Hardness and Hardness Measurement:
Hardness is a measure of resistance of a material to deformation when an external force or load is applied to the material. There are several hardness scales, which use different methods of applying force and quantifying the resistance to deformation. Hardness is closely correlated to other mechanical characteristics. It is, like many other mechanical characteristics, a relative value that has no fundamental quantity or absolute standard and is different from physical quantities such as length, time, and force. Because of this, hardness values are determined using a standard testing machine under standard conditions.Today the most popular hardness scales are Brinell hardness (HB), Vickers hardness (HV), Rockwell and Rockwell superficial hardness (HR) and Knoop hardness (HK). Most hardness tests determine hardnessfrom the area of the indentation made in a specimen by the indenter under a known load. The Brinell test was devised in Sweden, the Vickers test in the United Kingdom, and the other hardness tests in the United States. Although hardness is a relative value, Brinell, Vickers, and Knoop hardness are expressed in a unit of stress (1 kgf/mm2 or 9.8MPa).
Rockwell Hardness and Rockwell Superficial Hardness: Mitutoyo Rockwell Hardness Tester click for brochure on Rockwell Machines
In the Rockwell hardness and the Rockwell superficial hardness tests, a conical diamond indenter witha 120° angle and a radius of curvature of 0.2mm, or a steel or carbide ball indenter is pressed into thespecimen. First, a preliminary test force is applied, then a total test force is applied, and then the test load is reduced to the preliminary test force. The hardness number is determined from the difference, h,of the indentation depth of the indenter between the first and second applications of the preliminarytest force.The Rockwell hardness test uses a preliminary load of 10kgf, and the Rockwell superficial hardness test uses a preliminary test force of 3kgf. The Rockwell and Rockwell superficial hardness have multiple scales to indicate specific combinations of the indenter type, test force, and formula to obtain the hardness. A unique symbol is given to each scale.
In the Vickers hardness test, a diamond pyramid indenter with a 136° angle between opposite faces is pressed into the specimen under a test force F (kgf). The hardness number (HV) is obtained by dividing F by the area, A (mm2), of contact between the indenter and specimen.This area is calculated from the diagonal length, d (mm), of the indention when the indenter is removed.The Vickers hardness test is the most versatile hardness testing method of those that use different load settings. The Micro-Vickers hardness test, which accepts load settings of 1kgf (9.807N) or less, is especially well suited for industrial production today, where accuracy requirements are increasing due to technology improvements.Vickers hardness testing at loads of 1 kilogram and up is also known as heavy load Vickers or Macro Vickers. The other testing parameters are similar to lighter load vickers testing. This type of testing may be used to meet the requirements of international specifications or to replace Rockwell testing.
In the Knoop hardness test, a diamond pyramid indenter, which has a rhombic base withincluded angles of 172°30’ and 130°, is pressed onto the specimen under a test force F (kgf).The hardness number (HK) is obtained by dividing the test force F by the projected area, A(mm2), of the indentation. This area is calculated from the longer diagonal length, d (mm), of the indentation when the indenter is removed.The Knoop hardness scale is generally used when shallower depth indentions are required.Knoop hardness can be measured by installing a Knoop indenter on the Macro-Vickers hardness testing machine.
Durometers: For Rubber and Plastics Hardness Testing Mitutoyo Durometer click for brochure
Digital / Dial Durometers are suitable for testing the nature of the following materials— natural rubber, neoprene, polyesters, P.V.C., leather, Thiokol, nitrite rubber, wax,vinyl, cellulose acetates, glass polystyrene, etc. Units are available in both Shore A and Shore D scales, and will test a wide variety of applications.