Indentation tests Rockwell test, depth reading

An alternative to the measurement of the dimensions of the indentation by means of a microscope is the direct reading method, of which the Rockwell method is an example. The Rockwell hardness is based on indentation into the sample under the action of two consecutively applied loads - a minor load (initial) and a standardised major load (final). In order to eliminate zero error and possible surface effects due to roughness or scale, the initial or minor load is first applied and produce an initial indentation. The Rockwell hardness is based on the increment in the indentation depth produced by the major load over that produced by the minor load. Rockwell hardness scales are divided into a number of groups, each one of these corresponding to a specified penetrator and a specified value of the major load. The different combinations are designated by different subscripts used to express the Rockwell hardness number. Thus, when the test is performed with 150 kg load and a diamond cone indentor, the resulting hardness number is called the Rockwell C (Rc) hardness. If the applied load is 100 kg and the indentor used is a 1.58 mm diameter hardened steel ball, a Rockwell B (RB) hardness number is obtained. The facts that the dial has several scales and that different indentation tools can be filled, enable Rockwell machine to be used equally well for hard and soft materials and for small and thin specimens. Rockwell hardness number is dimensionless. The test is easy to carry out and rapidly accomplished. As a result it is used widely in industrial applications, particularly in quality situations. 

The Rockwell testing machine is thus a framework permitting stable support of the workpiece on one side and means to impress the indenter under specified load on the other. A dial indicator attached to the indenter spindle is used to read direcdy the depth of indentation in hardness numbers. 

The standard test methods for determining Rockwell hardness are listed in Table 12.6 and the Rockwell scales are given in Table 12.7. For example, ISO 2039 employs a hardened steel ball, 5 mm diameter. The ball is pressed into the specimen under a specified load selected to give an indentation between 0.07 and 0.10 mm (Method A) or between 0.15 and 0.35 mm (Method B). The recommended thickness of the specimen is 4 mm and the suggested time of application of the load is 30 s before the depth reading is taken. 

The Rockwell test differs fi om the other three tests because the depth of the indent rather than its surface area is taken as a measure of hardness. A hardened steel ball is used as the indentor. A major advantage of the Rockwell test is that no visual measurement of the indentation is necessary, and the depth of the indent is read directly as a hardness value on the scale. 

Indentation Depth Reading Rockwell Test, in this test, the depth of the indentation is read from a dial indicator no microscope is required (34). In the Rockwell hardness tests, a load of 98 N is first applied to the surface and the depth of penetration is thereafter reckoned as the zero of measurement. A further load of 588, 980, or 1470 N is applied and removed leaving the additional depth of indentation recorded on a dial. The hardness is then expressed in terms of the dial reading on an arbitrarily numbered scale. The indenter used may be a steel spherical penetrator or a diamond cone with a hemispherical tip. The scales, indenter, and loads employed are chosen to adapt to the material properties. Results given by different testers are not readily interconverted. 

In this test the depth of indentation is read from a dial (ASTM, 1978) no microscope is required. In the most frequently used procedure, the Rockwell hardness does not measure total indentation but only the non-recoverable indentation after a heavy load is applied for 15 s and reduced to a minor load of 98 N for 15 s. Rockwell hardness data for a variety of polymers are reported by Maxwell (1955) and Nielsen (1963). 

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