Rockwell, UCI, and Leeb hardness measurements are widely used in quality control across a variety of industries. A common factor among these applications is the need to test large, bulky metallic materials (often converted to the Rockwell HRC scale and other common scales like HRB).
Although all three methods provide indications of hardness, their operating principles and application methods are significantly different.
Let’s take a closer look at the substantial differences identified in academic literature.
ROCKWELL
PORTABLE HARDNESS
This is based on measuring the difference between the indentation depth of a indenter (120° diamond) under the action of two successive loads (preload and main load). This is the classic Rockwell hardness test principle.
UCI HARDNESS
(or “Ultrasonic”)
This leverages ultrasound to measure the propagation speed of an ultrasonic wave within the material, providing an indirect calculation of hardness. The value is often directly converted to the Vickers scale.
LEEB HARDNESS
(or “Rebound Hardness”)
This is based on measuring the rebound height of a indenter dropped onto the sample. It’s a rapid test but subject to many variables that can influence measurement accuracy due to its dynamic, rather than static, calculation method.
This is, incidentally, the primary differentiating factor of this method, introducing additional errors to which all Leeb results are unfortunately subject.
Despite their differences, these three tests share a fundamental common element: the perpendicularity of the instrument relative to the sample being analyzed.
This seemingly trivial condition is actually crucial for obtaining high accuracy, repeatable and reliable hardness values. After all, it’s the condition required by every hardness test known to Academic literature, from Brinell method to Knoop and Vickers method.
Remember, portable hardness testers generally apply a force significantly lower than benchtop counterparts, regardless of the different methods and scales used.
KINEMATIC ERROR IN LEEB TESTING
Due to the analysis method it relies on, the sphere’s movement is deflected by the force of gravity. This makes the Leeb hardness tester not entirely suitable for measuring in any direction.
IRREGULARITIES IN THE SAMPLE SURFACE
If the sample surface is not flat, the indenter may not penetrate perpendicularly, altering the measurement.
IMPRECISE INSTRUMENT ALIGNMENT
Incorrect alignment of the instrument relative to the sample due to an imprecise grip can cause significant measurement errors.
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