PROPERTIES OF MATERIALS

Different materials possess different properties in varying degree and therefore behave in different ways under given conditions. These properties include Mechanical properties, Electrical properties, Thermal properties, Chemical properties, Magnetic properties and Physical properties. A design engineer is interested in the behaviour of materials under load which is mechanical in nature, for the design of machines & structures. Any material subjected to a load either deforms, yield, or break, depending upon the magnitude of the load. We are basically interested in knowing as to how a particular material will behave under applied load i.e., in knowing the mechanical properties.

PRINCIPAL MECHANICAL PROPERTIES

Those characteristics of the materials which describe their behaviour under external loads are known as Mechanical Properties. The most important and useful mechanical properties are:

Strength

It is the resistance offered by a material when subjected to external loading. So, stronger the material the greater the load it can withstand. Depending upon the type of load applied the strength can be tensile, compressive, shear or torsional. The maximum stress that any material will withstand before destruction is called its ultimate strength.

Elasticity

Elasticity of a material is its power of coming back to its original position after deformation when the stress or load is removed. Elasticity is a tensile property of its material. The greatest stress that a material can endure without taking up some permanent set is called elastic limit.

Stiffness (Rigidity)

The resistance of a material to deflection is called stiffness or rigidity. Steel is stiffer or more rigid than aluminium. Stiffness is measured by young’s modulus E. The higher the value of the young’s modulus, the stiffer the material. E is the ratio of stress over strain and is given by the slope of line O–A.

Plasticity

The plasticity of a material is its ability to undergo some degree of permanent deformation without failure. Plastic deformation will take place only after the elastic range has been exceeded, beyond point b. Plasticity is an important property and widely used in several mechanical processes like forming, shaping, extruding and many other hot and cold working processes. In general, plasticity increases with increasing temperature and is a favourable property of material for secondary forming processes. Due to these properties various metal can be transformed into different products of required shape and size. This conversion into desired shape and size is affected either by the application of pressure, heat or both.

Ductility

Ductility of a material enables it to draw out into thin wire on application of the load. Mild steel is a ductile material. The wires of gold, silver, copper, aluminium, etc. are drawn by extrusion or by pulling through a hole in a die due to the ductile property. The ductility decreases with increase of temperature. The per cent elongation and the reduction in area in tension is often used as empirical measures of ductility.

Malleability

Malleability of a material is its ability to be flattened into thin sheets without cracking by hot or cold working. Aluminium, copper, tin, lead, steel, etc. are malleable metals. Lead can be readily rolled and hammered into thin sheets but cannot be drawn into wire. Ductility is a tensile property, whereas malleability is a compressive property. Malleability increases with increase of temperature.

Brittleness

The brittleness of a material is the property of breaking without much permanent distortion. There are many materials, which break or fail before much deformation take place. Such materials are brittle e.g., glass, cast iron. Therefore, a non-ductile material is said to be a brittle material. Usually, the tensile strength of brittle materials is only a fraction of their compressive strength. A brittle material should not be considered as lacking in strength. It only shows the lack of plasticity. On stress-strain diagram, these materials don’t have yield point and value of E is small.

Toughness

The toughness of a material is its ability to withstand both plastic and elastic deformations. It is a highly desirable quality for structural and machine parts to withstand shock and vibration. Manganese steel, wrought iron, mild steels are tough materials. For Ex: If a load is suddenly applied to a piece of mild steel and then to a piece of glass the mild steel will absorb much more energy before failure occurs. Thus, mild steel is said to be much tougher than a glass. Toughness is a measure of the amount of energy a material can absorb before actual fracture or failure takes place. “The work or energy a material absorbs is called modulus of toughness” Toughness is also resistance to shock loading. It is measured by a special test on Impact Testing Machine.

Hardness

Hardness is closely related to strength. It is the ability of a material to resist scratching, abrasion, indentation, or penetration. It is directly proportional to tensile strength and is measured on special hardness testing machines by measuring the resistance of the material against penetration of an indentor of special shape and material under a given load. The different scales of hardness are Brinell hardness, Rockwell hardness, Vicker’s hardness, etc. Hardness of a metal does not directly relate to the hardenability of the metal. Hardenability is indicative of the degree of hardness that the metal can acquire through the hardening process. i.e., heating or quenching.

Hardenability

Hardenability is the degree of hardness that can be imparted to metal by process of hardening. A metal capable of being hardened throughout its structure is said to have high hardenability. The material is heated above a certain temperature and then suddenly quenched in a cold oil or water bath.

Impact Strength

It can be defined as the resistance of the material to fracture under impact loading, i.e., under quickly applied dynamic loads. Two standard tests are normally used to determine this property.

1. The IZOD impact test.

2. The CHARPY test.

Resilience

Resilience is the capacity of material to absorb energy elastically. On removal of the load, the energy stored is released as in a spring. The maximum energy which can be stored in a body up to elastic limit is called the proof resilience. The quantity gives capacity of the material to bear shocks and vibrations. The strain energy stored in a material of unit volume gives proof resilience and is measured by work stretching.

FATIGUE AND FATIGUE TEST

The fatigue strength of a material is the maximum stress at which failure may occur after a certain number of cyclic load applications. A component is designed to give a certain length of service under a specified loading cycle. Many components of high speed aero and turbine engines are designed for fatigue strength. The fatigue strength or endurance limit of material is used in the design of parts subjected to repeated alternating stresses over an extended period of time. Specimens are tested to failure using different loads. The number of cycles is noted for each load. The results of such tests are plotted as graphs of applied stress against the logarithm of the number of cycles of failure. The curve is known as S-N curve. The tests are carried out on special fatigue testing machines.

CREEP AND CREEP TESTING

The slow and continuous elongation of a material with time at constant stress and high temperature below elastic limit is called creep. At high temperatures, stresses even below the elastic limit can cause some permanent deformation on stress-strain diagram. There are three stages of creep. In the first stage the material elongates rapidly but at a decreasing rate. In the second stage, the rate of elongation is constant. In third stage, the rate of elongation increases rapidly until the material fails. The stress for a specified rate of strain at a constant temperature is called creep strength. Creep test is carried out at high temperature. A creep curve is a plot of elongation of a tensile specimen versus time, for a given temperature and under constant stress. Tests are carried out for a period of a few days to many years. The test can be carried out on Universal Testing Machine with special attachments.

Creep curve shows four stages of elongation:

(a) Instantaneous elongation on application of load.

(b) Primary creep: Work hardening decreases and recovery is slow.

(d) Tertiary Creep: Grain boundary cracks. Necking reduces the cross-sectional area of the test specimen. The creep strength is used for the design of blades and other parts of steam and gas turbines working at high temperatures.

PROPERTIES OF MATERIALS | PRINCIPAL MECHANICAL PROPERTIES