Terms of Engineering Materials and Practices for Textile Engineering Students

Terms of Engineering Materials and Practices (EMP) for Textile Engineering Students
Rezaul Rasel
Dept. of Textile Engineering
Mawlana Bhashani Science and Technology University, Tangail, Bangladesh
Email: rasel10002@gmail.com
 


Introduction:
Engineering Material & Practices (EMP) is an important subject for textile engineering students. From this subject student get knowledge of mechanical. In this article, I have explained important terms of Engineering Material & Practices for students of textile technology.

Stress:

The resistance or force per unit area of deformation is called stress.


Stress, σ = External force or Load (P) / Cross sectional area of body (A)

Load: The external force acting on the body is called load.

Strain:
The measure of deformation in the body is known as strain.


Example: If a bar is subjected to a direct load and hence a stress, the bar will change in length. If the bar has an original length L and changes in length δL, the strain produce as follows:
Strain
Strain is thus a measure of the deformation of metal and is non- dimensional. That means it has no unit, it is simply a ratio of two quantities with the same unit.

Types of stress:
  1. Tensile stress
  2. Compressive stress
  3. Shear stress
1. Tensile stress: The stress to which a bar or structure is subjected when in tension.

2. compressive stress: The stress to which a bar or structure is subjected when in compression.

3. Shear stress: Consider a block of a material (fig:A) subjected to a set of equal and opposite force/ shear load Q. There is then a tendency for one layer of the material to slide over another to produce the form of a failure (fig:B). If this failure is restricted then a shear stress is set up, defined as follows.
Fig: shear force and resulting shear stress showing typical form of failure by relative sliding of planes.
Shear stress, τ = Shear load (Q) / Area resisting shear(A)

Strain:
Strain is defined as the ratio of change in dimension of a body, to the original dimension.


Types of Strain:

  1. Tensile strain
  2. Compressive strain
  3. Shear strain
  4. Volumatric strain
1. Tensile strain: If the change in dimension is due to tensile stress which means increase length then the ratio of increase of length to the original length is known as tensile strain.

2. Compressive strain: If the change in dimension due to the compressive stress which means decrease in length, then the ratio of decrease in length to the original length is known as compressive strain.

3. Shear strain: If the change in dimension is due to the shear stress which means the angle through which a body is distorted then the angle is known as shear strain.

4. Volumatric strain: It is the ratio of the change of volume of body to the original volume.

Example of shear strain:
If the block (fig:a) considers to be bi-cycle brake block, it is clear that the rectangular shape of the block will not be retained as the brake is applied and the sfear force introduce. The block will change its shape or strain into the form ( shown in fig:B) . The angle of deformation γ is then termed as shear strain. It is measured in radians.
Fig:a                                                      Fig:b
Double shear:
Consider the simple riveted lap join (shown in fig:a) when load is applied to the plates the rivet is subjected to shear forces tending to shear it on one plane is indicated.

In the butt joint with two cover plate however each revet is subjected to possible shearing on two faces that mean double shear. In such cases twice the area of metal is resisting the applied forces so that the shear stress set up is given by,

Shear stress, τ(In double shear) = P / 2A

Mohr’s circle of stresses
Mohr’s circle of stresses is used for finding the normal, tangential and resultant stresses on an inclined plane.

The Mohr’s circle of stresses is drawn for following cases:
  • A bar is subjected to two unequal like principal stresses
  • A bar is subjected to two unequal unlike principal stresses
Hook’s Law
When a material is loaded within its elastic limit, the stress is proportional to strain.

Mathematically, Stress/Strain =const.

Hook’s law equally holds good for tension as well as compression.

Elastic limit:
In order to preserve the elastic property of a material having limited physical strength, deformations and the stresses which accompany the certain limit, appropriately referred to its elastic limit.

Permanent set:
A material stressed beyond its elastic limit will return only patially to its original form upon complete removal of the deforming force. The deformation remaining is called permanent set.

Elasticity :
Elasticity is that property of a material which enables it within certain limit of stress to recover it’s original form after removal of a deforming force.

Youngs modulus/ Modulus of elasticity:
The ratio of unit stress to unit strain of any given material which may be determined experimentally, then gives us a measure of its stiffness or elasticity, which we call the modulus of elasticity of material.

It is denoted by E,

E= Unit stress(σ) / Unit strain(€)

Some values of E for different material

Serial no. ...........Material ..............Modulus of Elasticity(Gpa or GN/m2)
  1. .................Steel ..................200-300
  2. ............Wrought iron ...........190-200
  3. ...............Cast iron ..............100-600
  4. .............Copper ..................90-100
  5. .............Brass .....................80-90
  6. ............Aluminium............... 60-80
Modulus of rigidity:
Modulus of rigidity is defined as the ratio of the shear stress to the corresponding shear strain within the elastic limit. It is denoted by C, G or N.

Ultimate stress or strength:
It is defined as the greatest unit stress that a material can withstand without rupture.

Allowable stress:

Allowable stress is that portion of the ultimate stress or strain which may safely be used in design.

Factor of safety:
It is the ratio of failure stress to allowable stress. The appropriate

Factor of safety = Failure stress/ Allowable stress

Factor of safety for any given material design depends upon a number of considerations such as (a) The degree of safety required
  • The degree of economy desire
  • Depend ability of the material
  • Permanency of design ( in some cases lower factor of safety are permitted in temporary design)
  • Load condition
  • Accessibility of parts for inspection and maintenance.
Poisons ratio
The ratio of unrestrained unit contraction (or expansion) to the unit longitudinal elongation ( or contraction) is called Poisons Ratio.It is denoted by µ

Poisons ratio (µ) = Laternal strain / Longitudinal strain

ALLOY
An alloy is a coherent metallic mass formed by intimate association of two or more substances, at least one of them bing metal.

Fe + c = Steel
Cu + Zn = brass

Effect of alloy:
  1. Increase strength
  2. Reduction in cost
  3. Lower melting point
  4. Development of certain desirable properties by heat treatment
  5. Decrease ductility(except brass)
  6. Improve corrosion resistance
  7. Improve magnetic properties of alloy metal
  8. Increase hard enability
Alloying of steel:
1. Steel is an alloy of iron and carbon containing less than 2% carbon

2. Stainless steel contains a maximum of 1.2% carbon a minimum of 10.5% chromium(Standard EN-10088-1) and other alloying elements.

The presence of chromium confers on stainless steel. It is principal quality that is corossion resistance.The alloying elements, depending on their percentages, give stainless steel their physical, chemical and their mechanical properties.

The presence of alloying elements is the starting point for obtaining the desired properties, various production processes facilitate this issue. The carbon and iron steel brass together with addition of various alloying elements ,provide the balance of each grade and determine which stainless family it belongs.

3. The most frequently used alloying elements are Ni, M0, Mn,Ti, N,C,Si,V(vanadium), Al.
Classification of metal
Composition
Percentage of carbon
C.I(cast iron)
Iron, carbon
>2%
C.S
Iron, carbon
<2%
SPS
Iron, carbon, Cr or Ni or M0 (min 5%)
<2%
SS
Iron,carbon,Cr >=5%(STD.EN 10088-1)
< or = 1.2%
 

SPRING:
A spring is a device in which the material is arranged in such a way that it can undergo a considerable change without getting permanently distorted.

Spring is used to absorb energy due to resilience, which may be restorted as an when required..

The quality of spring is judged from the energy it can absorb. For example,
In a watch the spring is wound to absorb strain energy. This energy is released to run the watch, when the spring regains its original shape.

(Resilience: The stored energy of a strained or elastic material such as in a compressed spring or in a rubber damper which have inherent damping properties.)

Types of spring:
  1. Bending spring
  2. Torsion spring
Stiffness of spring:
The load required to produce a unit deflection in a spring is called the stiffness of spring.

Bending spring:
A spring which is subjected to bending only and resilience is also due to bending is called bending spring.

Torsion spring:
A spring which is subjected to torsion or twisting moment and the resilience is also due to torsion is called torsion spring.

Leaf spring / The carriage spring /Semi elliptical leaf spring:
They are widely use in railway wagons, buses and road vehicles. Those are used to absorb shocks which give an unpleasant feelings to the passengers. The energy absorbed by a laminated spring during a shock, is released immediately without doing any useful work.

A laminated spring is very simple consists of a number of parallel stripes of a metal having different lengths but same width and placed one over the other in lamination. All the plates are initially bend to the same radius and are free to slide one over the other.

When the spring is loaded to the designed load all the plates become flat and the central deflection disappears. The purpose of this type of arrangement of plates is to make the spring of uniform strength throughout. This is achieved by tapering the ends of the lamination. The semi elliptical type spring rests on the axis of the vehicle and its top plate is pinned at the ends to the chassis of the vehicle.

Springs in series and parallel:
Sometimes two or more springs are used at one place. Though there are many ways of using this spring, yet the springs in series and parallel are important from the subject point of view.

Spring in series:
In this case the two springs are connected in series. Each spring is subjected to the same load applied at the end of one spring. A little consideration will show that the total extension of the assembly is equal to the algebraic summation of the extension of the two springs.

Spring in parallel:
In this case the two springs are connected in parallel. The extension of each spring is the same. A little consideration will show that the load applied on the assembly is shared by the two springs.

Helical spring
It is a torsion spring and made up of a wire coil into a helix. Though there are many types of helical spring the following two types are vastly discussed.
  1. Closely coiled helical spring
  2. Open coiled helical spring
Closely coiled helical spring:
In a closely coiled helical spring the spring wire is coiled so close that the each turn is practically a plane at right angles to the axis of the helix and the wire is subjected to torsion. The bending stress is negligible is compared to the torsional stress.a closely coiled helical stress is subjected to
  1. Axial loading
  2. Axial twist
Closely coiled helical spring subjected to an axial load:

Let,

d = diameter of the spring wire
R = Mean radius of the spring coil
n = no. of turns of coil
C = modulus of rigidity
W = Axial load on the spring
τ = Maximum shear stress induced in the wire due to twisting
ϴ = Angle of twist in the spring wire
δ = Deflection of the spring as a result of axial load

Consider a closely coiled helical spring subjected to an axial load as shown in the figure (a)

A little consideration will show that the load W will cause a twisting moment. Therefore twisting moment T = WR…(1)

We know that the twisting moment

T = π/16*d3*τ….(2)

We also know that the length of the wire L = length of one coil * no. of coils
                                                                 = 2πR*n ….(4)

For one half turn if one cross section twists through an angle ϴ (fig:b) relative to the other then from the torsion theory we get,


Equation (6) is the required equation for closely coiled helical spring when it is subjected to an axial load.

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