Hardness Testing
Hardness is a characteristic of a material, not a fundamental physical property.It is defined as a measure of the resistance to localised plastic deformation induced by either mechanical indentation or abrasion.
It is determined by measuring the permanent depth of the indentation. Some materials (e.g. metals) are harder than others (e.g. plastics). More simply put, an indenter is pressed into the surface of the material to be tested under a specific load for a definite time interval, and a measurement is made of the size or depth of the indentation, hence, the smaller the indentation, the harder the material.
Hardness has a variety of meanings:
» To the Metals industry, it may be thought of as resistance to permanent deformation.
» To the Metallurgist, it means resistance to penetration.
» To the Lubrication Engineer, it means resistance to wear.
» To the Design Engineer, it is a measure of flow stress.
» To the Mineralogist, it means resistance to scratching,
» To the Machinist, it means resistance to machining.
Hardness may also be referred to as mean contact pressure. All of these characteristics are related to the plastic flow stress of materials.
Purpose of Hardness Testing
The principal purpose of the hardness test is to determine the suitability of a material for a given application, or the particular treatment to which the material has been subjected. Determining these material properties provides valuable insight to the durability, strength, flexibility, and capabilities of a variety of component types from raw materials to prepared specimens, and finished goods.
Why is Hardness testing so Valuable?
The hardness test is, by far, the most valuable and most widely used mechanical test for evaluating the properties of metals as well as certain other materials. The ease with which the hardness test can be made has made it the most common method of inspection for metals and alloys. Certainly, as a material characteristic, its value and importance cannot be understated.
Principally, the importance of hardness testing has to do with the relationship between hardness and other material properties, that is, the information from a hardness test complements and is often used in conjunction with other material verification techniques such as tensile or compression to provide critical performance information. For example, both the hardness test and the tensile test measure the resistance of a metal to plastic flow, and results of these tests may closely parallel each other. In the end, the hardness test is ,therefore, preferred because it is simple, easy, and relatively Non-Destructive.
