Hardness is a complex concept that generally pertains to the resistance of a solid material to permanent deformation when force is applied. The term can apply to deformation from indentation,
scratching, cutting or bending. Materials exhibit hardness in different ways making it not an intrinsic physical property of the material but, rather, a composite property that includes a
number of factors such as the ductility, elastic stiffness, plasticity, strain, strength, toughness, viscoelasticity, and viscosity of the material being tested.
Hardness measurements are widely used for the quality control of materials. From an engineering standpoint, resistance to wear by friction or erosion generally increases with hardness.
Proper testing ensures materials are suitable for their intended use.
Since hardness can be characterized in many ways, a number of tests have been developed that will each determine a different hardness value for the same material. The most common types
of hardness tests are those which test for indentation hardness and those that test for rebound hardness.
Indentation hardness measures the ability of a material to resist deformation due to a constant compression load from a sharp object. Indentation hardness correlates
linearly with tensile strength or the ability of the material to be stretched without failing.
Common tests for indentation hardness include Rockwell, Vickers, Shore, and Brinell. Each of these tests use a specifically shaped indenter which is much harder than the material being
tested. The indented is pressed into the surface of the test material using a specific amount of force. The depth or size of the indent left in the material is measured to indicate a
hardness value.
Indentation tests often rely on complicated mathematical formulas and require microscopes to assess the indentation. Ultrasonic contact impedance (UCI) is a variation
of the Vickers test which has proven popular due to the ease at which it is administered. The UCI method uses a Vickers diamond attached to a metal rod. Piezoelectric elements excite
the rod into oscillations. The resonant frequency of the rod changes as the diamond penetrates the material being tested and increases as the indentation becomes larger. Since the size
of the indentation is used to determine hardness, there is a direct relationship between the frequency change and the material hardness.
Rebound hardness, on the other hand, measures the speed of an impact body as it’s propelled into, and rebounds from, the surface of the material being tested. This type of
hardness is related to elasticity. Common measurement techniques for rebound hardness include the Leeb rebound hardness test and Bennett hardness scale.
Rebound hardness testers generally use a spring-loaded impact body tipped with a hardened ball. The impact body is accelerated into the material under test at a defined speed. The impact
causes an indentation which absorbs the kinetic energy of the impact body. The speed of the impact body is measured as it rebounds from the impact. The hardness value is calculated by the
ratio of the impact and rebound speeds. The harder the material, the higher the rebound velocity and the displayed hardness value.
Whether choosing to test for rebound or indentation hardness is mostly a function of the material being tested. The UCI method is particularly effective for testing fine-grained materials in
almost any shape or size. They are also capable of testing to close tolerances. Rebound hardness testing is primarily for large components having rough, coarse-grained surfaces.
Calibration
Like most test and measurement equipment, hardness testers need to be periodically calibrated to maintain accuracy. In the case of NDT equipment, calibrations are performed
using precision test blocks that simulate conditions and serve as reference standards. Different types of test blocks simulate different conditions. Test blocks can include steps of different
thicknesses for use with thickness gauges, holes and other “flaws” for use with flaw detectors, or predetermined hardness for hardness detectors.
Things to Consider When Selecting a Hardness Tester:
- What parameter (thickness, flaw detection, etc.) is being tested for?
- Does the equipment require specialized training?
- What accessories (transducers, couplant, calibration blocks) are required?
- Are there professional standards which must be adhered to?
If you have any questions regarding hardness testers please don't hesitate to speak with one of our engineers by e-mailing us at sales@instrumart.com or calling 1-800-884-4967.
