Hardness generally refers to the resistance of a material to scratching or indentation, and a qualitative measure of the strength of the material. The most commonly used hardness tests are the Rockwell test, Brinell test and the Vickers hardness test. This article will provide an introduction into durometer hardness and the gauge used to measure it. Hardness is used as a qualitative basis for material comparison, manufacturing specifications, heat treatment, quality control, and association with other material properties. It is commonly used to determine wear resistance, such as in gear teeth, or the material used in the grinding of ores for example.
Durometer Hardness and Testing
The term durometer has been used both to describe hardness measurement, as well as the actual instrument or gauge used to test the material. A durometer gauge measures Shore hardness, which is also referred to as “durometer hardness”.
The depth of penetration of the indenter into the material determines the hardness value of the material. The test is not a good prediction of material properties like resistance to scratch, wear, or abrasion, and should only be used as a supplement when it comes to specifications for design.
The Shore Scales
A common way of measuring hardness of plastics is through Shore durometer testing. Shore is a measure of the resistance of the plastic material to indentation. Shore A or Shore D scales are used as scales for shore hardness with Shore A being used for soft material, and Shore D being used for harder material.
The Shore A scale is a measure of the Shore A hardness of elastic materials such as rubber or soft plastics, and the Shore D scale is a measure of the Shore D hardness of hard plastics and rubbers. The dimensionless reading of the Shore A and D readings range between 0 and 100; where 0 represents complete penetration, and 100 represents no penetration. Basically, the higher the number, the higher resistance the material has to indentation.
Measure of Modulus
Durometer is as much a measure of the modulus of an elastomeric material. Modulus of elasticity is the ratio of stress to strain in a material. This is shown in the slope of the stress-strain-curve (link). Stress-strain is non-linear for elastomers, which makes it very dependent on the conditions of measurement. If the hardness is low then this would imply a low modulus or stiffness. If the hardness is high then this would imply a high modulus or stiffness. It’s worth noting that durometer is an indirect measurement of the modulus and they are not directly proportional.
Quality control is not always achieved since durometers allow for large variations in modulus even though stiffness may be tightly controlled during measurement. Sensitivity wise, the durometer measurement is very sensitive at lower hardness and almost non-dependent on modulus at higher hardness values. So as the hardness increases, the sensitivity of the device decreases and in fact goes to zero.
For rubbers, stress relaxation and compression are important properties in the life of a seal for example, which are functions of the stiffness of the material and are dependent on time. The modulus of rubber is a measure of how much spring force the rubber will have when subject to deformation. Durometer measurements are taken on rubber to ensure the material remains intact both mechanically and chemically, to make certain the rubber element seals over the life of the product.
Durometer Gauges
The depth of surface penetration is measured using a durometer gauge which uses predetermined geometry. The durometer gauge is a machine shop dial indicator which measures deflection between 0.000” inches, and 0.100” inches. Durometer is used as a stiffness indicator since the depth of the penetration is directly related to the force required to make the penetration. Both force and deflection are measured. Stiffness is derived from the relationship between force and deflection. Durometer gauges are inexpensive and do not require a lot of training, making them affordable and easy to use.
In summary
Durometer is a standard in industry for measuring the hardness (or indirectly, modulus) of a material. It is commonly used to measure the hardness in rubber and plastic materials and a durometer gauge is a quick and easy way to obtain a measurement of stiffness of a material. The Shore scale is a common way of measuring the hardness of materials with Shore A being used for softer materials and Shore D being used for harder materials. Durometer is more of a measure of the modulus of a material and can be greatly affected by ambient conditions, and so care should be taken that comparable measurements are carried out in the same environment.
