Engineering material(notes)
Chapter=1 introduction
1.1 Classification of Engineering Materials Basically Engineering Materials Can be classified into two categories1-Metals 2-Non-Metals 1-Metals Metals are polycrystalline bodies which are having number of differentially oriented fine crystals. Normally major metals are in solid states at normal temperature. However, some metals such as mercury are also in liquid state at normal temperature. All metals are having high thermal and electrical conductivity. All metals are having positive temperature coefficient of resistance. Means resistance of metals increase with increase of temperature. Examples of metals – Silver, Copper, Gold, Aluminum, Iron, Zinc, Lead, Tin etc. Metals can be further divided into two groupsFerrous Metals – All ferrous metals are having iron as common element. All ferrous materials are having very high permeability which makes these materials suitable for construction of core of electrical machines. Examples: Cast Iron, Wrought Iron, Steel, Silicon Steel, High Speed Steel, Spring Steel etc. Non-Ferrous Metals- All non-ferrous metals are having very low permeability. Example: Silver, Copper, Gold, Aluminum etc. 2-Non-Metals Non-Metal materials are non-crystalline in nature. These exist in amorphic or mesomorphic forms. These are in both solid & gases forms at normal temperature. Normally all non-metals are bad conductor of heat and electricity. Examples: Plastics, Rubber, Leathers, Asbestos etc. As these non-metals are having very high resistivity which makes them suitable for insulation purpose in electrical machines.
Difference between Metals and Non Metals Sl. No. Property
Metals Non-Metals
1. Structure All metals are having crystalline structure amorphic & mesomorphic structure
All Non-metals are having
2. State Generally metals are solid normal temperature State varies material to material. Some are gas state and some are in solid state at normal temperature. 3. Valance electrons and conductivity Valance electrons are free to move within metals which makes them good conductor of heat & electricity Valence electrons are tightly bound with nucleus which are not free to move. This makes them bad conductor of heat & electricity 4.
Density
High density
Low density
5.
Strength
High strength
Low strength
6.
Hardness
Generally hard
7.
Malleability
Malleable
8.
Ductility
Ductile
9.
Brittleness
Hardness is generally varies Non malleable Non ductile
Generally non brittle in nature
10. Lustre Metals possess metallic lustre lustre (Except graphite & iodine)
Brittleness varies material to material Generally do not possess metallic
Other classification of engineering materials: Engineering materials can also be classified as below1-Metals and Alloys 2-Ceramic Materials 3-Organic Materials 1-Metals and Alloys Metals are polycrystalline bodies which are have number of differentially oriented fine crystals. Normally major metals are in solid states at normal temperature. However, some metals such as mercury are also in liquid state at normal temperature. Pure metals are having very low mechanical strength, which sometimes does not match with the mechanical strength required for certain applications. To overcome this draw back alloys are used.
Alloys are the composition of two or more metals or metal and non-metals together. Alloys are having good mechanical strength, low temperature coefficient of resistance. Example: Steels, Brass, Bronze, Gunmetal, Invar. Super Alloys etc. 2-Ceramic Materials Ceramic materials are non-metallic solids. These are made of inorganic compounds such as Oxides, Nitrides, Silicides and Carbides. Ceramic materials possess exceptional Structural, Electrical, Magnetic, Chemical & Thermal properties. These ceramic materials are now extensively used in different engineering fields. Examples: Silica, glass, cement, concrete, garnet, Mgo, Cds, Zno, SiC etc. 3-Organic Materials All organic material are having carbon as a common element. In organic materials carbon is chemically combined with oxygen, hydrogen and other non-metallic substances. Generally organic materials are having complex chemical bonding. Example: Plastics, PVC, Synthetic Rubbers etc.
1.2 properties of material: 1.2.1 Physical Properties of Materials: To finalize the material for an engineering product or application, we should have the knowledge of physical properties of materials. The physical properties of a material are those which can be observed without change of the identity of material. Some of these typical properties of a material are listed below1-Density 2-Specific gravity 3-State Change temperatures 4-Coefficients of thermal expansion 5-Specific Heat 6-Latent heat 7-Fluidity 8-Weld ability
9-Elasticity 10-Plasticity 11-Porosity 12-Thermal conductivity 13-Electrical Conductivity
1-Density of Materials: Density of a material or substance is defined as “the mass per unit volume”. It is represented as the ratio of mass with volume of a material. It is denoted by “ρ”. Its unit in SI system is Kg/m3. If, m is the mass of material in Kg, V is the volume of materiel in meter3. Then the Density of material, 2-Specific Gravity of Materials: It is defined as the ratio of density of material with respect to density of a reference material or substance. It does not have any unite. Sometimes it is also called as relative density. For gravity calculation generally water is considered as a reference substance. State Change Temperatures Generally a substance is having three states called – solid state, liquid state, gaseous state. State change temperature is the temperature at which the substance changes from one state to another state. State change temperature are of following typesMelting point - It is the temperature (in °C or K) at which the substance changes from solid state to liquid state. Boiling point - It is the temperature (in °C or K) at which the substance changes from liquid state to gaseous state. Freezing point - It is the temperature (in °C or K) at which a liquid changes from liquid to solid state. Theoretically it is equal to the melting point. However, practically there may observed some difference. 3-Coefficient of Thermal Expansion: When a material is heated, it expends, due to which its dimensions change. Coefficient of thermal expansion, represents the expansion in material with increase of temperature. Thermal expansion coefficients are three types
Coefficient of Linear Thermal Expansion: The change in length of an object due change in temperature is related by “Coefficient of linear thermal expansion”. It denoted by “αL” Where, ‘l’ is the initial length of object, ‘Δl’ is the change in length, ‘Δt’ is the change in the temperature. Unit of αL is per °C. Coefficient of Area Thermal Expansion: The change in area of an object due change in temperature is related by “Coefficient of Area thermal expansion”. It denoted by “αA” Where, ‘l’ is the initial length of object, ‘ΔA’ is the change in length, ‘Δt’ is the change in the temperature. Unit of αA is per °C Coefficient of Volume Thermal Expansion: The change in volume of an object due change in temperature is related by “Coefficient of volume thermal expansion”. It denoted by “αV” Where, ‘l’ is the initial length of object, ‘ΔV’ is the change in length, ‘Δt’ is the change in the temperature. Unit of αA is per °C 3-Specific Heat of Materials: Specific heat of a material is defined as the amount of heat required to increase the temperature of unit mass of material by 1°C. It is denoted by ‘S’. Where, ‘m’ is the mass of material in Kg. ‘Q’ is the amount of heat given to material in Joule. ‘Δt’ is rise in temperature. Unit of specific heat in SI system is, Joule/Kg °C 4-Latent Heat of Materials: Latent Heat of a material is defined as the amount of heat required / released by change the unit mass of material from one state to another state (Phase change). It is denoted by ‘L’. Latent heat is given by, Where, ‘Q’ is the amount of heat required / released by material (in joule), ‘m’ is the mass of material (in Kg). Unit of Latent heat in SI system is Joule / Kg. 5-Fluidity of Materials: It is a property of material which represents that how easily a material can flow in liquid state. It is the reciprocal to viscosity of liquid material. 6-Weld Ability of Materials:
It is the property of a material which presents that how easily the two pieces of material can be welded together by applying pressure or heat or both. 7-Elasticity of Materials: It is the property of a material by which it regains its original dimensions on removal of load or force. 8-Plasticity of Materials: When we keep on increasing the load beyond limit of elasticity material retains it molded state. This property of material is called plasticity. 9-Porosity of Materials: When a material is in melting condition, it contains some dissolved gases with in the material. When the material is solidifies these gases get evaporate and leave behind voids. The Porosity of material represents the quantity of voids in solid materials. 10-Thermal Conductivity of Materials: It is the property of a material which represents that how easily the heat can be conducted by material. The thermal conductivity of a material can be defined as “the amount of heat transmitted by unit thickness of material normal to the unit area surface in unit time when the temperature gradient across the material piece is unity in steady state condition. Its unit in SI system is watts per meter per °K. 11-Electrical Conductivity of Materials: It is the property of material which represents that how easily the electricity can be conducted by the material. It is denoted by ‘σ’. It is the reciprocal of resistivity of material. It unit is mho/meter.
1.2.2-Mechanical Properties of Engineering Materials: To finalize the material for an engineering product / application, we should have the knowledge of Mechanical properties of materials. The mechanical properties of a material are those which effect the mechanical strength and ability of material to be molded in suitable shape. Some of the typical mechanical properties of a material are listed below1-Strength 2-Toughness 3-Hardness
4-Hardenability 5-Brittleness 6-Malleability 7-Ductility 8-Creep and Slip 9-Resilience 10-Fatigue
1-Strength: It is the property of material which opposes the deformation or breakdown of material in presence of external forces or load. Material which we finalize for our engineering product, must have suitable mechanical strength to be capable to work under different mechanical forces or loads. 2-Toughness: It is the ability of material to absorb the energy and gets plastically deformed without fracturing. Its numerical value is determined by the amount of energy per unit volume. It unit is Joule/ m3. Value of tough ness of a material can be determines by stress-strain characteristics of material. For good toughness material should have good strength as well as ductility. For example: brittle materials, having good strength but limited ductility are not tough enough. Conversely, materials having good ductility but low strength are also not tough enough. Therefore, to be tough, material should be capable to withstand with both high stress and strain. 3-Hardness: It is the ability of material to resist to permanent shape change due to external stress. There are various measure of hardness – scratch Hardness, indentation hardness and rebound hardness Scratch Hardness: Scratch Hardness is the ability of material to oppose the scratch to outer surface layer due to external force. Indentation Hardness: It is ability of material to oppose the dent due to punch of external had and sharp object. Rebound Hardness:
Rebound hardness is also called as dynamic hardness. It is determined by the height of “bounce” of a diamond tipped hammer dropped from a fixed height on the material. 4-Hardenability: It is the ability of a material to attain the hardness by heat treatment processing. It is determined by the depth up to which the material becomes hard. The SI unit of hardenability is meter (similar to length). Hardenability of material is inversely proportional to the weld-ability of material. 5-Brittleness: Brittleness of a material indicates that how easily it gets fractured when it is subjected to a force or load. When a brittle material is subjected to a stress is observes very less energy and gets fractures without significant strain. Brittleness is converse to ductility of material. Brittleness of material is temperature depended. Some metals which are ductile at normal temperature become brittle at low temperature. 6-Malleability: Malleability is property of solid material which indicates that how easily a materials gets deformed under compressive stress. Malleability is often categorized by the ability of material to be formed in the form of a thin sheet by hammering or rolling. This mechanical property is an aspect of plasticity of material. Malleability of material is temperature dependent. With rise of temperature, the malleability of material increases. 7-Ductility: Ductility is a property of a solid material which indicates that how easily a materials gets deformed under tensile stress. Ductility is often categorized by the ability of material to get stretched into a wire by pulling or drawing. This mechanical property is also an aspect of plasticity of material and temperature dependent. With rise of temperature, the ductility of material increases. 8-Creep and Slip: Creep is the property of material which indicates the tendency of material to move slowly and deform permanently under the influence of external mechanical stress. It results due to long time exposure to large external mechanical stress with in limit of yielding. Creep is more severe in material that are subjected to heat for long time. Slip in material is a plane with high density of atoms. 9-Resilience: Resilience is the ability of material to absorb the energy when it is deformed elastically by applying stress and release the energy when stress is removed. Proof resilience is defined as the maximum energy that can be absorbed without permanent deformation. The modulus of resilience is defined as the maximum energy that can be absorbed per unit volume without permanent deformation. It can be determined by integrating the stress-strain cure from zero to elastic limit. Its unit is joule/m3.
10-Fatigue: Fatigue is the weakening of material caused by the repeated loading of material. When a material is subjected to cyclic loading, and loading greater than certain threshold value but much below the strength of material (ultimate tensile strength limit or yield stress limit, microscopic cracks begin to form at grain boundaries and interfaces. Eventually the crack reached to a critical size. This crack propagates suddenly and the structure gets fractured. The shape of structure effects the fatigue very much. Square holes and sharp corners lead to elevated stresses where the fatigue crack initiates.
1.2.3 Electrical Properties of Engineering Materials: To finalize the material for an engineering product / application, we should have the knowledge of Electrical properties of materials. The Electrical properties of a material are those which determine ability of material to be suitable for a particular Electrical Engineering Application. Some of the typical Electrical properties of engineering materials are listed below1-Resistivity 2-Conductivity 3-Temperature coefficient of Resistance 4-Permittivity 5-Thermoelectricity
1-Resistivity: It the property of material which resists the flow of electric current through material. It is the reciprocal of conductivity. It is dented by ‘ρ’. Resistivity of a material of a conductor can be determined as below Where, ‘R’ is the resistance of conductor in Ω ‘A’ is the cross sectional area of conductor in m2 ‘l’ is the length of the conductor in meter SI unit of resistivity of is Ω¦-meter. Resistivity of some materials is listed below Sl. No. Element 1
Silver
Resistivity at 20°C in Ω - m 1.59 × 10-8
2
Copper
1.7 × 10-8
3
Gold
2.44 × 10-8
4
Aluminum
2.82 × 10-8
5
Tungsten
5.6 × 10-8
6
Iron
1.0 × 10-7
7
Platinum
1.1 × 10-7
8
Lead
2.2 × 10-7
9
Manganin
4.82 × 10-7
10
Constantan
4.9 × 10-7
11
Mercury
9.8 × 10-7
12
Carbon (Graphite)
3.5 × 10-5
13
Germanium
4.6 × 10-1
14
Silicon
6.4 × 102
15
Glass
1010 to 1014
16
Quartz (fused)
7.5 × 1017
2-Conductivity: It is the property of material with allow the flow of electric current through material. It is a parameter which indicates that how easily electric current can flow through the material. It is denoted by ‘σ’. Conductivity of material is the reciprocal of resistivity. Conductivity of material can be determined by, Its SI unit is 1/(Ω-meter) or ℧/meter. 3-Dielectric Strength:
It is the property of material which indicates the ability of material to withstand at high voltages. Generally it is specified for insulating material to represent their operating voltage. A material having high dielectric strength can withstand at high voltages. Generally, it is represented in the unit of KV/cm. Dielectric strength of some insulating materials are listed belowSl. No. Material 1
Air
Dielectric Strength [KV(max.)/cm] 30
2
Porcelain
80
3
Paraffin Wax
120
4
Transformer oil
160
5
Bakelite
220
6
Rubber
280
7
Paper
500
8
Teflon
600
9
Glass
1200
10
Mica
2000
4-Temperature Coefficient of Resistance: The temperature coefficient of resistance of a material indicates the change in resistance of material with change in temperature. Resistance of conductor changes with change of temperature. The rise in resistance of a material with rise in temperature depends on following things, R2 - R1 ∝ R1
R2 - R1 ∝ t2 - t1
Property of material of conductor: Where, R1 is the resistance of conductor at temperature of t1°C and R2 is the resistance of conductor at temperature of t2°C. Hence, from above, R2 - R1 ∝ R1 (t2 - t1)
Or, R2 - R1=α1 R1 (t2 - t1) ⇒ R2=R1 [1+α1 (t2 - t1)]
Where, α1 is temperature coefficient of resistance of material at temperature of t1°C. Its unit is /°C. Temperature coefficient of resistance of material is also depends on temperature. Temperature coefficient of some materials are listed below, Sl. No. Element
Temperature Coefficient of Resistance in /°C
1
Manganin
0.00002
2
Constantan
0.00017
3
Nichrome
0.0004
4
Mercury
0.0009
5
Silver
0.0038
6
Copper
0.00386
7
Annealed copper
0.000393
8
Platinum
0.003927
9
Aluminum
0.00429
10
Carbon ( Graphite)
- 0.0005
11
Germanium
- 0.05
12
Silicon
- 0.07
5-Thermoelectricity: If the junction, formed by joining to two metals, is heated, a small voltage in the range of millivolt is produced. This effect is called thermoelectricity or thermoelectric effect. This effect forms the basis of operation of thermocouples and some temperature based transducers. This effect can be used to generate electricity, to measure the temperature and to measure the change is temperature of objects.
1.2.4-Magnetic Properties of Engineering Materials: To finalize the material for an engineering product / application, we should have the knowledge of magnetic properties of materials. The magnetic properties of a material are those which determine the ability of material to be suitable for a particular magnetic Application. Some of the typical magnetic properties of engineering materials are listed below1-Permeability 2-Retentivity or Magnetic Hysteresis 3-Coercive force 4-Reluctance
1-Permeability: It is the property of magnetic material which indicates that how easily the magnetic flux is build up in material. Some time is also called as the magnetic susceptibility of material.
It is determined by the ratio of magnetic flux density to magnetizing force producing this magnetic flux density. It is denoted by µ. Hence, μ = B/H Where, B is the magnetic flux density in material in Wb/m2 H is the magnetizing force of magnetic flux intensity in Wb/ Henry-meter SI unit of magnetic permeability is Henry / meter Permeability of material is also defined as, μ=μ0 μr Where, µ0 is the permeability of air or vacuum, and μ0=4π × 10-7 Henry / meter and µr is the relative permeability of material. µr = 1 for air or vacuum. A material selected for magnetic core in electrical machines should have high permeability, so that required magnetic flux can be produced in core by less ampere- turns. 2-Retentivity: When a magnetic material is placed in an external magnetic field, its grains get oriented in the direction of magnetic field. Which results in magnetization of material in the direction of external magnetic field. Now, even after removal of external magnetic field, some magnetization exists, which is called residual magnetism. This property of material is called Magnetic retentively of material. A hysteresis loop or B-H cure of a typical magnetic material is shown in figure below. Magnetization Br in below hysteresis loop represents the residual magnetism of material. 3-Coercive Force: Due to retentivity of material, even after removal of external magnetic field some magnetization exists in material. This magnetism is called residual magnetism of material. To remove this residual magnetization, we have to apply some external magnetic field in opposite direction. This external magnetic motive force (ATs) required to overcome the residual magnetism is called “coercive force” of material. In above hysteresis loop, - Hc represents the coercive force. The material having large value of residual magnetization and coercive force are called magnetically hard materials. The material having very low vale of residual magnetization and coercive force are called magnetically soft materials. 4-Reluctance: It is a property of magnetic material which resists to buildup of magnetic flux in material. It is denoted by R. Its unit is “Ampere-turns / Wb”. Reluctance of magnetic Material is given by,
A magnetic material suitable for core of electrical machines should have low reluctance.
1.2.5-Chemical Properties of Materials: Being an Engineers it importance to have the knowledge of chemical properties of engineering materials. Because most the engineering materials come into contact of other materials react chemically to each other. Due to this chemical reaction they may suffers from chemical deterioration. Some of the chemical properties of engineering materials are listed below – 1-Chemical composition 2-Atomic bonding 3-Corrosion resistance 4-Acidity or Alkalinity
1-Chemical Composition: The chemical composition of engineering material indicates the elements which are combined together to form that material. Chemical composition of material effects the properties of engineering material very much. The strength, hardness, ductility, brittleness, corrosion resistance, weldability etc. depend on chemical composition of material. Hence, we should also have the knowledge of chemical composition of engineering material. For Example the Chemical compositions of some materials are listed below-
Sl. No. Material
Chemical Composition
1.
Steel
Fe, Cr, Ni
2.
Brass
Cu = 90%, Ni = 10%
3.
Bronze
90% Cu, 10% Ni
4.
Invar
Fe = 64%, Ni = 36%
5.
Gun Metal
Cu = 88%, Tin = 10%, Zn = 2%
6.
German Silver
Cu = 50%, Zn = 30%, Ni = 20%
7.
Nichrome
Ni = 60%, Cr = 15%, Fe = 25%
8.
Phosphor Bronge
Cu = 89 – 95.50% , Tin = 3.50 -10%, P = 1%
9.
Manganin
Cu = 84%, Mn = 12%, Ni = 4%
10.
Constantan
Cu = 60%, Ni = 40%
2-Atomic Bonding: Atomic bonding represents how atoms are bounded to each other to form the material. Many properties, such as melting point, boiling point, thermal conductivity & electrical conductivity of materials are governed by atomic bonding of materials. Hence, to understand the properties of materials, it is very important to study the atomic bonding of materials. Atomic bonds in materials are of following types, Ionic bond – from by exchanging of valence electrons between atoms.
Covalent bonds – from by sharing of electrons between atoms.
Metallic bonds – found in metals. 3-Corrosion Resistance: Corrosion is a gradual chemical or electrochemical attack on a metal by its surrounding medium. Due to the corrosion, metal starts converted into an oxide, salt or some other compound. Corrosion of a metals is effected by many factors such as air, industrial atmosphere, acid, bases, slat solutions & soils etc. Corrosion has a very adverse effect on material. Due to corrosion, the strength & life of material is reduced, which results. Corrosion resistance of a material is the ability of material to resist the oxidation in atmospheric condition. Generally the pure metal such as iron, copper, aluminum etc. get corroded in slowly in atmosphere. To avoid the corrosion of these metal in pure form, we use these metals in the form of alloys such as stainless steel, brass, bronze, German silver, Gunmetal etc. 4-Acidity or Alkalinity: Acidity or Alkalinity is very important chemical property of engineering materials. Material is acetic or Alkane, it is decided by the ph value of material. Ph value of material varies from 0 to 14. Ph value of 7 is considered to be neutral. Ordinary water is having ph value of 7. The material which are having ph value below 7 are called Acetic & Materials which are having ph value greater than 7 are called alkane. Acidity of Alkalinity of material indicates that how the react with other materials.
