Aircraft materials lecture 2

T e n s i l e T e s t i n g a n d C o m p r e s s i v e T e s t i n g M e t h o d s
B Y : K A N U P R I Y A J H A N J I
A S S T . P R O F E S S O R
S C H O O L O F A E R O N A U T I C A L S C I E N C E S
H I N D U S T A N U N I V E R S I T Y
k a n u p r i y a j @ h i n d u s t a n u n i v . a c . i n
AIRCRAFT MATERIALS
UNIT-1
LECTURE2

INSTRUCTIONAL OBJECTIVES
• By the end of lecture, the students will learn the working principle of
ultimate testing machine and procedure of performing the tensile
and compressive tests.
• Students will also have knowledge about the various impact tests.
SCHOOL OF AERONAUTICAL SCIENCES
HINDUSTAN UNIVERSITY

CONTENTS
• Tensile testing
• Equipment for testing
• Process of testing
• Components of Universal Testing Machine
• Use of UTM
• Compressive strength
• Compressive test
• Charpy impact test
• Izod impact test

TENSILE TESTING
• Tensile testing, also known as tension
testing, is a fundamental materials
science test in which a sample is
subjected to a controlled tension until
failure.
• The results from the test are commonly
used to select a material for an
application, for quality control, and to
predict how a material will react under
other types of forces.
• Properties that are directly measured via
a tensile test are ultimate tensile
strength, maximum elongation and
reduction in area.
• The test specimen is shown in the figure.

TENSILE TESTING
• From these measurements the following properties can also be
determined: Young's modulus, Poisson's ratio, yield strength,
and strain-hardening characteristics.
• Uniaxial tensile testing is the most commonly used for obtaining the
mechanical characteristics of isotropic materials.
• For anisotropic materials, such as composite materials and
textiles, biaxial tensile testing is required.

EQUIPMENT
• The most common testing machine
used in tensile testing is
the universal testing machine.
• This type of machine has
two crossheads; one is adjusted for
the length of the specimen and the
other is driven to apply tension to
the test specimen.
• There are two types: hydraulic
powered
and electromagnetically powered
machines

PROCESS
• The test process involves placing the test specimen in the testing machine and slowly
extending it until it fractures.
• During this process, the elongation of the gauge section is recorded against the applied
force.
• The data is manipulated so that it is not specific to the geometry of the test sample.
• The elongation measurement is used to calculate the engineering strain, ε, using the
following equation:[
• where ΔL is the change in gauge length, L0 is the initial gauge length, and L is the final
length.
• The force measurement is used to calculate the engineering stress, σ, using the following
equation:
• where F is the tensile force and A is the nominal cross-section of the specimen. The
machine does these calculations as the force increases, so that the data points can be
graphed into a stress–strain curve.

COMPONENTS OF UNIVERSAL TESTING
MACHINE
• Load frame - Usually consisting of two
strong supports for the machine. Some
small machines have a single support.
• Load cell - A force transducer or other
means of measuring the load is required.
Periodic calibration is usually required by
governing regulations or quality system.
• Cross head - A movable cross head
(crosshead) is controlled to move up or
down. Usually this is at a constant speed:
sometimes called a constant rate of
extension (CRE) machine.
• Some machines can program the
crosshead speed or conduct cyclical
testing, testing at constant force, testing at
constant deformation, etc.
Electromechanical, servo-hydraulic, linear
drive, and resonance drive are used.

COMPONENTS OF UNIVERSAL TESTING
MACHINE
• Means of measuring extension or
deformation - Many tests require a measure
of the response of the test specimen to the
movement of the cross head.
• Extensometers are sometimes used.
• Output device - A means of providing the
test result is needed. Some older machines
have dial or digital displays and chart
recorders.
• Many newer machines have a computer
interface for analysis and printing.
• Conditioning - Many tests require controlled
conditioning (temperature, humidity,
pressure, etc.). The machine can be in a
controlled room or a special environmental
chamber can be placed around the test
specimen for the test.
• Test fixtures, specimen holding jaws, and
related sample making equipment are
called for in many test methods.

USE OF UTM
• The set-up and usage are detailed in a test method, often published by a standards
organization.
• This specifies the sample preparation, fixturing, gauge length (the length which is under
study or observation), analysis, etc.
• The specimen is placed in the machine between the grips and an extensometer if
required can automatically record the change in gauge length during the test.
• If an extensometer is not fitted, the machine itself can record the displacement between
its cross heads on which the specimen is held.
• However, this method not only records the change in length of the specimen but also all
other extending / elastic components of the testing machine and its drive systems
including any slipping of the specimen in the grips.
• Once the machine is started it begins to apply an increasing load on specimen.
• Throughout the tests the control system and its associated software record the load and
extension or compression of the specimen.

COMPRESSIVE STRENGTH
• The compressive strength is the
capacity of a material or structure to
withstand loads tending to reduce
size.
• It can be measured by plotting
applied force against deformation in
a testing machine.
• Some materials fracture at their
compressive strength limit; others
deform irreversibly, so a given amount
of deformation may be considered as
the limit for compressive load.
• Compressive strength is a key value
for design of structures.
• Compressive strength is often
measured on a universal testing
machine.
• The compressive test specimen is
shown in figure.

COMPRESSIVE TEST
• . • Compression is the opposite of tensile
testing. This kind of testing is used for
brittle materials.
• The compressive strength is usually
obtained experimentally by means of
a compressive test.
• The apparatus used for this
experiment is the same as that used
in a tensile test.
• However, rather than applying a
uniaxial tensile load, a uniaxial
compressive load is applied.
• As can be imagined, the specimen
(usually cylindrical) is shortened as
well as spread laterally

IMPACT TEST
• The purpose of impact testing is to determine the ability of materials
to withstand impact or shock or suddenly applied load while in
service.
• It is usually thought of in terms of two objects striking each other at
high relative speeds.

INTRODUCTION
• The CHARPY Impact Tests are conducted on instrumented machines
capable of measuring less than 1 foot-pound. to 300 foot-pounds. at
temperatures ranging from -320°F to over 2000°F.
• Impact test specimen types include notch configurations such as V-
Notch, U-Notch, Key-Hole Notch, as well as Un-notched and ISO
(DIN) V-Notch, with capabilities of impact testing sub-size specimens
down to ¼ size.
• IZOD Impact Testing can be done up to 240 foot-pounds. on
standard single notch and type-X3 specimens.

TYPES OF IMPACT TEST SPECIMENS

IMPACT TESTING
• Notched-bar impact test of
metals provides information on
failure mode under high
velocity loading conditions
leading sudden fracture where
a sharp stress raiser (notch) is
present.
• The energy absorbed at
fracture is generally related to
the area under the stress-strain
curve which is termed as
toughness in some references

IMPACT TESTING
• Figure (b) below shows the
brittle fracture in mild steel
• Brittle materials have a small
area under the stress-strain
curve (due to its limited
toughness) and as a result, little
energy is absorbed during
impact failure.
• The fracture surfaces for low
energy impact failures,
indicating brittle behaviour, are
relatively smooth and have
crystalline appearance in the
metals

IMPACT TESTING
• Figure (a) below shows cup
cone fracture in aluminum
• As plastic deformation
capability of the materials
(ductility) increases, the area
under the curve also increases
and absorbed energy and
respectively toughness
increase.
• The fracture surfaces for high
energy fractures have regions
of shear where the fracture
surface is inclined about 45° to
the tensile stress, and have
rougher and more highly
deformed appearance, called
fibrous fracture.

IMPACT TESTING
• The stress- strain curve for both
ductile and brittle materials is
shown in figure.

CHARPY IMPACT TEST
• Although two standardized tests, the Charpy and Izod, were designed and
used extensively to measure the impact energy, Charpy v-notched impact
tests are more common in practice.
• The apparatus for performing impact tests is illustrated schematically in
Figure

CHARPY IMPACT TEST
• The Charpy Impact Test is
commonly used on metals, but
is also applied to composites,
ceramics and polymers.
• The standard Charpy Impact
Test specimen consist of a bar
of metal, or other material,
55x10x10mm having a notch
machined across one of the
larger dimensions.
• Typical v-notched specimen is
shown in figure

CHARPY IMPACT TEST
• The load is applied as an impact blow from a weighted pendulum
hammer that is released from a position at a fixed height h.
• The specimen is positioned at the base and with the release of
pendulum, which has a knife edge, strikes and fractures the
specimen at the notch.
• The pendulum continues its swing, rising a maximum height h ' which
should be lower than h naturally.
• The energy absorbed at fracture E can be obtained by simply
calculating the difference in potential energy of the pendulum
before and after the test such as,
E = m.g.(h-h ')
• where m is the mass of pendulum and g is the gravitational
acceleration.

IZOD IMPACT TEST
• Izod test is carried out on a cantilever test specimen 10 X 10 X 75mm
long having a standard 45⁰ notch 2 mm deep.
• In the presence of notch ductile material behave like a brittle one so
that rupture can takes place during impact. This property is called
notch sensitivity.

IMPACT TESTING
MACHINE
• The same testing machine is
used for both charpy and izod
impact testing.
• The impact testing machine is
shown in figure.

TESTING PROCEDURE
• The specimen is held vertically
as a cantilever between two
jaws in such a way that the
striking hammer in the swinging
pendulum strikes the specimen
on the same face as that of
notch.
• The specimen is broken by
means of that pendulum which
is allowed to fall from a certain
height to cause an impact
load on the specimen
• The angle of rise of the
pendulum after the rupture of
the specimen or the energy to
rupture the specimen is
indicated on the graduated
scale by the pointer.
• The energy required to rupture
the specimen is a function of
angle of rise.

TEST CALCULATIONS
• Let “W” be the weight of
pendulum and “α” be the
angle through which
pendulum falls and “β”
be the angle through
which pendulum rises
and “R” is the distance
between the center of
gravity of pendulum and
axis of rotation.
• From the geometry of the
testing machine,
• Initial energy of the
pendulum before
striking= WR(1-cos α)
• Final energy of the
pendulum after breaking
the specimen=WR(1-cos
β)
• Assuming there are no
losses, energy required to
breakaway the specimen
=WR(1-cos α)-WR(1-cos β)
= WR(cos β - cos α)
= W(h1 – h2)

DUCTILE-BRITTLE TRANSITION IN
IMPACT TEST
• The notched bar impact test is
also used to determine the
ductile brittle transition
temperature of a material at
which there is a big change in
the energy absorbed.
• A curve plotted between
impact energy and
temperature of the specimen
and using that the transition
temperature can be
determined as shown in figure

Aircraft materials lecture 2

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