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Lecture _– Mechanical Properties of Introduction
Metals 1. Often materials are subjected to forces (loads) when they
are used. Mechanical engineers calculate those forces and
I. Introduction material scientists how materials deform (elongate,
II. Elastic Deformation compress, twist) or break as a function of applied load,
III. Engineering Stress time, temperature, and other conditions.
IV. Engineering Strain
V. Modulus of Elasticity 2. Materials scientists learn about these mechanical
VI. Poisson’s Ratio properties by testing materials. Results from the tests
VII. Plastic Deformation depend on the size and shape of material to be tested
(specimen), how it is held, and the way of performing the
test.
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Introduction Elastic Deformation
Force may be applied in the following ways;
A temporary shape change that is self-reversing after the
- Tension force is removed, so that the object returns to its original
- Compression shape, is called elastic deformation.
- Shear
- Torsion In other words, elastic deformation is a change in shape of a
- Bending material at low stress that is recoverable after the stress is
removed.
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Elastic Deformation Engineering Stress
At any load, the engineering stress is the load divided by this
initial cross- area.
While you are pulling, the length increases, but the width and
thickness shrink. At any load, the true stress is the load divided
by the cross-area at that instant.
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Engineering Stress Engineering Stress
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Engineering Strain Modulus of Elasticity
Normal in normal strain does not mean common, or usual Young's modulus ( E ) describes tensile elasticity, or the
strain. It means a direct length-changing stretch (or tendency of an object to deform along an axis when opposing
compression) of an object resulting from a normal stress. It is forces are applied along that axis; it is defined as the ratio of
commonly defined as; tensile stress to tensile strain.
ϵ = ΔL /Lo It is often referred to simply as the elastic modulus.
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Modulus of Elasticity Example
A nylon thread is subjected to a 8.5-N tension force. Knowing
that E = 3.3 GPa and that the length of the thread increases by
1.1%, determine;
(a) the diameter of the thread,
(b) the stress in the thread.
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Example Poisson’s Ratio
Two gage marks are placed exactly 250 mm apart on a 12-mm- The Poisson ratio is the fraction (or percent) of expansion
diameter aluminum rod. Knowing that, with an axial load of divided by the fraction (or percent) of compression, for small
6000 N acting on the rod, the distance between the gage values of these changes.
marks is 250.18 mm, determine the modulus of elasticity of
the aluminum used in the rod. Conversely, if the material is stretched rather than
compressed, it usually tends to contract in the directions
transverse to the direction of stretching.
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Poisson’s Ratio Poisson’s Ratio
Definition of Poisson's ratio
Poisson's ratio is the ratio of transverse contraction strain to
longitudinal extension strain in the direction of stretching
force.
Tensile deformation is considered positive and compressive
deformation is considered negative.
Poisson's ratio, also called Poisson ratio or the Poisson
coefficient, or coefficient de Poisson, is usually represented as
a lower case Greek nu, n.
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Poisson’s Ratio Plastic Deformation
Definition - What does Plastic Deformation mean?
Plastic deformation is a process in which permanent
deformation is caused by a sufficient load. It produces a
permanent change in the shape or size of a solid body
n = - etrans / elongitudinal without fracture, resulting from the application of sustained
stress beyond the elastic limit.
Plastic deformation can be applied in the creation of a variety
of items constructed with metal or plastic, and can be
conducted under controlled circumstances, or may occur
unintentionally.
Plastic deformation is also known as plasticity.
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Plastic Deformation Plastic Deformation
Plastic deformation is a technique in which enough stress is
placed on metal or plastic to result in the object changing its
size or shape in a way that is not reversible.
Plastic deformation is seen in most materials, including;
- Metals
- Plastics
- Soils
- Rocks
- Concrete
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Plastic Deformation
In plastic deformation an object first goes through elastic
Lecture Summary
deformation, which is reversible, so the object may regain its
I. Introduction
original shape.
II. Elastic Deformation
III. Engineering Stress
Soft thermoplastics have a large plastic deformation range, as do
IV. Engineering Strain
ductile metals like copper, silver and gold. Steel does, too, but not
V. Modulus of Elasticity
cast iron. Hard thermosetting plastics like rubber, crystals and
VI. Poisson’s Ratio
ceramics have minimal plastic deformation ranges.
VII. Plastic Deformation
Under tensile stress, plastic deformation is characterized by a
strain-hardening region and a necking region and finally, fracture, BIBLIOGRAPHY:
which is also called rupture. 1. Hannah & Hillier (1995) Applied Mechanics, 3rd edn., Essex, England: Pearson
Education Limited.
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