Shore durometer
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Durometer is one of several ways to indicate the hardness of a material, defined as the material's resistance to permanent indentation. It is named for instrument maker Albert F. Shore, who developed a measurement device called a durometer in the 1920s. The term durometer is often used to refer to the measurement, as well as the instrument itself. Durometer is typically used as a measure of hardness in polymers, elastomers and rubbers.[1]
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[edit] Durometer Scales
There are several scales of durometer, used for materials with different properties. The two most common scales, using slightly different measurement systems, are the ASTM D2240 type A and type D scales. The A scale is for softer plastics, while the D scale is for harder ones. However, the ASTM D2240-00 testing standard calls for a total of 12 scales, depending on the intended use; types A, B, C, D, DO, E, M, O, OO, OOO, OOO-S, and R. Each scale results in a value between 0 and 100, with higher values indicating a harder material.[2]
[edit] Method of measurement
Durometer, like many other hardness tests, measures the depth of an indentation in the material created by a given force on a standardized presser foot. This depth is dependent on the hardness of the material, its viscoelastic properties, the shape of the presser foot, and the duration of the test. ASTM D2240 durometers allows for a measurement of the initial hardness, or the indentation hardness after a given period of time. The basic test requires applying the force in a consistent manner, without shock measuring the hardness (depth of the indentation). If a timed hardness is desired, force is applied for the required time and then read. The material under test should be a minimum of 6.4 mm (.25 inch) thick.[3]
| Durometer | Indenting foot | Applied mass [kg] | Resulting force [N] |
|---|---|---|---|
| Type A | Hardened steel rod 1.1 mm - 1.4 mm dia, with a truncated 35º cone, 0.79 mm dia. | 0.822 | 8.064 |
| Type D | Hardened steel rod 1.1 mm - 1.4 mm dia, with a 30º conical point, 0.1 mm radius tip | 4.550 | 44.64 |
The final value of the hardness depends on the depth of the indenter's penetration. If the indenter penetrates 2.5mm or more into the material, the durometer is 0 for that scale. If it does not penetrate at all, then the durometer is 100 for that scale. It is for this reason that multiple scales exist. Durometer is a dimensionless quantity, and there is no simple relationship between a material's durometer in one scale, and its durometer in any other scale, or by any other hardness test.[1]
[edit] Durometer of various common materials
| Material | Durometer | Scale |
|---|---|---|
| Bicycle gel seat | 15-30 | OO |
| Chewing Gum | 20 | OO |
| Sorbothane | 40 | OO |
| Sorbothane | 0 | A |
| Rubber band | 25 | A |
| Door seal | 55 | A |
| Automotive tire tread | 70 | A |
| Soft skateboard wheel | 75 | A |
| Hydraulic O-Rings | 70-90 | A |
| Hard skateboard wheel | 98 | A |
| Ebonite Rubber | 100 | A |
| Solid truck tires | 50 | D |
| Hard Hat | 75 | D |
| Urethane Bandsaw Tires | ? | D |
[edit] Patents
- US1,770,045 (1930-07-08) A.F. Shore Apparatus for Measuring the Hardness of Materials
- US2,421,449 (1947-06-03) J.G. Zuber Hardness Measuring Instrument
[edit] References
- ^ a b Shore (Durometer) Hardness Testing of Plastics. Retrieved on 2006-07-22.
- ^ Material Hardness. CALCE and the University of Maryland (2001). Retrieved on 2006-07-22.
- ^ a b Rubber Hardness. National Physical Laboratory, UK (2006). Retrieved on 2006-07-22.
[edit] External links
ASTM Standard D2240 - Standard Test Method for Rubber Property - Durometer Hardness
