TITLE
Tensile testing of materials
OBJECTIVE
To analyze and evaluate the tensile properties of materials such as values of Tensile
strength, Yield strength, Percentage elongation, fracture strain and Young's Modulus of
the selected metal (steel and aluminum ) when subjected to uniaxial tensile loading.
To explain deformation and fracture characteristics of different materials such as
aluminum, steel or any other metal.
INTRODUCTION
Tensile testing is one of the most fundamental tests for engineering, and provides
valuable information about a material such as how they will behave under load and its associated
properties. These properties can be used for design and analysis of engineering structures, and
for developing new materials that better suit a specified use. Most of the time this test is used to
evaluate material parameters such as ultimate strength, yield strength, percentage elongation,
percentage area of reduction and Young's modulus.
In this lab experiment the tensile testing is carried out by applying longitudinal or axial
load at a specific extension rate to a standard tensile specimen with known dimensions (gauge
length and cross sectional area perpendicular to the load direction) till failure. The applied tensile
load and extension are recorded during the test for the calculation of stress and strain.
The equipment used for tensile testing ranges from simple devices to complicated
controlled systems. In this lab experiment a device called a Tensometer is used to measure,
calculate and draw the Stress- Strain graph. In order to use this device, material to be tested
must be cut to a specific shape so as to fit the grips, most usually in the form of a dog-bone shape
when flat sheet is being tested.
THEORY
Stress � =�/� Strain � = Δ�/�0
F- Applied load ΔL- Elongation of the specimen after the test
A0- Initial cross sectional area L0- Initial gauge length
Young’s modulus E = �/ �
Young’s Modulus (E) or the modulus of elasticity is a measure of a materials stiffness.
The higher the Young’s modulus value the stiffer the material.
Young’s modulus can be calculated from tensile test stress (�)/strain (�) graphs–derived
from load/extension graphs. The slope of the graph is used to calculate E when the
material is obeying Hooke’s law.
Yield strength σy = Py/ A0
It is often difficult to precisely define yielding due to the wide variety of stress–strain
curves exhibited by real materials. However by considering the stress-strain curve beyond
the elastic portion, if the tensile loading continues, yielding occurs at the beginning of
plastic deformation. The yield stress, σy, can be obtained by dividing the load at yielding
(Py) by the original cross-sectional area of the specimen (Ao)
Ultimate Tensile Strength σTS = Pmax / A0
Ultimate tensile strength (UTS), often shortened to tensile strength (TS) is the maximum
stress that a material can withstand while being stretched or pulled before failing or
breaking. Tensile strength is not the same as compressive strength and the values can be
quite different.
If the load is continuously applied, the stress-strain curve will reach the maximum point,
which is the ultimate tensile strength (UTS, σTS). At this point, the specimen can
withstand the highest stress before necking takes place. This can be observed by a local
reduction in the cross sectional area of the specimen generally observed in the center of
the gauge length.
When a metal is subjected to an external tensile loading, the metal will undergo elastic and
plastic deformation. , the metal will elastically deform giving a linear relationship of load and
extension. These two parameters are then used for the calculation of the stress and strain to give
a relationship
MATERIALS AND APPARATUS
Material
Specimen – Steel
Equipment
Tensometer
Percentage Elongation gauge
Percentage Area gauge
Micrometer
A partial segment of the
Tensometer
Figure of Micrometer while measuring Figure of Percentage Elongation
the length of the steel specimen after the gauge while measuring the percentage
experiment change of the steel specimen after the
experiment
PROCEDURE
A sample of medium carbon steel was tested by using a manual Tensile-Testing Machine.
Horizontally fixed sample is subjected to a uniaxial tensile force with a testing speed of about
20mm/min. The force acting on the sample and the extension will be measured and recorded in
the machine. Basically meaning, placing the test specimen in the testing machine and slowly
extending it until it fractures and during this process, the elongation of the gauge section is
recorded against the applied force.
Metallic specimen used for the tensile test
A- Length of reduced section
D- Diameter
G- Gage length
R- Radius of fillet
First choosing and measured the gauge length, width and the cross sectional area of it by
using the micrometer, elongation gauge and area gauge. Then before loading the
specimen to the Tensometer (tensile test machine) the computer system connected to the
machine was setup by inputting the necessary information of gauge length and width of
the specimen. The computer system was then prepared to record data and output
necessary graphs. Specimen was loaded into the Tensometer and the paddle was rotated
in a constant speed until the specimen was fractured. The final graph was finally given as
an output from the computer.
OBSERVATIONS
o Observations
We observed that when the metal was subjected to an external tensile loading, the
necking point was unseen until it was about to break. There for the elastic deformation
was invisible to the eye while doing the experiment. But after the Necking occurred, we
were able to witness the deformation the metal took.
Figure; The metal specimen after the necking occurred
o Observation sheet Attached next page
o Graph (Stress vs. Strain)
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CALCULATIONS
Initial diameter of the mild metal specimen - 16.1 mm
Initial length of the mild metal specimen - 4.5 mm
Initial are of the mild metal specimen - π (d/2)2 15.9043 mm2
RESULTS
After experiment measurements;
Elongation Percentage - 30%
Percentage of reduction area - 20%
Percentage of reduction area - 60%
at the necking point
Stress vs Strain curve of
Percentage of Elongation - 31% mild Steel
Final length of specimen - 21.1 mm
Ultimate tensile strength - 583.5 N/mm2
Stress vs Strain curve of
Aluminum
CONCLUTION
This tensile test experiment is important for determining a material’s properties, limits and its
potential application in a wide range of industries. If a material is to be used in an engineering
structure it will be subjected to various loads and it is important to know that the material is
strong enough to withstand the loads that it will experience during its service life. In summary,
tensile properties should be considered as important design parameters for the selection of
engineering materials for their desired application. Engineers have played a significant role in
that they should be able to analyze and understand material behavior and properties through
these mechanical testing parameters.
REFERENCE
Tensile testing https://en.wikipedia.org/wiki/Tensile_testing
Stress and its effect on Materials http://www.scribd.com/doc/22919957/Tensile-Test-#scribd
Davis, Joseph R. (2004). Tensile testing ( 2nd Ed.). ASM International
Ashby, M. (2006).Engineering Materials 1: An Introduction to Properties, Applications and
Design. 3rd ed. Butterworth-Heinemann
