2.0 PROPERTIES OF MATERIALS
Introduction
Property of a material (or material property) is a factor that influences qualitatively or quantatively the response of given a material to imposed stimuli and constraints. e.g.: - forces, temperature, etc.
Some properties are dependent on actual crystal structure and are called structure sensitive properties. In these crystal defects will affect their property.
e.g.:- strength, ductility, di-electric strength etc…
Structure sensitive material properties are independent on crystal structure.
E.g.:-stiffness, density, and electrical conductivity.
Important material properties are:-
- Physical properties
- Thermal properties
- Magnetic properties
- Electrical properties
- Mechanical and Technological properties
- Chemical properties
- Optical properties
- Physical Properties
These are employed to describe a material under conditions in which external forces are not concerned. This include
Dimension: - Implies its size and shape
Appearance: - This includes lustre, color and finish of a material. Lustre is the ability of a material to reflect light when finely polished. Colour of a material depends upon the wavelength of the light that it can absorb.
Density: - It is the mass of unit volume of material and its unit is Kg/m3.
i.e. Density= Mass / Volume
Specific gravity: - It is the ratio of the density of the material to the density of a substance taken as a standard. As it the ratio of same parameters, it has no dimension. Solids and liquids are referred to water as standard (at maximum density of 4degree Celsius, standard atmospheric pressure). If we use weight of body instead of mass the density is called weight density.
Melting Point: - It is that temperature at which the solid material changes into the molten state. At this state solid and liquid are in equilibrium.
Porosity: - A material is said to be porous if it has the pores within it. Pores can absorb the lubricant as in a sintered self-lubricating bearing.
Structure: - It implies geometric relationship of material components.
2.2 Thermal Properties
This means the response of a material to the application of heat, e.g.:- Heat capacity, specific heat, thermal expansion etc.
Heat capacity: - It indicates the materials ability to absorb heat from external surroundings. Its unit is J/K. in other words it is the amount of heat absorbed by the given mass of substance in raising its temperature by unit degree.
Specific heat: - Quantity of heat that must be added to a unit mass of the solid to raise its temperature by one degree. Its unit is J/ (Kg.K). It increases with temperature.
Thermal expansion: - The increase in volume of material as temperature increases is called thermal expansion.
Thermal conductivity: - This determines the rate at which heat can flow through a material under the influence of a given temperature gradient. It is measured by thermal conductivity.
- Magnetic Properties
Some of the magnetic properties are,
Permeability: - This measures the relative ease with which magnetism may be developed in a material. It is the ratio of the magnetic induction to the magnetic field.
Coercive force: - It is the opposing magnetic force, which is necessary to remove previous magnetization or residual magnetization.
Hysteresis: - It may be defined as the lag in the changes in the magnetization behind the variation of magnetic field.
Super conductivity: - It is a phenemnon referring to the state at which the abrupt drop in electrical resistance of certain materials at temperature below a certain critical temperature.
2.4 Electrical Properties
Resistivity: -It is that electrical property of a material owing to which it resists the flow of electricity through it. Its unit is Ohmmeter.
Conductivity: - It is due to this property the electrical current flows easily through the material. It is reciprocal of the resistivity. Its unit is Mho/meter.
Temperature coefficient of resistance: - It is employed to specify the variation of resistivity with temperature.
Dielectric strength: -This means the insulating capacity of a material against high voltages.
Thermo electricity: - This forms the basis for the thermocouple operation. If two dissimilar metals are joined and this junction is then heated, a small voltage in milli-volt range is produced and this is known as thermo electric effect.
2.5 Mechanical Properties
The mechanical properties of materials define the behavior of materials under the action of external forces, called loads. They are the measure of the strength and lasting characteristics of a material in service, and are of great importance in the design of tools, machines and structures.
Mechanical properties are structure sensitive in the sense that they depend up on the crystal structure and its bonding forces, and especially upon the nature and behavior of the imperfections, which exists within the crystal itself or at the grain boundaries.
The most important and useful mechanical properties are briefly explained below to ensure that the readers will be able to choose thee proper material for a given design quickly and wisely.
Strength: - The strength of a material is its capacity to withstand destruction under the action of external forces or loads. The stronger the material the greater the load it can withstand. The stronger the material the greater the load it can withstand. It therefore determines the ability of a material to withstand stress without failure.
The maximum stress that any material will withstand before destruction is called its ultimate strength.
Elasticity: - Elasticity is that property of a material by virtue of which deformation caused by applied loads disappears upon the removal of the load. In other words 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 a material.
Stiffness: -The resistance of a material to elastic deformation or deflection is called stiffness or rigidity. A material which suffers slight deformation under load has a high degree of stiffness or rigidity. For instance suspended beams of steel and aluminium may be strong enough to carry the required load but the aluminium beam will “sag” or deflect further. In other words, the steel beam is stiffer or more rigid than aluminium beam.
Plasticity: - The plasticity of a material is its ability to undergo some degree of permanent deformation without rupture or failure. Plastic deformation will take place only after the elastic range has been exceeded.
Plasticity is important for forming, shaping, extruding, and many other hot or cold working processes. Materials such as clay, lead, etc… are plastic at room temperature and steel is plastic when at bright heat. In general, plasticity increases with increasing temperature.
Ductility: - Ductility is that property of a material, which enables it to draw out into thin wire. Mild steel is a ductile material. The percentage elongation and the reduction in area in tension are often used as empirical measure 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.
Resilience: -Resilience is the capacity of the material to absorb energy elastically. On removal of the load, the energy stored is given off exactly as in spring when the load is removed.
The maximum energy, which can be stored in a body up to elastic limit, is called proof resilience, and the proof resilience per unit volume is called modulus of resilience. This property reveals capacity of material to bear shocks and vibrations. The resilience of a material is of great importance in the selection of materials that is used for various types of springs.
Toughness: - Toughness is a measure of the amount of energy a material can absorb before actual fracture or failure takes place. For example, if a load is suddenly applied to a piece of mild steel and then to a piece of a glass, the mild steel will absorb much more energy before failure occurs. Thus mild steel is said to be much tougher than a glass.
The toughness of a material is its ability to withstand both plastic and elastic deformations. It is, therefore, a highly desirable quality for structural and machine parts, which have to withstand shock and vibration. Manganese steel, wrought iron, mild steel, etc are tough materials.
Hardness: - Hardness is a fundamental property, which is closely related to strength. Hardness is usually defined in terms of the ability of a material to resist to scratching, abrasion, cutting, indentation, or penetration.
Hardenability: - Hardenablity indicates the degree of hardness that can be imparted to metal, particularly steel, by the process of hardening. It determines the depth and distribution of hardness induced by quenching. A metal that is capable of being hardened throughout its structure is said to have high hardenability.
Brittleness: - Brittleness of a material is the property of breaking without much distortion. There are many materials, which break or fail before much deformation takes place. Such materials are brittle, e.g. glass, cast iron. Therefore a non-ductile material is said to be brittle material.
Creep: - The slow and progressive deformation of a material with the time at constant stress is called creep. Creep can take place and leads to fracture at static stresses much smaller than those, which will break the specimen when loaded suddenly. For example creep occurs in steel at higher temperatures. This is always considered while designing I C engines, boilers, turbines, etc.
Fatigue: - It has been recognized that a metal part subjected to repetitive or fluctuating stress will fail at a stress at a much lower than that are required to cause fracture on a single application of load. The term “fatigue” applied to materials refers to premature failure under the action of repeated stresses. A component subjected to that type of stresses develops characteristics behavior, fundamentally different from the behavior of metal part subjected to steady loads. The behavior is called fatigue and is marked by
- Loss of strength
- Loss of ductility
- Increased uncertainly in both strength and service life.
Actually fatigue causes over 85% of the operating failures of the machine elements. Even in ductile materials, the failure of materials by fatigue will be sudden i.e. brittle like fracture. It is important in design of crankshafts, axles, aircraft wings, etc. It is found that for most materials there is a limiting stress below which a load may be repeatedly applied an indefinitely large number of times without causing failure. This limiting stress is called endurance or fatigue limit. So endurance limit is therefore used as a principle criterion of the resistance to fatigue of materials. The presence of stress raises such as notches at the surface lower the endurance limit.
The events that lead to fatigue i.e. mechanism include crank nucleation, crack growth, and fracture. Fatigue is a structure sensitive property. Surface condition, temperature, range of stress, frequency of stress cycle, environment and metallurgical factors can be considered as variables affecting fatigue life.
2.6 Technological Properties
The technological properties of a material are those properties, which completely define its behavior in shaping, forming and fabrication operations during manufacturing operations. The main five technological properties are discussed below.
Malleability: - It is the property of the material by virtue of which it can be deformed in to thin sheets by rolling or hammering without rupture. It depends on the crystal structure of the material.
e.g. Gold, silver, aluminium, copper etc…
Machinability: - This property indicates the ease with which it can be cut or removed by cutting tools in various operations such as turning, drilling, boring, etc. E.g. copper aluminium alloy, grey cast iron etc.
Weldability: - This property indicates the ease with which two similar or dissimilar metals are joined by fusion with or without the application of pressure as well as filler material.
Formability: - The term formability or workability may be defined as the property of the metal, which indicates the ease with which it can be formed into different shapes and sizes. e.g. Low carbon steel.
Castability: - It may be defined as the property of the metal, which indicates the ease with which it is formed into different shapes and sizes form its liquid state.
The factors affecting the mechanical properties are grain size, temperature, heat treatment, and atmospheric exposure.
2.7 Chemical properties
This describes the combining tendencies, corrosion characteristics, reactivity, solubility etc…of substances. e.g. corrosion resistance, chemical composition, acidity or alkalinity.
2.8 Optical properties
Refractive Index: - It is the ratio of the velocity of light in vacuum, to the velocity of light within the material.
Absorptivity: - The energy incident on any material gets partially transmitted, reflected or absorbed. The decimal fraction absorbed is known as the absorptivity.
Resistivity: - It is the decimal fraction reflected.
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