In this article we will discuss about: 1. General Properties of Engineering Materials 2. Physical Properties of Engineering Materials 3. Mechanical Properties.
General Properties of Engineering Materials:
The economists mainly concerned with the industries manufacturing engineering materials are interested in finding out ways in which the engineering materials can be used or applied in the best economic way. The economist should have working knowledge of properties and manufacturing processes of various engineering materials.
The services of such an economist can be of extreme use in making choice of engineering materials under specific conditions or in deciding the line of manufacture of engineering materials from the available local raw materials.
The fields of application of a particular engineering material are governed by the characteristics and various properties of that engineering material.
Such properties may be classified into various categories as follows:
(1) Chemical Properties:
The chemical properties of the material suggest the tendency of the material to combine with other substances, its reactivity, solubility and effects like corrosion, chemical composition, acidity, alkalinity, etc. Corrosion is one of the serious problems faced by engineers in selecting engineering materials, caused due to chemical properties of metal.
In metals, valence electrons are loosely bound to their atoms and can be easily removed during chemical reactions. So, when metals are exposed in the atmosphere and come in contact with gases like oxygen, chlorine etc., the chemical reactions take place. When iron reacts with oxygen, iron oxide is formed which is red in colour and iron metal is coated with it. This is called corrosion.
(2) Electrical Properties:
These properties signify the ability of the material to resist the flow of an electric current and they include conductivity, dielectric strength and resistivity.
(3) Magnetic Properties:
The study of magnetic properties of the material like permeability, hysteresis and coercive force is required when.it is to be used for generators, transformers, etc.
(4) Mechanical Properties:
The characteristics governing the behaviour of material when external forces are applied are included in these properties. Some of the important mechanical properties are elasticity, hardness, plasticity, strength, etc.
(5) Optical Properties:
When the material is to be used for the optical work, the knowledge of its optical properties like colour, light transmission, refractive index, reflectivity, etc. is necessary. When light strikes any material, it interacts with its atoms and causes various types of effects. The light may be reflected, refracted, scattered or absorbed. The study of light in materials and how to use this behaviour to control the various light effects is called optics.
(6) Physical Properties:
These are required to evaluate the condition of the material without any external force acting on it and they include bulk density, durability, porosity, etc.
(7) Thermal Properties:
The knowledge of thermal properties of the material like specific heat, thermal expansion and conductivity is helpful in knowing the response of the material to the thermal changes. Thus suitable materials can be selected to withstand fluctuating and high temperatures.
(8) Technological Properties:
The properties of metals and alloys which have a bearing on their processing or application are called technological properties. Castability, machinability, weldability and workability are some of the significant technological properties of metals and alloys.
Out of all such properties, the physical properties and mechanical properties are particularly very important to a construction engineer.
Physical Properties of Engineering Materials:
Following terms in connection with the physical properties of engineering materials are defined and explained:
(1) Bulk density
(2) Chemical resistance
(3) Coefficient of softening
(5) Density index
(7) Fire resistance
(8) Frost resistance
(12) Spalling resistance
(13) Specific heat
(14) Thermal capacity
(15) Thermal conductivity
(16) Water absorption
(17) Water permeability
(18) Weathering resistance.
(1) Bulk Density:
The term bulk density is used to mean the mass of a unit volume of material in its natural state i.e. including pores and voids. It is obtained by finding out the ratio of mass of specimen to the volume of specimen in its natural state.
The technical properties of the material such as strength, heat, conductivity, etc. are greatly influenced by its bulk density and hence the performance efficiency of a material will depend upon its bulk density.
For most of the materials, the bulk density is less than its density except for dense materials, liquids and materials obtained from the molten masses.
Table 1-1 shows the bulk densities of some of the important building materials.
(2) Chemical Resistance:
The ability of material to withstand the action of acids, alkalies, gases and salt solutions is known as its chemical resistance.
This property is carefully examined while selecting material for sewer pipes, hydraulic engineering installations, sanitary facilities, etc.
(3) Coefficient of Softening:
The ratio of compressive strength of material saturated with water to that in dry state is known as the coefficient of softening. The materials such as glass and metal are not affected by the presence of water and their coefficient of softening is unity. On the other hand, the materials like clay easily loose their strength when soaked in water and hence, their coefficient of softening is zero.
The materials having coefficient of softening equal to 0.8 or more are referred to as the water-resisting materials. It is advisable to avoid the use of materials with coefficient of softening less than 0.8 for situations which are likely to be exposed permanently to the action of moisture.
The term density of a material is defined as the mass of a unit volume of homogeneous material. It is obtained by working out the ratio of mass of material to the volume of material in homogeneous state. The physical properties of a material are greatly influenced by its density.
(5) Density Index:
The ratio of bulk density of a material to its density is known as its density index and it thus denotes the degree to which its volume is filled up with solid matter.
As there are practically no dense substances in nature, the density index of most of the building materials is less than unity.
The property of a material to resist the combined action of atmospheric and other factors is known as its durability.
The running or maintenance cost of a building will naturally depend upon the durability of the materials of which it is composed.
(7) Fire Resistance:
The term fire resistance is used to mean the ability of a material to resist the action of high temperature without losing its load-bearing capacity i.e., without substantial loss of strength or deformation in shape.
This property of a material is of great importance in case of a fire and as the operation of fire-fighting is usually accompanied by water, this property of a material is tested by the combined actions of high temperature and water. The material should be sufficiently fireproof to afford safety and stability in case of a fire.
(8) Frost Resistance:
The ability of a water-saturated material to resist repeated freezing and thawing without considerable decrease of mechanical strength or visible signs of failure is known as the frost resistance. The frost resistance of a material depends upon the density of material and its degree of saturation with water.
In general, the dense materials are frost resistant. The porous materials whose pores are closed or filled with water to less than 90% of their volume are frost resistant.
The property of a material to absorb water vapour from air is known as the hygroscopicity and it is governed by the nature of substance involved, number of pores, air temperature, relative humidity, etc. The water-retaining or hydrophilic substances readily dissolve in water.
The term porosity is used to indicate the degree by which the volume of a material is occupied by pores. It is expressed as a ratio of volume of pores to that of the specimen. The porosity of a material is indicative of its various properties such as strength, bulk density, water absorption, thermal conductivity, durability, etc., and hence it is to be carefully studied and analysed.
The ability of a material to withstand prolonged action of high temperature without melting or loosing shape is known as its refractoriness.
(12) Spalling Resistance:
The ability of a material to endure a certain number of cycles of sharp temperature variations without failing is known as its spalling resistance and it mainly depends on the coefficients of linear expansion of its constituents.
(13) Specific Heat:
The term specific heat is defined as the quantity of heat, expressed in kilocalories, required to heat 1 N of material by 1°C. The specific heat of a material is to be considered when heat accumulation is to be taken into account.
The specific heats of steel, stone and wood are as follows:
Steel – 0.046 x 103 J/N °C
Stone – 0.075 to 0.09 x 103 J/N °C
Wood – 0.239 to 0.27 x 103 J/N °C.
(14) Thermal Capacity:
The property of a material to absorb heat is known as its thermal capacity and it is worked out by the following equation –
T = H / M (T2 – T1)
Where, T = Thermal capacity in J/N °C
H = Quantity of heat required to increase the temperature of material from T1 to T2 in J M = Mass of material in N
T1 = Temperature of material before heating in °C
T2 = Temperature of material after heating in °C.
(15) Thermal Conductivity:
The thermal conductivity of a material is the amount of heat in kilocalories that will flow through unit area of the material with unit thickness in unit time when difference of temperature on its faces is also unity. The unit of thermal conductivity is J per m hr °C and it is usually denoted by K. The thermal conductivity of a material depends on its density, porosity, moisture content and temperature.
The term thermal resistivity of a material is used to mean the reciprocal of its thermal conductivity. The thermal resistance of a material is equal to thermal resistivity multiplied by its thickness.
(16) Water Absorption:
The ability of a material to absorb and retain water is known as its water absorption. The dry material is fully immersed in water and then the water absorption is worked out either as percentage of weight or percentage of volume of dry material. It mainly depends on the volume, size and shape of pores, present in the material.
(17) Water Permeability:
The capacity of a material to allow water to pass through it under pressure is known as its water permeability and it is described as the quantity of water that will pass through the material in one hour at constant pressure, the cross-sectional area of the specimen being 1 cm. The dense materials like glass, steel, etc. are water-proof or impervious to the water.
(18) Weathering Resistance:
The term weathering resistance is used to express the ability of a material to resist alternating wet and dry conditions without seriously affecting its shape and mechanical strength. It thus, indicates the behaviour of materials when exposed to changing conditions of humidity.
Mechanical Properties of Engineering Materials:
The mechanical properties of materials like their rigidity, ductility and strength are of vital importance in determining their fabrication and possible practical applications.
The building materials show a wide range of the mechanical properties ranging from hardness of diamond to the ductility of pure copper and the astonishing elastic behaviour of rubber. In a similar way, many materials behave quite differently when stressed in different ways. For instance, cast-iron, cement and bricks are much stronger in compression whereas wood and steel are stronger in tension.
Following terms in connection with the common mechanical properties of building materials are defined and explained:
(6) Impact strength
(7) Plasticity and brittleness
The resistance of a material to the abrasion is found out by dividing the difference in weights of specimens prior to and after abrasion with the area of abrasion.
In many applications, the building materials are required to sustain steady loads for long periods. Under such conditions, the material may continue to deform until its usefulness is seriously reduced. Such time-dependent deformations of a structure can grow large and may even result in final fracture without any increase in load. If the deformation continues even when the load is constant, such additional deformation is known as the creep.
Most of the building materials creep to a certain extent at all temperatures. However the engineering metals such as steel, aluminium and copper creep very little at room temperature. The high temperatures lead to rapid creep which is often accompanied by microstructural changes. The phenomenon of creep is important in polymers at room temperature, in alloys of aluminium at 100°C and in steels above 300°C.
When a load is applied to a material, there is change in its shape and dimension. The term elasticity is used to indicate the ability of a material to restore its initial form and dimensions after the load is removed.
The difference between the following two terms should be noted:
(i) Elastic Deformation:
A deformation is said to be elastic when the solid deforms when it is loaded but returns to its original position when unloaded. A change in pressure or an application of load results in the elastic deformation. The term ideal deformation is used to mean the deformation that takes place instantaneously upon application of force and disappears completely on removal of the force.
Such deformations obey Hooke’s law and the elastic strain of the metal is directly proportional to the applied force. The ideal deformation occurs with comparatively smaller deformation forces which can keep the working stresses within the elastic limit.
(ii) Plastic Deformation:
A deformation is said to be plastic when the solid retains full or partly the change in shape after the load is removed. The plastic deformation is observed when the stress exceeds the elastic limit and its rate is controlled by the strain rate, applied stress and temperature. It can occur under tensile, compressive and torsional stresses. It is intentionally carried out in processes like rolling, forging, etc. so as to make useful products.
When the materials are subjected to a repetitive or fluctuating stress, they will fail at a stress much lower than that required to cause fracture under steady loads.
This behaviour is called the fatigue and it is distinguished by the following three features:
(i) Increased uncertainty in strength and service life;
(ii) Loss of ductility; and
(iii) Loss of strength.
Following are the reasons of the fatigue failures:
(i) Corroding environments resulting in the reduction of the fatigue strength;
(ii) Stress concentration points;
(iii) Surface imperfections like machining marks and surface irregularities; and
(iv) Temperature, the fatigue strength being high at low temperatures and decreasing gradually with the rise in temperature.
The ability of a material to resist penetration by a harder body is known as its hardness. It is a major factor in deciding the workability and use of a material for floors and road surfaces. The hardness is not a fundamental property. But it is a combined effect of compressive, elastic and plastic properties relative to the mode of penetration, shape of penetrator, etc.
The hardness bears a fairly constant relationship to the tensile strength of a given material. It can therefore be used as a practical non-destructive test to get roughly an idea of the tensile strength of the material and the state of the metal near the surface.
The hardness of stone materials can be determined with the help of Mohs’ scale of hardness. It is a list of ten materials arranged in the order of increasing hardness. The hardness of a material lies between the hardness of two materials i.e. the one which scratches and the other which is scratched by the material to be tested.
Table 1-2 shows the Mohs’ scale of hardness.
(6) Impact Strength:
The impact strength of a material is the quantity of work required to cause its failure per its unit volume. It thus indicates the toughness of a material and the materials are tested in an impact testing machine to determine their impact strength.
The impact strength is a complex characteristic which takes into account both the toughness and strength of a material.
It varies with the following factors:
(i) If the dimensions of the specimen are increased, there is also increase in the impact strength.
(ii) If the sharpness of the notch increases, the impact strength required to cause failure decreases.
(iii) The angle of the notch also improves the impact strength after certain values.
(iv) The impact strength is also affected to a certain extent by the velocity of impact.
(v) The temperature of the specimen under test gives an indication about the type of fracture that is likely to occur i.e. ductile, brittle or ductile to brittle transition.
(7) Plasticity and Brittleness:
The term plasticity of a material is defined as its ability to change its shape under load without cracking and to retain its shape after the removal of load.
The materials can broadly be divided into two groups, namely, plastic materials and brittle materials. The steel, copper, hot bitumen, etc. are plastic materials. The brittle materials fail suddenly under pressure without appreciable deformation preceding the failure. The rock materials, ceramic materials, glass, cast-iron, concrete and some other materials are brittle and they offer poor resistance to bending, impact and tension.
The ability of a material to resist failure under the action of stresses caused by a load is known as its strength. The loads to which a material is commonly subjected to are compression, tension and bending. The corresponding strength is obtained by dividing the ultimate load with the cross-sectional area of the specimen.
The stresses in the building materials are not allowed to exceed a certain percentage of their ultimate strength. Thus, a margin of safety is provided and the term factor of safety is used to denote the ratio of ultimate stress to safe stress. For instance, if the factor of safety is two, the stress to be adopted for design purposes would be one-half of the ultimate stress.
The values of factors of safety are specified by design standards and they are framed by taking into account various factors such as nature of work, quality of material, service conditions, economic considerations, etc.
The failure of a material under the combined actions of abrasion and impact is known as its wear. The wear resistance is usually expressed as a percentage of loss in weight and it is of great importance in deciding the suitability of a material for use of road surfaces, railway ballast, etc.