Mechanical Properties of Metals

Mechanical Properties of Metals Points : Mechanical Properties of Metals, Hardness, Toughness or (tenacity), Ductility, Malleability, Brittleness, Elasticity, Plasticity, Stiffness, Fatigue, Creep, 1. Hardness
2. Toughness or (tenacity)
3. Ductility
4. Malleability
5. Brittleness
6. Elasticity
7. Plasticity
8. Stiffness
9. Fatigue
10. Creep 1. Hardness Hardness has been variously defined as resistance to local penetration, to scratching, to machining, to wear as abrasion, and to 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, indults ,of elasticity and others. Thus, hardness is the property of th6 metal which gives it the ability to resist being permanently deformed when a load is applied. The hardness is ‘judged by the ease with which the object can be scratched. One metal is said to be harder if it scratches the other. The greater the hardness, the greater is the resistance towards the scratch, When the heat treated steel cannot he field, it is termed as file hard. Hardness testing machines are employed for more exact measurements of hardness. 2. Toughness It is the property of the material which enables it to be twisted, bent or stretched under a high stress before rupture It is measured by the amount of energy that a unit volume of the material has absorbed after being stressed up to the point of fracture. Toughness decreases when material is heated. An elastic or ductile material requiring great forces for deformation and having resistance against break called tough. 3. Ductility it is the property of the material which enables it to be drawn out or elongated to an appreciable extent before rupture occurs. Ductility of material can be measured by the percentage of elongation and the percentage of reduction of area before rapture of a test piece. This property depends very largely upon tenacity and to some extent upon hardness. Ductility of a metal is usually much less when hot than when cold, hence wires are drowning cold. Glass on the other hand, is extremely ductile when hot, and may be drawn out into a very fine thread. In doing a tensile test, ductile material will show quite an appreciable amount of strength of elongation before fracture takes place, but brittle material, on the other hand, exhibits no perceptible increase in length under similar circumstances. 4. Malleability This is the property by virtue of which a material may be hammered or rolled into thin sheets without rupture. This property generally increases with the increase of temperature. 5. Brittleness It is the property of the material which is opposite to ductility. Material, having very, little property of deformation, either elastic or plastic is called brittle. Although they may resist pretty every loads whet) smoothly applied, they will readily break at any point. The lack ductility is commonly called brittleness. Therefore, a non-ductile material is said to be brittle material. Usually the tensile strength of brittle materials is only a fraction of their compressive strength. 6. Elasticity It is the property of material which a enables it to regain its original shape after deformation within the elastic limit. This property is desirable in materials used in tools and machines. 7. Plasticity Plasticity is the property that enables the formation of permanent deformation material. It is reverse of elasticity a plastic material will retain exactly the shape it this under load, even after the load is removed. Gold and lead are the highly plastic material Plasticity is used in stamping images on coins and ornamental work. 8. Stiffness It is the property of material which enables it t resist deformation. Modulus of elasticity is the measure of stiffness, A material which offers slight deformation under load has a high degree of stiffness. For instance, suspended beams of steel and Aluminum may both be strong enough to carry the required load but the Aluminum beam will sag or deflect further. In other words, the steel beam is stiffer than Aluminum. 9. Fatigue When subjected to fluctuating or repeating loads (or tresses), materials tend to develop a characteristic behavior which is different from that (or materials) under steady loads. Fatigue is the phenomenon that leads to fracture under such conditions. Fracture takes place under repeated. or fluctuating stresses whose maximum value is less than the tensile strength of the materials (under steady load). Fatigue fracture is progressive, beginning as minute cracks that grow. 10. Creep Creep is the slow plastic deformation of metals under constant stress or under prolonged loading usually at high temperature. It can take place and lead to fracture at static stresses much smaller than those which will break the specimen by loading it quickly. Creep is specially taken care of while designing I,C. engines, boilers and turbines.