Types of Tests

Hardness

Simply stated, hardness is the resistance of a material to permanent indentation. There are several different hardness tests, using differing indentation techniques, that give different hardness values for the same piece of material. This means that hardness measurements are method dependent and every test result must have a label identifying the test method used.

Hardness is, however, used extensively to characterize materials and to determine if they are suitable for their intended use. This is primarily because of the advantages of the hardness test, namely they are quick and easy, finished parts can be tested and used since the test is a non-destructive test and almost any shape and size of specimen can be tested.

All of the hardness tests use of a specifically shaped indenter, significantly harder than the test sample, that is pressed into the surface of the sample using a specific force. Either the depth or size of the indent is measured to determine a hardness value. The most common uses for hardness tests is to verify the heat treatment of a part and to determine if a material has the properties necessary for its intended use.

There are five major hardness scales, which in alphabetical order are:

Brinell - HB

Knoop - HK

Rockwell - HR

Shore - HS

Vickers - HV

Each involve the use of a specifically shaped diamond, carbide or hardened steel indenter pressed into the material with a known force using a defined test procedure. The hardness values are determined by measuring either the depth of indenter penetration or the size of the resultant indent – the smaller the indent, the harder the materials, the higher the hardness number. The hardness values must also be reported with the proper test scale symbol, HB, HK, HR, etc.