An Introduction To Mechanical properties Of Material
Introduction
The describe the physical properties of a material, this means how it is likely to behave whilst it is being processed and when it is in service. such as strength, hardness, toughness and corrosion resistance. These now need to be carefully defined so that wherever possible, they can be measured.
Physical properties can be sub-divided into mechanical properties, thermal properties, electrical and magnetic properties and durability.now we.ll explain mechanical properties
Mechanical Properties
These include density, tensile strength, hardness, toughness, ductility, malleability, elasticity and brittleness...etc (Figure 1) .They are defined as follows.
Density
Density is the mass in kilograms, contained in a cubic meter of a material or substance. When used in calculations, it is usually denoted by the Greek letter 𝜌 (rho) and its units are kilograms per cubic meter (kg.m³).
The density of steel is around 7800 kgm³ and that of cast iron is around 7300 kgm³. The densities of the major non-ferrous metals used in engineering are listed in Table 1.
When designing aircraft, engineers need to be able to estimate the weight of the different components and then the total weight. Knowing the densities of the different materials helps them do this.
Specific Gravity
Specific gravity is the ratio of the density of the metal to the density of a reference material, usually water at a specified temperature and pressure. Specific gravity is unitless.
Tensile strength
The tensile strength of a material is a measure of its ability to withstand tensile forces. The Ultimate Tensile Strength (UTS) is the tensile stress that causes a material to fracture. If you have completed the unit Science for Technicians, you will know that tensile stress is the load carried per square meter of cross-sectional area. Its units are thus Newtons per square meter or Pascals (Pa)
The UTS of mild steel is around 500 MPa. The values of UTS for other ferrous and non-ferrous metals are given in Tables 2,3, and 4. Sometimes material suppliers quote the UTS of a
material in Newtons per square millimeter (N mm²). It is useful to know that the value is numerically the same in both mega pascals and Newtons per square millimeter, i.e. the UTS of mild steel
can be written as 500 MPa or 500 N mm².
The Ultimate Tensile Strength of a material can be determined by carrying out a destructive tensile test on a prepared specimen (Figure 3). The specimen is gripped in the chucks of a tensile
testing machine and increasing values of tensile load are applied up to the point of fracture. Depending on the type of machine, the load is applied by means of hydraulic rams, a lever system or a lead screw. Its value is read off from an analogue or a digital display.
The extension of the specimen can be measured by means of an extensometer that is attached to the specimen. This can be of the Lindley type, which records the extension on a dial test indicator,
or the Monsanto type, which operates on the micrometer principle.
There is also an electrical type, which is a linear variable differential transformer. This can be interfaced with an x–y plotter or a computer to produce a graph of load v. extension (FIG 4)
The extension of a specimen is usually measured over a gauge length of 50 mm. The initial diameter is checked using a micrometer and the initial cross-sectional area A, is calculated. The UTS of the material can then be calculated using the formula :
Ductility
The ductility of a material is a measure of the amount by which it can be drawn out in tension before it fractures. As can be seen from the typical load v. extension graphs in Figure 4., copper has a high degree of ductility and this enables it to be drawn out into long lengths of wire and tube. There are a number of different
ways in which ductility can be measured. One method is to carry out a bend test in which a sample of material is bent through an angle and observed for the appearance of cracks. The angle at which cracking or breaking occurs gives a measure of ductility.
Ductility measurement can also be incorporated in a tensile test. Before a tensile test, the gauge length over which extension is measured is centre punched. After the test, the two pieces of the
fractured specimen are placed in contact and the elongation of the gauge length is measured. The ductility can then be calculated as the percentage increase in the gauge length.
Alternatively, the diameter at the point of fracture can be measured and the cross-sectional area calculated. The ductility can then be calculated as the percentage reduction in area.
When a ductile material fails, the appearance of the fracture is as shown in Figure 5
As the maximum load is approached, a neck or waist is seen to form at the point where fracture will occur. When the fracture surfaces are examined it is found that they have a characteristic cup and cone appearance which is typical of ductile materials.
When ductile materials are loaded, a point is reached known as the elastic limit. If this is exceeded, elastic failure is said to have occurred. This is followed by plastic deformation up to the point
of fracture. If a material is unloaded within the plastic extension range, it does not return to its original shape and there is permanent deformation (Figure 6).
Mild steel has the characteristic load v. extension graph shown in Figure 4. Its elastic limit is followed closely by the yield point where the material suddenly gives way under the load. Elastic
failure is then said to have occurred. If the material is loaded beyond this point there will be some permanent deformation when the load is removed.
Mild steel behaves in this way because of the interstitial carbon atoms. They are said to ‘pin’ the planes of iron atoms within the grains, which stops them from slipping under load. Eventually
they can no longer hold the iron atoms and yielding takes place.
For a short time, extension occurs at a reduced load but the material soon becomes work hardened and the load v. extension graph then rises in a curve until the maximum load is reached. It is
important to know the stress at which yielding takes place in steel components so that they can be designed to operate below the yield point load.
Compressive strength
This is the ability of a material to withstand compressive (squeezing) loads without being crushed or broken. The material is in compression
Brittleness
This is the opposite of ductility. When metals are loaded in tension they first undergo elastic deformation which is proportional to the load applied. When unloaded from within the elastic range, the material returns to its original shape.
Brittle materials tend to fail within the elastic range, or very early in the plastic range, before
very much deformation has taken place. Cast iron is brittle, as can be seen from its load v. extension graph in Figure 4. Brittle materials very often fracture across a plane at right angles to the direction of loading. Sometimes however, the fracture plane is at 45⁰ to the direction of loading as shown in Figure 7. This indicates that the material has a low shear strength.
Toughness: impact resistance
This is the ability of a material to resist shatter. If a material shatters it is brittle (e.g. glass). If it fails to shatter when subjected to an impact load it is tough (e.g. rubber). Toughness should not be confused with strength. Any material in which the spread of surface cracks does not occur or only occurs to a limited extent is said to be tough.
Elasticity
This is the ability of a material to deform under load and return to its original size and shape when the load is removed. Such a material would be required to make the spring as shown.
If you have completed the unit Science for Technicians you will know that the modulus of elasticity of a material is given by the formula
Plasticity
This property is the exact opposite of elasticity. It is the state of a material which has been loaded beyond its elastic state. Under a load beyond that required to cause elastic deformation (the elastic limit) a material possessing the property of plasticity deforms permanently. It takes a permanent set and will not recover when the load is removed.
Malleability
Whereas ductility is the ability of a material to be drawn out in tension, malleability is the ability of a material to be deformed or spread in different directions. This is usually caused by compressive
forces during rolling, pressing and hammering operations.
Copper is both ductile and malleable but the two properties do not necessarily go together. Lead is extremely malleable but not very ductile, and soon fractures when loaded in tension.
A malleable material combines the properties of plasticity and compressibility, so that it can be squeezed to shape by such processes as forging, rolling and rivet heading.
Hardness
This is the ability of a material to withstand scratching (abrasion) or indentation by another hard body. It is an indication of the wear resistance of a material.
Processes which increase the hardness of materials also increase their tensile strength. At the same time the toughness of the material is reduced as it becomes more brittle.
Hardenability must not be confused with hardness. Hardenability is the ability of a metal to respond to the heat treatment process of quench hardening. To harden it, the hot metal must be chilled at a rate in excess of its critical cooling rate. Since any material cools more quickly at the surface than at the center there is a limit to the size of bar which can cool quickly enough at its center to achieve uniform hardness throughout. This is the ruling section for the material. The greater its hardenability the greater will be its ruling section.
Shear strength
This is the ability of a material to withstand offset or transverse loads without rupture occurring. The rivet connecting the two bars shown is in shear whilst the bars themselves are in tension. Note that the rivet would still be in shear if the bars were in compression.
Elongation
Elongation is a measure of ductility, as measured by the percentage of elongation. Increasing the gauge length of a specimen will decrease the percent of elongation to fracture. This is because after the neck forms all subsequent deformation takes place in the vicinity of the neck. The behavior around the neck is the same regardless of the length of the specimen. Therefore in shorter gauge lengths, a larger fraction of the specimen’s length is deforming during the test. In longer specimens, the portion away from the neck is not continuing to deform after the onset of necking, therefore a smaller percentage of the specimen’s length is contributing to the total deformation.
Because of this it is necessary to compare percents of elongation of various metals with the same gauge length when comparing ductility.
Creep
Creep is a time-dependent deformation of a material while under an applied load that is below its yield strength. It is most often occurs at elevated temperature, but some materials creep at room temperature. Creep terminates in rupture if steps are not taken to bring to a halt.
Creep data for general design use are usually obtained under conditions of constant uniaxial loading and constant temperature. Results of tests are usually plotted as strain versus time up to rupture. As indicated in the image, creep often takes place in three stages. In the initial stage, strain occurs at a relatively rapid rate but the rate gradually decreases until it becomes approximately constant during the second stage. This constant creep rate is called the minimum creep rate or steady-state creep rate since it is the slowest creep rate during the test. In the third stage, the strain rate increases until failure occurs.
Creep in service is usually affected by changing conditions of loading and temperature and the number of possible stress-temperature-time combinations is infinite. While most materials are subject to creep, the creep mechanisms is often different between metals, plastics, rubber, concrete.
resilience
resilience is the ability of a material to absorb energy when it is deformed elastically, and release that energy upon unloading. Proof resilience is defined as the maximum energy that can be absorbed up to the elastic limit, without creating a permanent distortion. The modulus of resilience is defined as the maximum energy that can be absorbed per unit volume without creating a permanent distortion. It can be calculated by integrating the stress–strain curve from zero to the elastic limit. In uniaxial tension, under the assumptions of linear elasticity,
where
Ur is the modulus of resilience,
σy is the yield strength,
εy is the yield strain,
and E is the Young's modulus This analysis is not valid for non-linear elastic materials like rubber, for which the approach of area under the curve till elastic limit must be used.
Introduction
The describe the physical properties of a material, this means how it is likely to behave whilst it is being processed and when it is in service. such as strength, hardness, toughness and corrosion resistance. These now need to be carefully defined so that wherever possible, they can be measured.
Physical properties can be sub-divided into mechanical properties, thermal properties, electrical and magnetic properties and durability.now we.ll explain mechanical properties
Mechanical Properties
These include density, tensile strength, hardness, toughness, ductility, malleability, elasticity and brittleness...etc (Figure 1) .They are defined as follows.
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| FIG 1 |
Density is the mass in kilograms, contained in a cubic meter of a material or substance. When used in calculations, it is usually denoted by the Greek letter 𝜌 (rho) and its units are kilograms per cubic meter (kg.m³).
 |
| FIG 2 |
The density of steel is around 7800 kgm³ and that of cast iron is around 7300 kgm³. The densities of the major non-ferrous metals used in engineering are listed in Table 1.
When designing aircraft, engineers need to be able to estimate the weight of the different components and then the total weight. Knowing the densities of the different materials helps them do this.
 |
| TABLE 1 |
Specific gravity is the ratio of the density of the metal to the density of a reference material, usually water at a specified temperature and pressure. Specific gravity is unitless.
Tensile strength
The tensile strength of a material is a measure of its ability to withstand tensile forces. The Ultimate Tensile Strength (UTS) is the tensile stress that causes a material to fracture. If you have completed the unit Science for Technicians, you will know that tensile stress is the load carried per square meter of cross-sectional area. Its units are thus Newtons per square meter or Pascals (Pa)
 |
| FIG 3 |
material in Newtons per square millimeter (N mm²). It is useful to know that the value is numerically the same in both mega pascals and Newtons per square millimeter, i.e. the UTS of mild steel
can be written as 500 MPa or 500 N mm².
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| TABLE 2 |
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| TABLE 3 |
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| TABLE 4 |
testing machine and increasing values of tensile load are applied up to the point of fracture. Depending on the type of machine, the load is applied by means of hydraulic rams, a lever system or a lead screw. Its value is read off from an analogue or a digital display.
The extension of the specimen can be measured by means of an extensometer that is attached to the specimen. This can be of the Lindley type, which records the extension on a dial test indicator,
or the Monsanto type, which operates on the micrometer principle.
There is also an electrical type, which is a linear variable differential transformer. This can be interfaced with an x–y plotter or a computer to produce a graph of load v. extension (FIG 4)
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| FIG 4 |
The ductility of a material is a measure of the amount by which it can be drawn out in tension before it fractures. As can be seen from the typical load v. extension graphs in Figure 4., copper has a high degree of ductility and this enables it to be drawn out into long lengths of wire and tube. There are a number of different
ways in which ductility can be measured. One method is to carry out a bend test in which a sample of material is bent through an angle and observed for the appearance of cracks. The angle at which cracking or breaking occurs gives a measure of ductility.
Ductility measurement can also be incorporated in a tensile test. Before a tensile test, the gauge length over which extension is measured is centre punched. After the test, the two pieces of the
fractured specimen are placed in contact and the elongation of the gauge length is measured. The ductility can then be calculated as the percentage increase in the gauge length.
 |
| FIG 5 |
When ductile materials are loaded, a point is reached known as the elastic limit. If this is exceeded, elastic failure is said to have occurred. This is followed by plastic deformation up to the point
of fracture. If a material is unloaded within the plastic extension range, it does not return to its original shape and there is permanent deformation (Figure 6).
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| FIG 6 |
Mild steel has the characteristic load v. extension graph shown in Figure 4. Its elastic limit is followed closely by the yield point where the material suddenly gives way under the load. Elastic
failure is then said to have occurred. If the material is loaded beyond this point there will be some permanent deformation when the load is removed.
Mild steel behaves in this way because of the interstitial carbon atoms. They are said to ‘pin’ the planes of iron atoms within the grains, which stops them from slipping under load. Eventually
they can no longer hold the iron atoms and yielding takes place.
For a short time, extension occurs at a reduced load but the material soon becomes work hardened and the load v. extension graph then rises in a curve until the maximum load is reached. It is
important to know the stress at which yielding takes place in steel components so that they can be designed to operate below the yield point load.
Compressive strength
This is the ability of a material to withstand compressive (squeezing) loads without being crushed or broken. The material is in compression
Brittleness
This is the opposite of ductility. When metals are loaded in tension they first undergo elastic deformation which is proportional to the load applied. When unloaded from within the elastic range, the material returns to its original shape.
Brittle materials tend to fail within the elastic range, or very early in the plastic range, before
very much deformation has taken place. Cast iron is brittle, as can be seen from its load v. extension graph in Figure 4. Brittle materials very often fracture across a plane at right angles to the direction of loading. Sometimes however, the fracture plane is at 45⁰ to the direction of loading as shown in Figure 7. This indicates that the material has a low shear strength.
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| FIG 7 |
This is the ability of a material to resist shatter. If a material shatters it is brittle (e.g. glass). If it fails to shatter when subjected to an impact load it is tough (e.g. rubber). Toughness should not be confused with strength. Any material in which the spread of surface cracks does not occur or only occurs to a limited extent is said to be tough.
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| FIG 8 |
This is the ability of a material to deform under load and return to its original size and shape when the load is removed. Such a material would be required to make the spring as shown.
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| FIG 9 |
This property is the exact opposite of elasticity. It is the state of a material which has been loaded beyond its elastic state. Under a load beyond that required to cause elastic deformation (the elastic limit) a material possessing the property of plasticity deforms permanently. It takes a permanent set and will not recover when the load is removed.
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| FIG 10 |
Whereas ductility is the ability of a material to be drawn out in tension, malleability is the ability of a material to be deformed or spread in different directions. This is usually caused by compressive
forces during rolling, pressing and hammering operations.
Copper is both ductile and malleable but the two properties do not necessarily go together. Lead is extremely malleable but not very ductile, and soon fractures when loaded in tension.
A malleable material combines the properties of plasticity and compressibility, so that it can be squeezed to shape by such processes as forging, rolling and rivet heading.
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| FIG 11 |
This is the ability of a material to withstand scratching (abrasion) or indentation by another hard body. It is an indication of the wear resistance of a material.
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| FIG 12 |
Hardenability must not be confused with hardness. Hardenability is the ability of a metal to respond to the heat treatment process of quench hardening. To harden it, the hot metal must be chilled at a rate in excess of its critical cooling rate. Since any material cools more quickly at the surface than at the center there is a limit to the size of bar which can cool quickly enough at its center to achieve uniform hardness throughout. This is the ruling section for the material. The greater its hardenability the greater will be its ruling section.
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| FIG 13 |
The hardness test is the most utilized mechanical property test of all methods available. These tests do not require much time and are very informative since hardness is related to strength. See Table 5.
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| TABLE 5 |
Shear strength
This is the ability of a material to withstand offset or transverse loads without rupture occurring. The rivet connecting the two bars shown is in shear whilst the bars themselves are in tension. Note that the rivet would still be in shear if the bars were in compression.
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| FIG 14 |
Elongation is a measure of ductility, as measured by the percentage of elongation. Increasing the gauge length of a specimen will decrease the percent of elongation to fracture. This is because after the neck forms all subsequent deformation takes place in the vicinity of the neck. The behavior around the neck is the same regardless of the length of the specimen. Therefore in shorter gauge lengths, a larger fraction of the specimen’s length is deforming during the test. In longer specimens, the portion away from the neck is not continuing to deform after the onset of necking, therefore a smaller percentage of the specimen’s length is contributing to the total deformation.
Because of this it is necessary to compare percents of elongation of various metals with the same gauge length when comparing ductility.
 |
| FIG 15 |
Creep is a time-dependent deformation of a material while under an applied load that is below its yield strength. It is most often occurs at elevated temperature, but some materials creep at room temperature. Creep terminates in rupture if steps are not taken to bring to a halt.
Creep data for general design use are usually obtained under conditions of constant uniaxial loading and constant temperature. Results of tests are usually plotted as strain versus time up to rupture. As indicated in the image, creep often takes place in three stages. In the initial stage, strain occurs at a relatively rapid rate but the rate gradually decreases until it becomes approximately constant during the second stage. This constant creep rate is called the minimum creep rate or steady-state creep rate since it is the slowest creep rate during the test. In the third stage, the strain rate increases until failure occurs.
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| FIG 16 |
Stiffness
It is defined as the property of a material which is rigid and difficult to bend. The example of stiffness is rubber band. If single rubber band is stretch by two fingers the stiffness is less and the flexibility is more. Similarly, if we use the set of rubber band and stretched it by two fingers, the stiffness will be more, rigid and flexibility is less.
The expression of stiffness for an elastic body is as below.
The expression of stiffness for an elastic body is as below.
Here, the stiffness is k, applied force is F, and deflection is δ.
The unit of stiffness is Newtons per meter.
The unit of stiffness is Newtons per meter.
resilience
resilience is the ability of a material to absorb energy when it is deformed elastically, and release that energy upon unloading. Proof resilience is defined as the maximum energy that can be absorbed up to the elastic limit, without creating a permanent distortion. The modulus of resilience is defined as the maximum energy that can be absorbed per unit volume without creating a permanent distortion. It can be calculated by integrating the stress–strain curve from zero to the elastic limit. In uniaxial tension, under the assumptions of linear elasticity,
The area under the linear portion of a stress–strain curve is the resilience of the material (figure 17)
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| FIG 17 |
Ur is the modulus of resilience,
σy is the yield strength,
εy is the yield strain,
and E is the Young's modulus This analysis is not valid for non-linear elastic materials like rubber, for which the approach of area under the curve till elastic limit must be used.
fatigue
fatigue is the progressive and localised structural damage that occurs when a material is subjected to cyclic loading. The maximum stress values are less than the ultimate tensile stress limit, and may be below the yield stress limit of the material.
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| FIG 18 |
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