Other Methods of Hardness Measurement

Dynamic hardness of a material is measured via dropping a minute weight from a fixed height upon the surface of the sample and measuring its rebound height. The instrument whether this measurement is made is termed as shore scelero-scope.

Now another dynamic hardness measurement method was enhanced via Martel. He employed a pyramidal indenter upon that a ram falls to generate an indentation of sample surface beneath dynamic loading state. The Martel Hardness Number or MHN is explained as:

                                                    MHN = Wh/V ..................Eq(6)
 
Here,     W = weight of falling arm,
h = height of fall, and
V = volume of indentation.

Relationship between Hardness and Other Properties

The property of hardness is concerned to the binding forces in between molecules and atoms. The hardness likes other strength, increase along with the binding forces. The molecular solids, as plastics are soft and contain low strength whilst ionic and metallic solids are stronger and also harder. The covalent solids are hardest. Diamond, the suitably known hardest solid has covalent bonding.

The dependence of both hardness and strength upon the binding forces among the atoms will certainly correlate the hardness and strength, a fact that was pointed out in previously section. However, since the mechanism of indentation is rather complex and occupies such factors as triaxial stress state, creep, strand hardening and friction, this is not theoretically probable to correlate hardness along with properties of strength, 32 creep and friction etc. Through, attempts have been made to generate closely fairly accurate relations and one among hardness and such ultimate tensile strength is just ultimate tensile strength equivalents 3.4 BHN in N/mm2. Figure in given below describes the link between hardness on various scales and ultimate tensile strength of structural steels. Same relationship exists for various materials except then them will be not same than the ones shown in given figure.

                       Figure: Approximate Relationship between Tensile Strength and different Hardness Numbers

The procedures used for enhancing the material strength, as like: heat treatment, alloying or treatments in mechanical result in also the raise in hardness. Thus, hardness check of specimens is the rapid and easy technique of that treatment of ascertaining the success. The hardness test is most generally employed for all manufacturing operations like a measure of quality and uniformity.