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 YA J H A N J IA 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 SH I N D U S TA 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 MATERIALSUNIT-1

LECT

URE

2

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 SCIENCESHINDUSTAN 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

SCHOOL OF AERONAUTICAL SCIENCES

HINDUSTAN UNIVERSITY

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.

Impact test

IzodCharpy


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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