hardship level
In the Shore hardness test, the hardness is based on the return value of a hammer with a sharp pointed diamond placed at the end and released from a certain standardized height (about 250mm) perpendicular to the surface of the test piece.
High compressive strength
Hardness test of polymers
Hardness measurement is one of the most basic and common physical and mechanical tests of materials. Hardness is defined as a material’s resistance to penetration and indentation caused by a harder solid object, or in other words, resistance to creating permanent dents on the surface of the material and changing the shape of the surface.
According to the definition of difficulty, standard tests have been defined to measure this property. According to Figure 1, in these tests, a hard penetrating object is pressed to the surface of the tested sample. By applying pressure, a three-way stress (tensile, compressive and shear) is created and causes a change in the surface of the sample. This test is very common and used because of its simplicity. Due to the non-destructive nature of these tests, it is possible to measure the hardness of small components and thin layers. In Figure 2, a test equipment can be seen.
Rockwell hardness test
The Rockwell hardness test consists of a cone-shaped indenter made of diamond or steel. First, the initial force ̥F is applied by the penetrator for 10 seconds and penetrates to the extent of h1, then the force F1 is applied for 15 seconds and penetrates to the extent of h2. Rockwell hardness value is calculated according to Figure 4. The standard of this hardness test is ISO2039-2. It is mainly used for hard plastics in which the elasticity or creep of the polymer has little effect on the test result, such as polycarbonate, nylon, polystyrene, and acetate.
Shore Durometer hardness test
Perhaps the most popular and common method of measuring the hardness of materials is the Shore Durometer method, which is divided into two different types. Shore A where a needle is shaped like an incomplete cone and Shore D where a needle is shaped like a cone with a spherical tip. (Figure 5) This test can be performed according to ASTM D2240 or ISO 868 standards. In both cases, the force is applied by a weight and a spring, and the indentation depth is a measure of hardness. One of the advantages of the Shore method is the portability of this test equipment due to its small size.
The Shore A test is used to determine the hardness of soft rubber and very soft plastics such as softened PVC. In this method, the incomplete conical needle has an angle of 35 degrees and a flat surface diameter of 0.79 mm, and it is calculated according to the following equation, where F is the applied force in millinewtons and Ha is the hardness value.
F = 550 + 75 Ha
The Shore D test is used for hard rubbers and thermoplastics such as PTFE. The placement angle of the needle in this method is 30 degrees with a spherical radius of 0.1 mm, which is calculated according to the following equation.
F = 445 Hd
Table 1- Application of Shore A and Shore D hardness test in different materials
