Plastic testing

The test sequence can be described as follows:

1. Test step:

First the indenter is pressed with a test pre-force of 98.07 N (also referred to as pre-force or preload) to a penetration depth of h0 in the specimen to be tested. h0 defines the reference level (basis) for subsequent measurement of the residual indentation depth (h).

2. Test step:

Next the test force is applied for a dwell time defined in accordance with the standard (several seconds), whereby the indenter penetrates into the specimen to a maximum indentation depth of h1.

3. Test step:

On conclusion of the dwell time, the test force is removed, the indenter moves up by the elastic proportion of the penetration depth in the total test force and remains at the level of the residual indentation depth h. This is also referred to as the depth differential (difference in indentation depth before and after application of the test force). Now the Rockwell hardness (HR) can be calculated, using the residual indentation depth (h) and a formula defined in the standard, taking account of the applied Rockwell scale.

The individual methods are distinguished from each other by:

• Diameter of the indenter ball
• Magnitude of applied test force (or test load) 

The resulting test methods use two different ball diameters (steel balls with diameters of: 6.35 and 12.7 mm) and two different test loads (588.4 N and 980.7 N). The following table shows the test methods standardised according to ISO 2039-2.

Rockwell hardness scale	Preload [N]	Test load [N]	Indenter diameter [mm]
R	98,07	588,4	12,7
L	98,07	588,4	6,35
M	98,07	980,7	6,35
E	98,07	980,7	3,175

The hardness value consists of two components:

1. A numeric hardness value;
2. The two letters "HR", standing for "Hardness according to Rockwell";

Example of how to represent and read a hardness value:

65 HR
65 …hardness value
HR …according to Rockwell