Durometers are designed to be used with plastics while hardness testers can be used to test the hardness of most materials
Hardness testers may also be referred to as durometers. The main difference between these two instruments is the materials being tested. Durometers, generally test soft materials, while hardness testers are designed specifically to test the surface hardness of materials ranging from plastics to metals like steel. Some testers have been modeled and branded for their hardness testing scales. Hardness testers may be large and stay in a fixed location – bench testers, other testers are designed for portable use such as the Ames portable hardness tester. In 1812 Friedrich Mohs developed a scale to identify the hardness of minerals, ranking them on a scale of 1-10. The Mohs hardness scale has identified minerals’ hardness and known objects that minerals can be tested against to determine the hardness. This is the oldest known hardness measurement system.
The Shore Scales are used in testing the hardness of plastics – Consisting of Shore 00 for super soft materials like gum through Shore D for hard plastics and rubbers. The scale was designed by Albert Ferdinand Shore in 1920 although his tester was not the first device to be called a durometer. The Shore durometer was designed specifically to test polymers, elastomers, and rubbers. The shore scales provide a universal reference for a large scale of non-metal hardness testing.
The Scleroscope hardness tester was developed in the same time period as Brinell by Albert Shore of Shore Instrument Manufacturing Company, this instrument did not use traditional penetrators, but a scale to measure the material’s elasticity, this instrument was replaced by more accurate and efficient forms of testing as technology advanced.
Vickers durometer This use of the durometer refers to the hardness test, not the instrument. This scale is popular in the UK and Europe and is one of the widest scales for hardness testing. The Vickers hardness test also referred to as the microhardness test procedure was developed in 1921 by Robert L. Smith and George E. Sandland at Vickers Ltd as an alternative to the Brinell method to measure the hardness of materials from ceramics to metals. HBW (H from hardness, B from Brinell, and W from the material of the indenter, tungsten (wolfram) carbide). In former standards HB or HBS were used to refer to measurements made with steel indenters. The Vickers test uses a smaller indenter making it easier to use on materials like foil and other very thin materials. Also, the required calculations are independent of the size of the indenter, making calculations easier. The basic principle, observes a material’s ability to resist plastic deformation from standard sources. The Vickers test can be used for all metals and has one of the widest scales among hardness tests. The unit of hardness given by the test is known as the Vickers Pyramid Number (HV) or Diamond Pyramid Hardness (DPH). The hardness number is determined using the load over the surface area of the indentation, not the area normal to the force, so it is not actually pressure. The hardness number can be converted into units of pascals, but should not be confused with pressure, which uses the same units.
The UCI (Ultrasonic Contact Impedance or Portable Vickers Procedure) hardness test is newer, developed in 1968 by Claus Kleesattel. This portable testing method uses the Vickers hardness test method described above which requires the use of a diamond indenter according to ASTM standards.
This method uses a 136-degree diamond at the end of a vibrating rod. The rod is depressed into a fixed surface and a fixed load is applied, the ultrasonic vibration frequency is then measured and calculated into a hardness value. This hardness testing procedure is used for measuring the hardness of small items, objects with a wall as thin as 1 mm, can be used to test hardness on complex forms, and measures the hardness of surface hardened layers. The indentation is considered less destructive than other testers making this a popular option.
The Knoop method is an alternative to the Vickers technique. Developed by Fredrick Knoop at the National Bureau of standards (now known as NIST) in 1939. Primarily used for thin materials with the risk of cracking like ceramic and in cases where a thin layer like a coating needs to be tested. This microhardness testing method uses a narrow diamond-shaped indenter. Knoop is the preferred method for small long test specimens whereas the Vickers method is more effective on small round pieces. The Knoop method is time-consuming and requires preparation of the test piece. Readings can be impacted by load, temperature, and environment as well.
The Brinell hardness test is the oldest hardness test, developed by Sweetish engineer, Johan Brinell. this hardness testing method can be used on rough materials and is performed in a standard or non-standard method. Irregular surfaces do not impact the outcome of this hardness test and can be used to test the hardness of components made from powder and cast. This scale can be used to test both soft and hard materials hardness such as tin, steel, and iron. Some restrictions of this method include, it can not be used on smaller objects, it creates a deep impression and the test can only be performed on a flat surface. This method is also time-consuming and has a higher rate of error in conducting measurements. A variety of hardness testers are available on the market providing Brinell hardness tests.
A popular standard in the hardness testing industry is the Rockwell scale. Developed by Stanley Rockwell in the early 1900s, the scale uses the indentation of a smaller steel ball than the Brinell method (less than 4mm) or a diamond cone to create and measure penetration and determine hardness. This test can be performed quickly. No preparation is required on the material, the scale is easy to read and requires no extra equipment which makes it the most commonly used hardness testing method. You can test the hardness of soft and hard metals in the Rockwell scales, the portable testers allow onsite testing quickly and easily. The Rockwell hardness test is standard on a variety of testers, bench top models were developed by Stanley Rockwell and developed by the Wilson-Mauelen company, acquired by Instron corporation in 1993. The Ames portable hardness tester was developed in 1947 and sold by Ames Precision in Massachusetts until Electro Arc purchased the product line in 1975. These Rockwell hardness testers provide portable hardness readings in the Rockwell scale, and a conversion chart into the Brinell scale.