Shore durometer
Encyclopedia
Durometer is one of several measures of the hardness of a material. Hardness may be defined as a material's resistance to permanent indentation. The durometer scale was defined by 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 polymer
s, elastomer
s, and rubber
s.
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, and 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.
The ASTM D2240 standard recognizes twelve different durometer scales using combinations of specific spring forces and indentor configurations. These scales are properly referred to as durometer types; ie a durometer type is specifically designed to determine a specific scale, and the scale does not exist separately from the durometer. The table below provides details for each of these types, with the exception of Type R.
Note: "Type R is a designation, rather than a true ‘type’. The R designation specifies a presser foot diameter (hence the R, for radius, obviously D could not be used) of 18 ± 0.5 mm (0.71 ± 0.02 in) in diameter, while the spring forces and indentor configurations remain unchanged. The R designation is applicable to any D2240 Type, with the exception of Type M; the R designation is expressed as Type xR, where x is the D2240 type, eg aR, dR, etc; the R designation also mandates the employment of an operating stand".
The final value of the hardness depends on the depth of the indenter after it has been applied for 15sec on the material. If the indenter penetrates 2.54 mm (0.100 inch) 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.
for elastomer
s has been derived by Gent. This relation has the form
where
is the Young's modulus in MPa and 
is the Shore hardness. This relation gives a value of 
at 
but departs from experimental data for 
.
Another relation that fits the experimental data slightly better is
where
is the error function
and
is in units of Pa.
A first order estimate of the relation between Shore D hardness and the elastic modulus for a conical indenter with a 15 degree cone is
where
is the Shore-D hardness and 
is in MPa.
Another linear relation between the Shore hardness and the logarithm of Young's modulus is applicable over a large range of Shore A and Shore D hardnesses. This relation has the form:
where
is the Shore A hardness, 
is the Shore D hardness, and 
is the Young's modulus in MPa.
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
s, elastomer
Elastomer
An elastomer is a polymer with the property of viscoelasticity , generally having notably low Young's modulus and high yield strain compared with other materials. The term, which is derived from elastic polymer, is often used interchangeably with the term rubber, although the latter is preferred...
s, and rubber
Rubber
Natural rubber, also called India rubber or caoutchouc, is an elastomer that was originally derived from latex, a milky colloid produced by some plants. The plants would be ‘tapped’, that is, an incision made into the bark of the tree and the sticky, milk colored latex sap collected and refined...
s.
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.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 viscoelasticViscoelasticity
Viscoelasticity is the property of materials that exhibit both viscous and elastic characteristics when undergoing deformation. Viscous materials, like honey, resist shear flow and strain linearly with time when a stress is applied. Elastic materials strain instantaneously when stretched and just...
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, and 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.
| Durometer | Indenting foot | Applied mass [kg] | Resulting force [N] |
|---|---|---|---|
| Type A | Hardened steel rod 1.1 mm - 1.4 mm diameter, with a truncated 35º cone, 0.79 mm diameter | 0.822 | 8.064 |
| Type D | Hardened steel rod 1.1 mm - 1.4 mm diameter, with a 30º conical point, 0.1 mm radius tip | 4.550 | 44.64 |
The ASTM D2240 standard recognizes twelve different durometer scales using combinations of specific spring forces and indentor configurations. These scales are properly referred to as durometer types; ie a durometer type is specifically designed to determine a specific scale, and the scale does not exist separately from the durometer. The table below provides details for each of these types, with the exception of Type R.
| Durometer Type | Configuration | Diameter | Extension | Spring force |
|---|---|---|---|---|
| A | 35° truncated (frustum Frustum In geometry, a frustum is the portion of a solid that lies between two parallel planes cutting it.... ) cone |
1.4 mm (0.0551181102362205 in) | 2.54 mm (0.1 in) | 822 gf (8.1 N) |
| C | 35° truncated (frustum) cone | 1.4 mm (0.0551181102362205 in) | 2.54 mm (0.1 in) | 4536 gf (44.5 N) |
| D | 30° cone | 1.4 mm (0.0551181102362205 in) | 2.54 mm (0.1 in) | 4536 gf (44.5 N) |
| B | 30° cone | 1.4 mm (0.0551181102362205 in) | 2.54 mm (0.1 in) | 822 gf (8.1 N) |
| M | 30° cone | 0.79 mm (0.0311023622047244 in) | 1.25 mm (0.0492125984251969 in) | 78 gf (0.7649187 N) |
| E | 2.5 mm (0.0984251968503937 in) spherical radius | 4.5 mm (0.177165354330709 in) | 2.54 mm (0.1 in) | 822 gf (8.1 N) |
| O | 1.2 mm (0.047244094488189 in) spherical radius | 2.4 mm (0.094488188976378 in) | 2.54 mm (0.1 in) | 822 gf (8.1 N) |
| OO | 1.2 mm (0.047244094488189 in) spherical radius | 2.4 mm (0.094488188976378 in) | 2.54 mm (0.1 in) | 113 gf (1.1 N) |
| DO | 1.2 mm (0.047244094488189 in) spherical radius | 2.4 mm (0.094488188976378 in) | 2.54 mm (0.1 in) | 4536 gf (44.5 N) |
| OOO | 0.635 mm (0.025 in) spherical radius | 10.7 mm (0.421259842519685 in) - 11.6 mm (0.456692913385827 in) | 2.54 mm (0.1 in) | 113 gf (1.1 N) |
| OOO-S | 10.7 mm (0.421259842519685 in) radius disk | 12 mm (0.47244094488189 in) | 5 mm (0.196850393700787 in) | 197 gf (1.9 N) |
Note: "Type R is a designation, rather than a true ‘type’. The R designation specifies a presser foot diameter (hence the R, for radius, obviously D could not be used) of 18 ± 0.5 mm (0.71 ± 0.02 in) in diameter, while the spring forces and indentor configurations remain unchanged. The R designation is applicable to any D2240 Type, with the exception of Type M; the R designation is expressed as Type xR, where x is the D2240 type, eg aR, dR, etc; the R designation also mandates the employment of an operating stand".
The final value of the hardness depends on the depth of the indenter after it has been applied for 15sec on the material. If the indenter penetrates 2.54 mm (0.100 inch) 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.
| Material | Durometer | Scale |
|---|---|---|
| Bicycle gel seat | 15-30 | OO |
| Chewing gum Chewing gum Chewing gum is a type of gum traditionally made of chicle, a natural latex product, or synthetic rubber known as polyisobutylene. For economical and quality reasons, many modern chewing gums use rubber instead of chicle... |
20 | OO |
| Sorbothane Sorbothane Sorbothane is the brand name of a synthetic viscoelastic urethane polymer used as a shock absorber and vibration damper. It is manufactured by Sorbothane, Inc., based in Kent, Ohio.Sorbothane was invented and patented in 1982 by Dr. Maurice Hiles... |
40 | OO |
| Sorbothane Sorbothane Sorbothane is the brand name of a synthetic viscoelastic urethane polymer used as a shock absorber and vibration damper. It is manufactured by Sorbothane, Inc., based in Kent, Ohio.Sorbothane was invented and patented in 1982 by Dr. Maurice Hiles... |
30-70 | A |
| Rubber band Rubber band A rubber band is a short length of rubber and latex formed in the shape of a loop and is commonly used to hold multiple objects together... |
25 | A |
| Door seal | 55 | A |
| Automotive tire tread | 70 | A |
| Soft wheels of rollerskates and skateboard Skateboard A skateboard is typically a specially designed plywood board combined with a polyurethane coating used for making smoother slides and stronger durability, used primarily for the activity of skateboarding. The first skateboards to reach public notice came out of the surfing craze of the early 1960s,... |
78 | A |
| Hydraulic O-ring O-ring An O-ring, also known as a packing, or a toric joint, is a mechanical gasket in the shape of a torus; it is a loop of elastomer with a disc-shaped cross-section, designed to be seated in a groove and compressed during assembly between two or more parts, creating a seal at the interface.The O-ring... |
70-90 | A |
| Hard wheels of rollerskates and skateboard | 98 | A |
| Ebonite Rubber Ebonite Ebonite is a brand name for very hard rubber first obtained by Charles Goodyear by vulcanizing rubber for prolonged periods. It is about 30% to 40% sulfur. Its name comes from its intended use as an artificial substitute for ebony wood... |
100 | A |
| Solid truck tires | 50 | D |
| Hard hat Hard hat A hard hat is a type of helmet predominantly used in workplace environments, such as construction sites, to protect the head from injury by falling objects, impact with other objects, debris, bad weather and electric shock. Inside the helmet is a suspension that spreads the helmet's weight over the... (typically HDPE) |
75 | D |
Relation between Shore hardness and elastic modulus
A semi-empirical relation between the Shore hardness and the Young's modulusYoung's modulus
Young's modulus is a measure of the stiffness of an elastic material and is a quantity used to characterize materials. It is defined as the ratio of the uniaxial stress over the uniaxial strain in the range of stress in which Hooke's Law holds. In solid mechanics, the slope of the stress-strain...
for elastomer
Elastomer
An elastomer is a polymer with the property of viscoelasticity , generally having notably low Young's modulus and high yield strain compared with other materials. The term, which is derived from elastic polymer, is often used interchangeably with the term rubber, although the latter is preferred...
s has been derived by Gent. This relation has the form
where
is the Young's modulus in MPa and is the Shore hardness. This relation gives a value of at but departs from experimental data for .Another relation that fits the experimental data slightly better is
where
is the error functionError function
In mathematics, the error function is a special function of sigmoid shape which occurs in probability, statistics and partial differential equations...
and
is in units of Pa.A first order estimate of the relation between Shore D hardness and the elastic modulus for a conical indenter with a 15 degree cone is
where
is the Shore-D hardness and is in MPa.Another linear relation between the Shore hardness and the logarithm of Young's modulus is applicable over a large range of Shore A and Shore D hardnesses. This relation has the form:
where
is the Shore A hardness, is the Shore D hardness, and is the Young's modulus in MPa.See also
- Brinell hardness test
- Bloom (test)Bloom (test)Bloom is a test to measure the strength of a gel or gelatin. The test was originally developed and patented in 1925 by O. T. Bloom. The test determines the weight needed by a probe to deflect the surface of the gel 4 mm without breaking it. The result is expressed in Bloom . It is usually between...
- Hardness
- Knoop hardness testKnoop hardness testThe Knoop hardness test is a microhardness test - a test for mechanical hardness used particularly for very brittle materials or thin sheets, where only a small indentation may be made for testing purposes...
- Leeb Rebound Hardness TestLeeb rebound hardness testThe Leeb rebound hardness test is one of the four most used method for testing metal hardness. This portable method is mainly used for testing sufficiently large workpieces .-History:...
- Rockwell hardness test
- Vickers hardness testVickers hardness testThe Vickers hardness test was developed in 1924 by Smith and Sandland at Vickers Ltd as an alternative to the Brinell method to measure the hardness of materials. The Vickers test is often easier to use than other hardness tests since the required calculations are independent of the size of the...