Hardness , Creep tests

Hardness has already been defined as the resistance of a material to indentation or abrasion by another hard body ( good hardness generally means that the material is resistant to scratching and wear) . It is by indentation that most hardness tests are performed. A hard indenter is pressed into the specimen by a standard load, and the magnitude of the indentation (either area or depth) is taken as a measure of hardness.
Hardness tests are commonly used for assessing material properties because they are quick and convenient. However, a variety of testing methods is appropriate due to differences in hardness among different materials. The most well known hardness tests are Brinell and Rockwell.
Hardness is a measure of the material’s resistance to localized plastic deformation (e.g. dent or scratch)
A qualitative Moh’s scale, determined by the ability of a material to scratch another material: from 1 (softest = talc) to 10 (hardest = diamond).
Diamond 10
Corundum 9
Topaz 8
Quartz 7
Orthoclase (Feldspar) 6
Apatite 5
Fluorite 4
Calcite 3
Gypsum 2
Talc 1
Different types of quantitative hardness test have been designed (Rockwell, Brinell, Vickers, etc.).
Usually a small indenter (sphere, cone, or pyramid) is forced into the surface of a material under conditions of controlled magnitude and rate of loading. The depth or size of indentation is measured.
The tests somewhat approximate, but popular because they are easy and non-destructive (except for the small dent).
Some limitation for hardness test as follows :-
•?Type of material
•?Specimen thickness
•?Test location
•?Scale limitations

1. The Brinell hardness test

J.A. Brinell introduced the first standardized indentation-hardness test in 1900.
The Brinell hardness test consists in indenting the metal surface with a 10-mm diameter steel or tungsten carbide ball at a load range of 500-3000 kg, depending of hardness of particular materials.
The load is applied for a standard time ( between 10 - 30 s), and the diameter of the indentation is measured. Giving an average value of two readings of the diameter of the indentation at right angle.

• The Brinell hardness number (BHN or HB) is expressed as
2. the load P divided by surface area of the indentation
In this test, hardness is measured by pressing a hard steel ball into the surface of the test piece, using a known load. It is important to choose the combination of load and ball size carefully so that the indentation is free from distortion and suitable for