Material hardness and hardness testing...

Material hardness and hardness testing Material hardness is the property of the material which enables it to resist plastic deformation, usually by penetration or by indentation. The term of hardness is also referred to stiffness or temper, or to resistance to bending, scratching, abrasion, or cutting. It is the property of a material, which gives it the ability to resist being permanently, deformed when a load is applied. The greater the hardness of the material, the greater the resistance it has to deformation. Hardness has been variously defined as resistance to local penetration, scratching, machining, wear or abrasion, and yielding. The multiplicity of definitions, and corresponding multiplicity of hardness measuring instruments, together with the lack of a fundamental definition, indicates that hardness may not be a fundamental property of a material, but rather a composite one including yield strength, work hardening, true tensile strength, modulus of elasticity, and others. In mineralogy, hardness is normally described as the resistance of a material to being scratched by another material. The ability of materials to resist scratching by another material can be ranked by referring to the Mohs scale which assesses relative hardness of the materials. In metallurgy hardness is defined as the ability of a material to resist plastic deformation. It is sometimes known as indentation hardness which is the resistance of a material to indentation. The usual type of hardness test is where a pointed or rounded indenter is pressed into a surface of the material under a substantially static load. Hardness measurement can be carried out at macro scale, micro scale or nano scale according to the forces applied and displacements obtained. Measurement of the macro hardness of the material is a quick and simple method of finding mechanical property data for the bulk...

Mechanical Properties of Steels...

Mechanical Properties of Steels The most important properties of steels which account for their widespread use are their mechanical properties. These properties include a combination of very high strength with the ability to bend rather than break. Different tests have been developed to describe the strength and ductility (a measure of bendability) of steels. A number of these tests which are used to describe the mechanical properties of steels are described below. Tensile testing Tensile testing of steel is a kind of a testing done for the evaluation of the strength of steels. A length of the steel material, usually a round cylindrical rod, is pulled apart in a machine that applies a known force, F. The machine has grips which are attached to the ends of the cylindrical steel rod, and the force is applied parallel to the axis of the rod, as shown schematically in Fig 1. As the force increases, the rod gets longer, and the change in length is represented as delta l (? l), where the symbol delta (?) means ‘a change in’ and the l refers to the original length of the rod. If a force of 50 kg is applied to two rods of the same steel material, where one is thin and the other thick then the thin rod elongate more. To compare their mechanical properties independent of rod diameter, the term ‘stress’ is used. Stress is simply the force divided by the cross-sectional area of the rod. When the same stress is applied to the thin and thick rods, they elongate the same amount, because the actual force applied to the thick rod is now larger than that applied to the thin rod by an amount proportional to its larger area. Because stress is force...