Durometer test

The radiation and temperature dependent mechanical properties of viscoelastic materials (modulus and loss) are of great interest throughout the plastics, polymer, and rubber from initial design to routine production. There are a number of laboratory research instruments are available to determine these properties. All these hardness tests conducted on polymeric materials involve the penetration of the sample under consideration by loaded spheres or other geometric shapes [1]. Most of these tests are to some extent arbitrary because the penetration of an indenter into viscoelastic material increases with time. For example, standard durometer test (the "Shore A") is widely used to measure the static "hardness" or resistance to indentation. However, it does not measure basic material properties, and its results depend on the specimen geometry (it is difficult to make available the identity of the initial position of the devices on cylinder or spherical surfaces while measuring) and test conditions, and some arbitrary time must be selected to compare different materials. 

Durometer Test Method for Rubber Property Durometer Hardness D2240... 

The hardness measured with the small pocket type of hardness meter is associated with the durometer tests. Although there is not an international standard [1] that covers durometers, there is one for plastics, ISO R868 [10], which was used [9]. The two durometers specified are the Shore A and D which are also described in ASTM D2240 [11] where they are intended to cover both rubbers and plastics. The A scale corresponds approximately to the IRHD (International Rubber Hardness Degrees) scale and the D scale can be conveniently be used for hard rubbers above about 90 IRHD. 

The performance of elastomers is of major interest and concern to the design engineer. The readily available data concern the tensile-elongation factor, the compression set, results from durometer tests, and information on oil resistance, heat aging, and the static modulus. 

The durometer test is based on the penetration of a specific type of indentor when forced into the material under specified conditions. The indentation hardness is inversely related to the penetration and is dependent on the elastic modulus and viscoelastic behavior of the material. The geometry of the indentor and the applied force influence the measurements such that no simple relationship exists between the measurements obtained with one type of durometer and those obtained with another type of durometer or ottrer instruments used for measuring hardness (10). 

The durometer test is an empirical test intended primarily for control purposes. No simple relationship exists between indentation hardness determined by this test method and any fundamental property of the material tested. Various types of indentors are in use, as shown in Figure 2.1. 

The hardness of plastics (and coatings may be considered to be very thin plastics) is most commonly measured by the Shore (durometer) test or Rockwell hardness test. Both of these hardness methods measure the resistance of plastics toward indentation. This provides an empirical hardness value. Shore hardness most often uses either the Shore A or the Shore D scale. These methods are best used for rubbers or elastomers and are also commonly used for softer plastics such as polyolefins, lluoropolymers, and polyvinyls. The Shore A scale is used for softer rubbers while the Shore D scale is used for harder ones. This method works better for thicker coatings, as with all hardness tests on coatings. The hardness of thin coatings can be influenced by the substrate, that is, an artificially hard measure may be measured. A picture of a durometer is shown in Fig. 2.23. 

The hardness range of this class of materials is from 60 Shore A to 60 Shore D. Hardness, modulus and tensile strength all increase as the PP content increases. (The hardness of rubber compounds is typically measured by the Shore Durometer test an indentor is pressed into the material, under a specified load, and the amount of penetration determines the hardness. A circular tip is used for the A scale and a pointed tip is used for the D scale. D means harder materids.)... 

For thermoplastic materials, hardness is often described using the term durometer. A common test is the Shore durometer test, with some of the common test protocols being A, B, C, and D. A comparison of hardness data for some common materials are given in the graph below (Figure 7.22). 

Duriron Duromer Durometer hardness test Duroquin Duro Sup er Glue Duro-Tak... 

Recommended Practice for Scleroscopic Hardness Testing of Metallic Materials Test Method for Rubber Property International Hardness Test Method for Rubber Property Durometer Hardness... 

Operator skih and experience are necessary to obtain consistent results usiag a Durometer. Speed of load appHcation, dweh time, and sample thickness can affect reproducibhity of results. Durometer cahbration prior to each test series is done usiag a test block provided with the iastmment. When large numbers of tests are required, improved consistency of results are obtained if the Durometer is used with the accessory vertical stand rather than hand held. 

International Rubber Hardness. The International mbber hardness test (ASTM D1415) (2) for elastomers is similar to the Rockwell test ia that the measured property is the difference ia penetration of a standard steel ball between minor and major loads. The viscoelastic properties of elastomers require that a load appHcation time, usually 30 seconds, be a part of the test procedure. The hardness number is read directly on a scale of 0 to 100 upon return to the minor load. International mbber hardness numbers are often considered equivalent to Durometer hardness numbers but differences ia iadenters, loads, and test time preclude such a relationship. 

Vickers and Knoop indentors, Barcol hardness, and Shore durometers (2) (b) to measure the resistance of a material to scratching by another material or by a sharp point, such as the Bierbaum hardness or scratch-resistance test and the Moh one for hardness and (c) to measure rebound efficiency or resilience, such as the various Rockwell hardness tests. The various tests provide different behavior characteristics for plastics, as described by different ASTM standards such as D 785. The ASTM and other sources provide different degrees of comparison for some of these tests. 

Figure 7-10 shows durometer scale relationships and hardness ranges. The letter designations refer to the Shore hardness test (Chapter 5, MECHANICAL PROPERTY, Hardness). 

The shore durometer is a simple instrument used to measure the resistance of a material to the penetration of a blunt needle. In the Barcol approach, a sharp indentor is used to measure the ability of a sample to resist penetration by the indentor (Figure 14.18). The values given in Table 14.3 are for one specific set of conditions and needle area for the Barcol and Brinell hardness tests. 

The so-called brittle point, associated with sample failure in impact tests, may be determined qualitatively using a penetrometer or a Shore durometer (an instrument used to measure resistance of a sample to penetration by a blunt needle) to measure the change in penetration hardness with temperature. Also, a thin film of a polymer may be readily folded at temperatures above T but may crack when folded at temperatures below Tr... 

The hardness of a polymer may be measured by the Rockwell tests, in which steel balls of specified diameter are used to indent the polymer specimen (ASTM-D785). The indentation hardness of rubbery polymers is measured by an indenter called a durometer (a rounded surface of specified weight and dimensions) (ASTM-D2240) which acts as the indenter onto test specimens. 

Durometer hardness - A value that indicates the indentation or resistance to indentation of the indentor point of a durometer. High values indicate harder materials. See ASTM D2280-Test Method for Rubber Property-durometer Hardness. 

Durometer. The Durometer hardness test was developed for and is used for determining the hardness of elastomers. The Durometer is a hand-held, spring-loaded instrument which when pressed against the sample forces a conical steel indenter into the surface. Durometer hardness numbers range from 0 to 100 and are read directly from the attached dial indicator. Several load scales are available, but the A scale (8 N = 822 gf) and the D scale (44.5 N = 4.54 kgf) are most common. Specifics of the test procedure are discussed in ASTM D2240 (2). Lighter load scales and larger diameter indenters are available for very soft materials such as foam. 

The standard briefly covers the significance of hardness in terms of its relation with modulus, and the practical use of hardness tests. The hardness tests for rubber that are standardized by ISO are introduced and the distinction between dead load and durometer type instruments is explained to help with selection of a test method for particular circumstances. The effect of test piece, use of standard hardness blocks and comparison of hardness scales is also outlined. 

The particular case of the hardness of rollers is covered by ISO 726734 36 which is in three parts dealing with the normal dead load method, Shore durometer and the Pusey Jones methods respectively. This last method is a very old hardness test which is now virtually never seen, although it is understood to still be popular in some circles for rollers. It is an amazing brass and chain contraption that uses a 3.175 mm indentor acting under a load of 1kg and without a surrounding foot. 

Hardness tests attract more interest in their accuracy, reproducibiliy and intercomparison than any other test - which is probably a result of them being simple tests which are carried out particularly frequently, and because the situation is confused with several scales. The fact that the situation is far from clear cut owes as much to history as to logic. If the Shore durometers had not been the first hardness meters, it is highly doubtful that we would now have spring loaded instruments mounted on stands or use damage prone indentors. Neither would the IRHD scale have been contrived to mimic the status quo. Such is the effect of powerful established interests. 

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