Mechanical Properties
> Mechanical properties determine the behavior of materials when mechanical force is applied to them.
> The study of mechanical properties is essential for designing and manufacturing purposes.
> The mechanical properties of a material refer to factors such as :
◆ Mechanical Properties of a Material ◆
1) Strain. 2) Stress
3) Strength. 4) Stiffness
5) Hardness. 6) Toughness
7) Elasticity. 8) Ductility
9) Malleability. 10) Brittleness
11) Fatigue. 12) Creep
13) Plasticity
● Strain
> Strain is measured as the deformation caused per unit length in the direction of force applied.
> Strain is divided into two types, elastic and plastic. Some materials, such as rubber, return to their original shape after the force applied on them is removed. This property of materials is called elastic strain. Plastic strain results in a permanent deformation.
● Stress
> When an external load is applied to a material, the material resists deforming effects. This interatomic force F per unit crosssectional area A of a material is referred as stress.
Mathematically
Stress = F/A
>Stress can be classified into different types depending on the load that is applied. These stresses are:
Classification of stress
1) Tensile stress
2) Compressive stress
3) Shear stress
4) Bending stress
1) Tensile stress : In tensile stress, two forces pull the material in opposite directions along the plane of stress.
2) Compressive stress : In compressive stress, adjacent parts of materials press against each other.
3) Shear stress: In shear stress, parts of materials slide across each other.
4) Bending stress: Bending stress is caused when the surfaces of materials are exposed to tension and compression. For example, when metal is bent, tension stretches one surface and the other surface is compressed,Displays how tensile, compressive, and shear stress act on a body.
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| Types of Stresses |
> A material can experience both tensile and compressive stress at a time, when in use. For example, in equipment such as a reactor, the wall is exposed to tensile stress at different locations. This is caused by the temperature and pressure exerted by the fluid against the wall. Compressive stress is applied on the wall from outside. This is caused due to outside pressure and temperature.
● Strength
> The strength of a material is its ability to withstand external forces applied on it during a test or when it is in use.
> These forces cause distortion. The resistance to this distortion is referred to as strength.
> Strength of a material is of the following types:
Types of Strength of a material
1) Tensile strength
2) Compressive strength
3) Yield strength
1) Tensile strength: Tensile strength is defined as the ability of a material to resist change caused by external forces, such as pulling and stretching.
For example, when an object is designed to carry heavy weights, such as bags, the tensile strength is considered.
2) Compressive strength : Compressive strength is the extent to which a material can redist compressive stress without freaking. For example, bricks have high compressive strength; therefore they are used to build chimneys, Concrete also has high compressive strength; therefore, it is used for construction purposes.
3) Yield strength: Strength is affected by increase in temperature. The strength of a material at a particular temperature is known as yield strength.
The yield strength of a material is the point at which a material deforms and does not retain its original shape even after external force is removed.
Stress-strain curve, showing various static strength of material.
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| Stress - Strain Curve |
● Stiffness
> Stiffness is defined as the resistance of a material to elastic deformation. It is also known as Young's modulus of elasticity.
Young's modulus of elasticity (E)=Stress / Strain
> Stiffness and plastic deformation are inversely proportional. To understand stiffness, consider an example of a metal spring. A spring returns to its original shape when the stress applied to it is removed. This property of a spring is known as stiffness
> Another example of stiffness is the wings of an airplane. which are required to maintain their shape in turbulent air.
> A bicycle is yet another example where the frame does not deform permanently for a long duration because the material used exhibits high stiffness.
● Hardness
> The hardness of a material indicates the strength of the material to resist penetration, abrasion and wear.
> Tensile strength is directly proportional to hardness Tensile strength increases with hardness.
> The hardness of a material must be considered when making objects such as knives. A knife can be made of stainless steel or carbon steel. Carbon steel has high hardness value. Therefore, knives made from carbon steel last longer.
> Diamond is the hardest material and is used to cut hard materials. Talc is the softest material.
> Ceramics have high hardness, metals have medium, and plastics low.
● Toughness
> Toughness is the ability of a material to absorb sudden external pressure that exerts force.
> It is measured as the energy absorbed per unit volume of material. In other terms, toughness is the amount of energy absorbed by a material before it develops a fracture. A fracture is a crack in the material.
> When pressure is applied on materials, both elastic and plastic deformations take place. Therefore, toughness can also be defined as the sum of elastic and plastic energy.
> Toughness is also the ability of a material to resist propagation of cracks in the material. Materials with high roughness offer high resistance to propagation of cracks when force is applied.
● Elasticity
> The ability of a material to return to its original shape after the applied load is removed is called elasticity. This is measured as the modulus of elasticity that is the ratio of stress and strain.
> Temperature lovers the modulus of elasticity. Elasticity is an important factor to be considered when selecting materials for high load applications, such as bridges.
> It is important that the material you use to create bridges have high elasticity.
● Ductility
> Ductility is the ability of a material to resist a high amount of plastic deformation before breaking under tension.
> A property by which material can be drawn into fine wires is known as ductility, eg copper, silver, etc.
> Ductility is denoted by percentage elongation, which is defined as the maximum elongation or the maximum.
>The formata to express ductility is given below:
Percentage elongation = Change in length of material x 100 Initial length
● Malleability
> Malleability is the ability of a material to exhibit deformation when compressive force is applied.
> A property by which a material can be beaten into thin sheets is known as malleability, e.g. plastic, gold, etc
● Brittleness
> It is the property of a material that shows little or no plastic deformation before fracture when a force is applied.
>Also it is usually said as the opposite of ductility and malleability.
● Fatigue
> Fatigue is defined as the behaviour of a material when exposed to fluctuating or periodic loads.
> Fatigue strength is the property of a material to withstand continuously varying and alterating loads.
> The level at which the fracture occurs when fluctuating load is applied is lower than the level at which fracture occurs when static load is applied. The level at which fracture takes place is referred to as fatigue failure.
● Creep
> Creep is the permanent plastic deformation of materials when subjected to constant stress or prolonged loading usually at high temperatures. Creep leads to a fracture in the material at static stresses.
>Creep is nothing but a deformation that occurs over a period of time when a material is subjected to constant stress at constant temperature.
> Creep at room temperature is known as low-temperature creep. Some materials in which a low-temperature creep may occur are load pipes, roofing, and glass. The occurrence of creep at high temperature is known as high-temperature creep.
> In metals, creep usually occurs only at elevated temperatures. Creep at room temperature is more common in plastic materials and is called cold flow or deformation under load.
> Data obtained in a creep test usually is presented as a plot of creep vs. time with stress and temperature constant. Slope of the curve is creep rate and end point of the curve is Time for Rupture.
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| Creep Curve |
1. First stage, or primary creep, starts at a rapid rate and slows with time.
2. Second stage, (secondary) creep has a relatively uniform rate.
3. Third stage, (tertiary) creep has an accelerating creep rate and terminates by failure of material at time for rupture.
● Plasticity
> Plasticity is the property of material, which retains the deformation, produced under load permanently. e.g gold. silver lead, etc.
> This property of material is important for forging and in ornamental work. Material deformation can be permanent or temporary. Permanent deformation is irreversible i.e. stays even after removal of the applied forces.
> This property is the opposite of strength. By careful alloying of metals, the combination of plasticity and strength is used to manufacture large structural members.
