STRESS STRAIN , EQUIPMENTS USED IN MATERIAL TESTING LAB

1. Assignment # 1
STRESS:
Stress is a measure of the internal forces acting within a deformable body.
Quantitatively, it is a measure of the average force per unit area of a surface within
the body on which internal forces act.
TYPES OF STRESS
Stresses occur in any material that is subject to a load or any applied force. There
are many types of stresses, but they can all be generally classified in one of six
categories
1. Residual Stress:
Residual stresses are due to the manufacturing processes that leave stresses in a
material. Welding leaves residual stresses in the metals welded.
2. Structural Stress:
Structural stresses are stresses produced in structural members because of the
weights they support. The weights provide the loadings. These stresses are
found in building foundations and frameworks, as well as in machinery parts.
3. Pressure Stress:
Pressure stresses are stresses induced in vessels containing pressurized
materials. The loading is provided by the same force producing the pressure
4. Flow Stress:
Flow stresses occur when a mass of flowing fluid induces a dynamic pressure
on a conduit wall. The force of the fluid striking the wall acts as the load. Water
hammer is an example of a transient flow stress
5. Thermal Stress:
Thermal stresses exist whenever temperature gradients are present in a material.
Different temperatures produce different expansions and subject materials to
internal stress. This type of stress is particularly noticeable in mechanisms
operating at high temperatures that are cooled by a cold fluid.
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2. Assignment # 1
6. Fatigue Stress:
Fatigue stresses are due to cyclic application of a stress. The stresses could be
due to vibration or thermal cycling.
The importance of all stresses is increased when the materials supporting them are
flawed. Flaws tend to add additional stress to a material. Also, when loadings are
cyclic or unsteady, stresses can effect a material more severely. The additional
stresses associated with flaws and cyclic loading may exceed the stress necessary
for a material to fail. Stress intensity within the body of a component is expressed
as one of three basic types of internal load. They are known as tensile,
compressive, and shear. Figure 1 illustrates the different types of stress.
Mathematically, there are only two types of internal load because tensile and
compressive stress may be regarded as the positive and negative versions of the
same type of normal loading.
As illustrated in Figure 1, the plane of a tensile or compressive stress lies
perpendicular to the axis of operation of the force from which it originates. The
plane of a shear stress lies in the plane of the force system from which it originates.
It is essential to keep these differences quite clear both in mind and mode of
expression.
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3. Assignment # 1
a) Tensile Stress:
Tensile stress is that type of stress in which the two sections of material on
either side of a stress plane tend to pull apart or elongate as illustrated in Figure
1(a).
b) Compressive Stress:
Compressive stress is the reverse of tensile stress. Adjacent parts of the material
tend to press against each other through a typical stress plane as illustrated in
Figure 1(b).
c) Shear Stress:
Shear stress exists when two parts of a material tend to slide across each other
in any typical plane of shear upon application of force parallel to that plane as
illustrated in Figure 1(c).
Assessment of mechanical properties is made by addressing the three basic stress
types. Because tensile and compressive loads produce stresses that act across a
plane, in a direction perpendicular (normal) to the plane, tensile and compressive
stresses are called normal stresses.
For tensile stresses: "+SN" (or "SN") or "s" (sigma)
For compressive stresses: "-SN" or "-s" (minus sigma)
The ability of a material to react to compressive stress or pressure is called
compressibility. For example, metals and liquids are incompressible, but gases and
vapors are compressible. The shear stress is equal to the force divided by the area
of the face parallel to the direction in which the force acts, as shown in Figure 1(c)
above.
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4. Assignment # 1
Stress strain curve for steel
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5. Assignment # 1
EQUIPMENTS USED IN MATERIAL TESTING LAB
1. Computerized Universal Tester:
Computerized Universal Tester to find out the following parameters
1. Tensile strength
2. Compression
3. Elongation
4. Fold-resisting
5. Bending
6. Flaking
7. Shearing
8. Adhesive
2. Material Testing Machines:
It has been specialy designed for
testing metals and other materials
under tension, compression bending,
transverse and shear loads. It is equally
suitable to carry out hardness test on
metals.
3. Mortar penetometer:
It is used for finding out the rate of hardening of
mortars sieved from concrete mixtures, by
means of penetraion needles of different cross-
sectional areas
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6. Assignment # 1
4. Auto tensile tester:
Auto tensile tester is applicable in
tensile, peeling, tearing, heat seal and
adhesivetest of plastic film, complex
film, flexible packaging materials,
adhesives, adhesive tapes, pressure
sensitive tape, medical plasters,
protective films, release paper, rubber
and paper, etc
5. Tensile Tester:
It is used for Adhesives, Peel Testing, Seal
Strength, 180° Peel Strength, Bond
Strength.
Determination of tensile strength, stretch at
break and tensile energy absorption
6. Impact Tester:
The impact test is a method for evaluating
the toughness and notch sensitivity of
engineering materials. It is usually used to
test the toughness of metals, but similar
tests are used for polymers, ceramics and
composites
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7. Assignment # 1
7. Nick Break and Weldability tester:
The principle of this test is to break the sample through the weld metal in order to
examine the fractured surface
8. Tensile bend compression flarring tester:
A tensile test measures the resistance of a material to a static or slowly applied
force.
9. Bend:
Bend testing is a procedure to determine the relative ductility of metal that is to be
formed (usually sheet, strip, plate or wire) or to determine soundness and
toughness of metal
10. Compression:
Compression testing is a method for assessing the
ability of a material to withstand compressive
loads.
11. Hardness Tester:
This is a simple indentation test for determining the
hardness of a wide variety of materials. The test
consists of applying a prescribed load, usually
between 500kg and 3000kg
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