an irregularly arranged surface.
The rebound shock generated by the collision between test specimen and ball creates considerable energy through sudden deformation. It is proportional to impact energy per unit area of the specimen surface. The hardness value of a material is obtained from the lost energy of the impacting body (the carbide ball) upon impact with the test material when using the Leeb Hardness Test. The Leeb quotient is obtained as a measure of the energy lost by the plastic deformation of the test material.
III. Leeb Testing Standards
- GB/T 17394: Leeb hardness test for metallic materials.
- DIN 50156: Leeb hardness test for metallic materials.
- DGZfP: Mobile Hartepr¨¹fung Guideline.
- ASTM A956: Leeb hardness testing standard method for testing steel products.
- ISO 16859: Leeb hardness test for metallic materials.
IV. Leeb Hardness Test Procedure/ How to Perform a Leeb Hardness Test?
Apparatus
The Leeb hardness test is usually performed using a Leeb hardness tester. This tester consists of a carbide ball-impact and a digital hardness display device.
Test sample: a material that you want to measure its hardness using a Leeb hardness tester.
Performing the test
The Leeb hardness test begins by placing the tester onto a flat surface and positioning the test sample inside the guide recess for it to be upright. Afterward, you place the carbide ball onto the stationary post and let it drop down vertically to reach its highest point, which is at its impact position. The moment it reaches this peak height, lower it to touch with the test sample and then quickly raise it again.
The rebound velocity of cavitation is measured using a timer to record the time taken by the measuring device while bouncing back from the bottom of the countertop surface after contacting a test sample.
Try to perform the test in a completely dry environment for best results and avoid any influence of moisture that can affect the accuracy of the result.
The Leeb Hardness tester should be calibrated on the impact return stroke, which is the initial downward movement of the ball after it has dropped off the countertop surface. The impact stroke of the tester should be between 0.3-0.6m/s (1m = 0.3ft, 1ft = 30cm).
V. Leeb Hardness Calculation and Formula
The rebound energy of the ball is a measure of the material’s hardness.
HL= VR-Impact velocity / VA-Rebound velocity
where HL is the Leeb Hardness Value.
The final Leeb hardness value is the ratio of impact and rebound energy. The impact energy is measured using a standard dynamometer, and the rebound energy is measured with the help of a proton displacement meter that determines how much voltage or current runs through it when it bounces back from the impact position.
The final values are displayed on the digital device.
Conclusion
The Leeb hardness test accurately determines what material can be used in specific applications, making it highly applicable in measuring materials with strict quality control requirements. The test is a highly accurate way to measure the hardness of materials and is suitable for any type of material in which specific requirements need to be met.