In order to prevent the test base (e.g. the test anvil) from affecting the test result in any way, a minimum specimen thickness must be observed with the Rockwell method (acc. to ISO 6508).
With cone-shaped indenters, this minimum thickness of the specimen or layer to be tested is ten times the residual indentation depth and fifteen times the residual indentation depth for ball-shaped indenters.
The minimum specimen thickness can be taken from the graphs below in relation to Rockwell hardness.The Rockwell graph includes the methods HRA, HRC, HRD. The Super Rockwell graph includes the methods HR15N, HR30N, HR45N, HR15TW, HR30TW, HR45TW.