Table of Contents
TABLE OF CONTENTS.........................................................................................................................................I
PREFACE ..........................................................................................................................................................II
LIST OF FIGURES..............................................................................................................................................III
LIST OF TABLES .............................................................................................................................................. IV
LIST OF ABBREVIATIONS ................................................................................................................................. V
LAB SAFETY .................................................................................................................................................... VI
LAB REPORT FORMAT .................................................................................................................................. VIII
EXPERIMENT 1: TENSILE TEST .......................................................................................................................... 1
EXPERIMENT 2: CREEP IN METALS ................................................................................................................... 9
i
Preface
This manual is prepared to cover the material and experiments of Material Science for
Engineering Lab (EC102) course. It covers a wide range of experiments that can be utilized
to introduce the most important concepts of material properties and its relation to the
microstructure of the material, also the lab introduce the ability of materials to withstand
external loads without failure which is the base of machine and components design. On the
other hand, material science lab will support student information in testing material
properties experimentally and understand the types of loadings and stresses induced in
members due to these loadings.
The most important experiments related to material properties are tensile test, creep test,
hardness test, impact test, torsion test, and fatigue test. Also other experiments conducted
related to study the micro structure of materials and the effect of engineering processes like
heat treatment on microstructure and properties.
The overall aim of the Laboratory is to strengthen and support the student information
and knowledge in materials and its roll in design of machine components and structural
designs and proper selection of materials
Also, the lab shall satisfy the following secondary objectives:
▪ Firsthand experience and practical information related to the apparatus and
equipment available on the laboratory.
▪ Studying and identifying the principle and the concept of operation of some
measurement instruments like force, torque deflection angular speed, …. etc., and
some control systems and safety devices.
▪ To increase the student skills in analyzing data and writing good lab reports.
▪ To enhance teamwork and leadership skills.
Students are encouraged to learn to do all experiments following the procedures
presented in this manual, and to document them in lab reports using similar styles and
format.
The experiments in this manual are designed to give engineering students an
introduction to the Experiment followed by theory and the physical principles that will be
demonstrated experimentally and laboratory procedures for performing the experiment.
ii
List of Figures
FIGURE 1: TYPICAL STRESS STRAIN DIAGRAM ...................................................................................... 3
FIGURE 2: STRESS STRAIN DIAGRAM FOR DIFFERENT MATERIALS AND CASES ..................................... 4
FIGURE 3: STATE OF STRESS FOR TENSILE TEST ................................................................................... 5
FIGURE 4: TYPES OF FRACTUR FOR TENSILE TEST ................................................................................. 5
FIGURE 5: TENSILE TESTING MACHINE ................................................................................................. 6
FIGURE 6: STRESS/STRAIN CURVES ..................................................................................................... 10
FIGURE 7: CREEP AT DIFFERENT TEMPERATURES AND STRESS. ......................................................... 11
FIGURE 8: CREEP CURVE ..................................................................................................................... 11
FIGURE 9: CREEP APPARATUS ............................................................................................................. 13
FIGURE 10: KEY DIMENSION OF THE SPECIMENS ................................................................................ 14
FIGURE 11: SOFTWARE INTERFACE...................................................................................................... 14
iii
List of Tables
TABLE 1: SPECIMEN DETAILS ................................................................................................................. 8
TABLE 2: TENSILE TEST RESULTS ........................................................................................................... 8
TABLE 3: SETUP AND SPECIMEN DATA ................................................................................................. 15
iv
List of Abbreviations
σ
σt
σy
ɛ
ɛt
ɛy
ɛf
E
F
A
Ao
l
li
lo
Ur
𝝂
𝜺̇
Q
R
T
∆L
Tensile test
Engineering stress - Stress
True stress
Yield stress
Engineering strain - Strain
True strain
Yield strain
Strain at fracture
Modulus of elasticity
Force - Load
Area – cross sectional area
Initial area
Length – Current length
Instantaneous length
Initial length
Modulus of resilience
Poisson’s ratio
Creep test
Strain rate
Activation energy
Universal gas constant
Absolute temperature
Extension – change in length
Torsion test
Torque
T
Shear stress
τ
Radius of shaft
r
Polar moment of inertia
J
Twist angle
θ
Shear strain
γ
Length
L
Ultimate shear stress
τu
Shear modulus of elasticity
G
Modulus of rigidity
Maximum shear stress
τmax
Buckling test
Critical buckling load
Pcr
Second moment of area
I
Effective length factor
K
Radius of gyration
r
Slenderness ratio
L/r
Quantitative Metallography
Flow stress
σ
Lattice friction stress
σo
Grain size
d
Mean intercept length
𝒍̅
Hardness test
LT
HR
Rockwell Hardness number
P
Fo
F1
e
E
D
BHN
HV
TS
Minor load
Major load
Permanent depth
Constant
Diameter of ball
Brinell Hardness Number
Vickers Hardness
Tensile Strength
Impact test
Mass
Hight
M
m
h
n
G
Af
Ao
%CW
Df
Do
v
Total length
Total number of
intersections
Magnifications
Average number of grain size
per square inch
Grain size number
Cold work
Final cross-sectional area
Original cross-sectional area
Percentage of cold work
Final diameter
Original diameter
Lab Safety
Safety Policy:
o Safety should be the primary concern when working in any laboratory. Unsafe
behavior will not be tolerated in the lab
o Your laboratory instructor will warn you of potentially dangerous situations which
might arise.
o At all times students will conduct themselves in safe manner.
o Safety instructions listed inside the lab should be followed when performing any
activity in the lab.
o Students not allowed to enter the lab unless they wear proper safety lab coat, also
open shoes is not allowed in lab sessions.
o Disciplinary action will be taken against violators.
•
Safety Instructions:
o No smoking is allowed in the laboratory.
o No food or beverages are allowed in the laboratory.
o Wear lab coat while conducting experiments all the time.
o Use appropriate personal protective equipment at all time, like Gloves, Safety
glasses, Skin Protection, Hearing Protection, and Foot Protection (don’t wear open
sandals)
o No running, playing, bantering, and kidding in the laboratory.
o Know locations of first aid and all emergency equipment, such as fire alarm, water
hoses, fire extinguishers, fire blankets, eyewash stations, and safety showers.
Know how to find and use them.
o Use laboratory equipment for its designed purpose.
o Always follow instructions and use only machines and equipment that you are
authorized and qualified to operate.
o Know and follow safety rules for specific experiments or tasks.
o Working alone should be avoided. Someone should always be within call when a
laboratory procedure is being performed.
o Avoid exposure to gases, vapors, and particulates by using a properly functioning
laboratory fume hood.
vi
o Use ground fault circuit interrupters where there is a risk of an operator coming in
contact with water and electrical equipment simultaneously.
o Follow electrical safety rules and make sure your hands are dry before using
electrical equipment, grounding portable electrical tools. Make sure electrical
wires are connected properly without short circuit before operating. Wear
protective clothing, well-insulated groves and boots, if required.
o Only trained, qualified personnel may repair or modify electrical or any
equipment.
o
•Reduce fire hazard.
•Use shower for fire victims.
•Smother flames by wrapping in fire blankets.
•Spills of flammable solvents can be a source of fire.
o Upon hearing fire alarm, you should evacuate the area and follow emergency
procedure.
o Report any damage to equipment or instrument and broken glassware to the
laboratory instructor as soon as such damage occurs.
o Wash hands upon completion of laboratory procedures and remove all protective
equipment including gloves and lab coats.
vii
Lab Report Format
A good lab. report should contain the following items:
1. COVER PAGE:
The followings are included in a cover page:
•
Name of University, Faculty and Department.
•
Student name and number.
•
Experiment name and number.
•
Instructor name.
•
Date.
2. TABLE OF CONTENT:
•
Number all pages, figures, and charts
•
Include main subjects and sub subjects.
3. TITLE:
•
Should tell the reader what report is about.
•
Should be as short as possible.
4. OBJECTIVE:
•
Should give the reasons for doing the work.
•
Should define the problem.
5. THEORY:
•
The main assumptions should be stated and justified.
•
Should be brief.
•
The necessary equations should be numbered.
6. APPARATUS:
•
Describe the experimental rig and the instruments used to perform the
experiment.
•
Include sketches of the test setup where appropriate.
7. PROCEDURE:
•
Should be brief.
•
Should be in short sentences.
•
Should be in passive voice.
8. RESULTS:
•
Each table should have a number and a title.
viii
•
The data should have units.
9. SAMPLE CALCULATIONS:
•
Should be clear.
•
Take care of units and make necessary conversions.
•
The calculated results should be tabulated, numbered, and named.
10. GRAPHS:
•
Scales and units shown clearly.
•
Points shown clearly.
•
The graphs should have necessary numerical working such as determination of
slope.
•
The title of the graph to explain what graph show.
11. DISCUSSION:
•
The results should be adequate.
•
Give an opinion on the reliability of the results.
•
State the importance of this experiment on our real life, and mention some of
applications.
•
Dose the experiment helps you to understand the theory.
12. CONCLUSION:
•
Compare results with what expected.
•
Estimate possible errors of the work.
•
The conclusions should be justified by the results.
13. REFERENCES:
•
List all references used in your report.
Note: lab. Reports are to be typed, in a neat and orderly fashion, with all pages
numbered at the bottom. Be concise – quality not quantity is important.
ix
EXPERIMENT 1: TENSILE TEST
OBJECTIVES:
•
Know which ASTM standards are used for the tensile test.
•
Know how the tensile test is used to establish mechanical properties.
•
Know how the tensile test is used to measure engineering and scientific data such as
strain hardening rates, modulus of elasticity, 0.2% yield stress, tensile strength,
fracture stress and energy to fracture a tensile specimen.
•
Know how to compute engineering stress and strain and true stress and strain.
•
Understand fracture types for tensile test
THEORY
1- Introduction
The tensile test is one of the most frequently tests used to determine some mechanical
properties by applying uniaxial load (static or quasi-static with very small rate) on a specimen
until it breaks. The tensile machine is instrumented to read the applying force, and we know
the cross- sectional area, so the stress at any load can be calculated. The gauge length of the
test specimen should be at least five times its diameter, and the specimen must have a
uniform cross-sectional area. The best way to represent the data is to plot a stress strain
diagram. From the stress strain diagram many properties related to the material tested can be
found. Also we can determine from this test whether the material is brittle or ductile, the
brittle material shows a little amount of deformation and under a small load a fracture will be
happened, on the other hand, the ductile material shows a considerable amount of
deformation before fracture. Moreover, the yield point in the ductile material is clearly seen
from the stress strain diagram, but in the brittle material the yield point is determined by the
offset method. From another point of the ductile and brittle material classified according to
the fracture strain as the following:
For ductile material
𝜀𝑓 ≥ 0.05
For brittle material 𝜀𝑓 < 0.05
The following standards are used for Tensile test: ASTM E8 and ASTM E 646.
1
Stress Strain relations
𝑨𝒑𝒑𝒍𝒊𝒆𝒅 𝑭𝒐𝒓𝒄𝒆
𝑭
•
Engineering Stress: 𝝈 = 𝑰𝒏𝒊𝒕𝒊𝒂𝒍 𝒄𝒓𝒐𝒔𝒔 𝒔𝒆𝒄𝒕𝒊𝒐𝒏𝒂𝒍 𝒂𝒓𝒆𝒂 = 𝑨
•
Engineering Strain: 𝜺 =
𝒐
𝒍𝒊 −𝒍𝒐
𝒍𝒐
=
∆𝒍
𝒍𝒐
Where: 𝑙𝑜 is initial gauge length, and 𝑙𝑖 is the instantaneous gauge length.
𝑨𝒑𝒑𝒍𝒊𝒆𝒅 𝒇𝒐𝒓𝒄𝒆
•
True Stress:
•
True Strain: 𝜺𝒕 = 𝒍𝒏 (𝒍 𝒊 ) = 𝒍𝒏(𝟏 + 𝜺)
𝑭
𝝈𝒕 = 𝑰𝒏𝒔𝒕𝒂𝒏𝒕𝒂𝒏𝒆𝒐𝒖𝒔 𝒄𝒓𝒐𝒔𝒔−𝒔𝒆𝒄𝒕𝒊𝒐𝒂𝒍 𝒂𝒓𝒆𝒂 = 𝑨 = 𝝈(𝟏 + 𝜺)
𝒊
𝒍
𝒐
Stress - Strain diagram
Stress strain diagram is a plot of engineering stress (y-axis) versus engineering strain (x-axis).
It is a very important plot, from which all mechanical properties for the test specimen can be
found. Figure 1 shows a typical stress strain curve where all details, points and properties are
illustrated
Elastic region:
•
Stress and strain are linear proportionally
•
Material returns to its original shape when load released
•
Modulus of elasticity or Young’s Modulus: it is a measure of a material’s stiffness,
and it is the slope of the linear part of the stress strain graph.
Proportional limit:
The end of the linear proportional between stress and strain
Elastic limit:
The end of the elastic region, for most materials proportional limit and elastic limit are
coincide
Yield stress:
At this point, the material no longer exhibits elastic behavior and permanent deformation
occurs. This onset of inelastic behavior is defined as the yield stress or yield strength.
Ductile materials will have a well-defined yield point that can be easily identified on the
stress strain curve. Other materials will not have a discernable yield point and other methods
must be employed to estimate the yield stress. One common method is the offset method,
where a straight line is drawn parallel to the elastic slope and offset an arbitrary amount, most
commonly for engineering metals, 0.2%
Ultimate tensile strength:
The highest stress the material is capable to withstand. It will be the highest measurable stress
on the graph. This is termed the ultimate strength or tensile strength.
2
Fracture strength:
The point at which the material actually fractures is termed the fracture stress. For ductile
materials, the Ultimate stress is greater than the fracture stress, but for brittle materials, the
ultimate stress is equal the fracture stress.
Figure 1: Typical Stress Strain diagram
Ductility:
Ductility is the materials ability to stretch or accommodate inelastic deformation without
breaking. Another phenomenon that can be observed of a ductile material undergoing tensile
testing is necking.
The ductility of the material can be measured by:
1- 𝑷𝒆𝒓𝒄𝒆𝒏𝒕 𝒐𝒇 𝒆𝒍𝒐𝒏𝒈𝒂𝒕𝒊𝒐𝒏 =
∆𝑳
𝑳
× 𝟏𝟎𝟎%
So the higher value the more ductile material.
2- 𝑷𝒆𝒓𝒄𝒆𝒏𝒕 𝒐𝒇 𝒓𝒆𝒅𝒖𝒄𝒕𝒊𝒐𝒏 𝒐𝒇 𝒂𝒓𝒆𝒂 =
∆𝑨
𝑨
× 𝟏𝟎𝟎%
Modulus of resilience Ur:
The amount of energy the material can absorb while still in the elastic region.
This is the area under the linear portion of the stress strain plot up to the yield point.
𝝈𝒚 𝟐
𝟏
𝑼𝒓 = × 𝝈𝒚 × 𝜺𝒚 =
𝟐
𝟐𝑬
3
Where the strain (εy) and stress (σy) are measured at the yield point.
Modulus of toughness:
Toughness is the amount of energy the specimen can absorb until failure. This is the area
under the stress-strain graph up to the failure point. This is easily done by counting the
squares under the line on graph paper. If you have electronic data, it toughness is easy to
calculate by numerical integration.
Poisson’s ratio ν
𝝂=
𝒍𝒂𝒕𝒆𝒓𝒂𝒍 𝒔𝒕𝒓𝒂𝒊𝒏
𝑨𝒙𝒊𝒂𝒍 𝒔𝒕𝒓𝒂𝒊𝒏
in the elastic region
At any point on diagram at elastic region
𝑙𝑎𝑡𝑒𝑟𝑎𝑙 𝑠𝑡𝑟𝑎𝑖𝑛 =
Δ𝐷
𝐷𝑖
and 𝐴𝑥𝑖𝑎𝑙 𝑠𝑡𝑟𝑎𝑖𝑛 =
Δ𝑙
𝑙𝑜
Note: use constant volume principle 𝐴 × 𝐿 = 𝐴𝑜 × 𝐿𝑜
Stress – strain diagram differs from ductile to brittle materials, figure 2 shows a typical stress
strain diagram for different materials
Figure 2: Stress Strain diagram for different materials and cases
4
Types of fracture
In tensile test the state of stress is shown in figure 3 where maximum normal stress occurs at
a plane perpendicular to the direction of applied load, but maximum shear stress occurs at a
plane inclined 45˚ to the axis of specimen. Ductile materials are strong in carrying normal
stress and weak in shear stress, on the contrary of brittle materials. So ductile material usually
fail at angle 45˚ like cup and cone shape especially in circular specimens, but brittle materials
fail at normal plane. See figure 4
Figure 3: State of Stress for tensile test
Figure 4: Types of fractur for tensile test
APPARATUS
Figure 5 shows a bench top tensile machine that allows simple tensile tests of metal
specimens up to a maximum load of 20 kN. The load application mechanism includes a handdriven worm-and-wheel gearbox, driving by large handwheel allow the user to apply axial
load to the specimen. The unit also has a smaller “quick advance” handwheel that allows the
user to set the distance between the chucks to fit the specimen. The load measuring
mechanism is a strain-gauged load cell that connects to a microprocessor-controlled digital
display. The load display unit has a ‘peak hold’ function to register the maximum load before
the specimen breaks. A sliding digital display measures the tensile displacement (extension)
5
over the entire movement. Both the load and extension displays can connect to Data
Acquisition System VDAS that gives accurate real-time data capture, monitoring and display
all important readings on a computer.
Figure 5: Tensile Testing Machine
PROCEDURE
1. Using a micrometer or a caliber, measure and record the diameter of the test specimen at
its center Do, and the initial gage length Lo
2. Fit the specimen on the apparatus
•
Remove the locking pin of the small handwheel to adjust the distance between the
chuck stubs.
• Fit the specimen in its place using the chuck ring and chuck pin.
• Refit the locking pin.
3. Switch on digital meter and press “press to zero”
6
4. Slowly turn the large handwheel CW while holding the specimen straight and slightly
shake the specimen until the load display just start to show small value, then “press to zero”
4. Set the Sliding indicator to display zero.
5. Press “Peak Hold” on the Digital Load Meter DLM.
6. Start loading the specimen by slow rate of turn the large handwheel CW [one turn every
4s]. Note: don’t turn CCW during the experiment if you do the test fail
7. In steps of (0.1 mm), record the extension sing the sliding indicator, and the load using
DLM.
8. After yield, take readings every (0.4mm).
9. Continue loading at constant rate CW until fracture.
ANALYSIS
1. Calculate Engineering Stress and Engineering Strain
2. Plot the stress – strain Diagram
3. From your plot find the followings
•
Modulus of elasticity or Young’s Modulus E.
•
Yield stress, using 0.2% offset.
•
Ultimate tensile strength.
•
Fracture strength.
•
Ductility [𝑷𝒆𝒓𝒄𝒆𝒏𝒕 𝒐𝒇 𝒆𝒍𝒐𝒏𝒈𝒂𝒕𝒊𝒐𝒏 𝒂𝒏𝒅 𝑷𝒆𝒓𝒄𝒆𝒏𝒕 𝒐𝒇 𝒓𝒆𝒅𝒖𝒄𝒕𝒊𝒐𝒏 𝒐𝒇 𝒂𝒓𝒆𝒂]
•
Modulus of resilience Ur:
•
Modulus of toughness
•
Poisson's ratio 𝝂
4. Compare your experimental values with the standard one.
5. Explain the importance of these mechanical properties to engineering design.
7
RESULTS
Specimen
Test specimens for tensile testing are generally either circular or rectangular with larger ends
to facilitate gripping the sample. In this experiment the specimen is circular with the
following details
Table 1: Specimen details
Material
Dimensions
Diameter D=
Initial test length lo=
Table 2: Tensile test results
Force
F(N)
Strain
∆𝑙
𝜀=
𝑙𝑜
Displacement
∆𝑙 (mm)
8
Stress
𝐹
𝜎 = 𝐴 (MPa)
EXPERIMENT 2: CREEP IN METALS
OBJECTIVES:
•
To demonstrate and investigate creep in metals
•
To construct creep curve and understand stages of creep
•
To be able to explain the causes of creep in metals, creep deformation and be able to
indicate factors influencing creep behaviour in metals.
•
To analyse the obtained creep data and how use it for the selection of appropriate
engineering materials to prevent creep failures.
•
To derive the creep constants from experimental data.
THEORY:
Elastic and Plastic Deformation
When a material is stressed so that it compresses or stretches (deforms), then returns to its
original shape when the stress is removed, the material is perfectly elastic. The atoms in the
material have not moved, but the bonds between them have stretched, then returned to their
original position.
When a material is stressed so that it compresses or stretches (deforms), then does not return
to its original shape when the stress is removed, the material is perfectly plastic. The atoms
have moved and will not return.
Most materials have both elastic and plastic properties. When stressed by a small amount,
they behave like an elastic material, up to their elastic limit. When stressed by a large amount
(that takes them past their elastic limit), they behave like a plastic material. Rubber and soft
plastic materials usually have more elasticity than more brittle materials like metal or
ceramics. (see figure 6)
9
Figure 6: Stress/Strain Curves
Creep
When a material like steel is plastically deformed at ambient temperatures its strength is
increased due to work hardening. This work hardening effectively prevents any further
deformation from taking place if the stress remains approximately constant. Annealing the
deformed steel at an elevated temperature removes the work hardening and restores the steel
to its original condition. However, if the steel is plastically deformed at an elevated
temperature, then both work hardening and annealing take place simultaneously. A
consequence of this is that steel under a constant stress at an elevated temperature will
continuously deform with time, that is, it is said to "creep".
Creep in steel is important only at elevated temperatures. In general, creep becomes
significant at temperatures above about 0.4Tm where Tm is the absolute melting temperature.
However, materials having low melting temperatures will exhibit creep at ambient
temperatures. Good examples are lead and various types of plastic. For example, lead has a
melting temperature of 326°C (599K) and at 20°C (293K, or about 0.5Tm) it exhibits similar
creep characteristics to those of iron at 650°C.
Three main things determine the speed and amount of creep:
• Applied stress - higher stresses will increase the speed of creep
• Type of material - softer materials creep more quickly for the same value of stress
• Temperature of the material - higher temperatures encourage faster creep
Figure 7 show Creep at Different Temperatures and Stress.
10
Figure 7: Creep at Different Temperatures and Stress.
Creep in metals
A creep test is carried out by applying a constant load to a specimen and observing the
increase in strain (or extension) with time. A typical extension - time curve is shown in
Figure 8.
Figure 8: Creep Curve
Three stages can be identified on the curve:
Primary Creep -creep proceeds at a diminishing rate due to work hardening of the metal.
Primary creep does not start until the material has passed its elastic limit.
11
Secondary Creep - creep proceeds at a constant rate because a balance is achieved between
the work hardening and annealing (thermal softening) processes.
Tertiary Creep - the creep rate increases due to necking of the specimen and the associated
increase in local stress until failure occurs.
In terms of dislocation theory, dislocations are being generated continuously in the primary
stage of creep. With increasing time, more and more dislocations are present, and they
produce increasing interference with each other’s movement, thus causing the creep rate to
decrease. In the secondary stage, a situation arises where the number of dislocations being
generated is exactly equal to the number of dislocations being annealed out. This dynamic
equilibrium causes the metal to creep at a constant rate. Eventually, however, the creep rate
increases, and the specimen fails due to localized necking of the specimen.
When in service, an engineering component should never enter the tertiary stage of creep. It
is therefore the secondary creep rate, which is of prime importance as a design criterion.
Components, which are subject to creep, spend most of their lives in the secondary stage, so
it follows that the metals or alloys chosen for such components should have as small a
secondary creep rate as possible. In general, it is the secondary creep rate, which determines
the life of a given component.
Calculation of Creep Rate (secondary creep)
Secondary creep rate is linear, so experimentally is equal to the slop of the line in secondary
stage.
𝜀̇ =
∆𝜀
… … … … … … … … … … … … … … … . . … … … … … … … … … (1)
∆𝑡
To most common equation used to predict creep rate in metals and alloys is:
𝜀̇ = 𝐴𝜎 𝑛 𝑒 −𝑄/𝑅𝑇 … … … … … … … … … … … … … … … … … … … … … . (2)
Where A and n are constants, Q is the activation energy and R is the universal gas constant
(8.31 J/mol K).
The equation shows that the creep rate is increased by raising either the stress or the
temperature. Taking natural logarithms gives:
𝑙𝑛𝜀̇ = 𝑙𝑛𝐴 + 𝑛𝑙𝑛𝜎 −
𝑄
… … … … … … … … … … … … … … … … … . (3)
𝑅𝑇
So for tests at constant temperature and varying stress, a plot of 𝒍𝒏𝜺̇ against 𝒍𝒏𝝈 gives n
Also, for tests at constant stress and varying temperature, a plot of 𝒍𝒏𝜺̇ against
produce the activation energy Q.
12
𝟏
𝑻
can
The fact that the exponent n varies with stress demonstrates the inadequacy of simple laws
for correlation of data over a wide range of stress levels. In practice, more complicated
equations are used to correlate experimental data. For our purposes, it is sufficient to use the
equation 2.
APPARATUS:
The Creep Apparatus uses a simple lever to apply a steady tensile load to a specimen (see
Fig. 9). The specimen is fixed at one end of lever and loads are applied by hanging weights
on the other end of the lever arm. A rest pin is provided to support the weight of the lever arm
when loading the specimen prior to a test.
Figure 9: Creep Apparatus
The Creep Apparatus uses a simple lever to apply a steady tensile load to a specimen (see
Fig. 9). The specimen is fixed at one end of lever and loads are applied by hanging weights
on the other end of the lever arm. A rest pin is provided to support the weight of the lever arm
when loading the specimen prior to a test.
A digital displacement indicator measures the change in length (extension) of the specimen
during the experiments.
A thermometer measures the ambient temperature around the specimen.
A clear enclosure fits around the specimen area to help keep the temperature constant and
provide some protection when specimens are tested to fracture.
Specimen
Lead specimen with dimensions shown in figure 10
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Figure 10: Key Dimension of the Specimens
Data Acquisition System (VDAS)
Figure 11: Software interface
The apparatus can be connected to a data Acquisition System with software installed on the
PC, so that data can be collected automatically from the experiment. Figure 11 show the
software interface.
PROCEDURE:
1. Accurately measure and record the width and thickness of the specimen
2. Put the weight hanger in position and fit its support pin in its highest hole to hold the arm
up and ready for the test specimen.
3. Fit the steel specimen support clips to the specimen.
4. Fit the specimen into place between the black support block.
5. Put the transparent cover into place around the specimen. Make sure that the thermometer
is in its hole in the top of the cover and its tip is near to the specimen.
6. Fit a suitable weight to the Weight Hanger, to give a stress needed.
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7. Set the digital indicator in its place and connect it to data logger.
8. Carefully remove the Weight Hanger support pin from the highest hole in the Weight
Hanger
9. Switch on the digital indicator and press its origin button to set its display to zero.
10. Run the software for sampling every 30 sec.
11. Apply the load by fix the hanger and weights at lower hole. Try to load the specimen and
sampling data at same time.
RESULTS:
Table 3: Setup and specimen data
Width
Specimen dimensions
Thickness
Gage length
Temperature
weight add to the Weight Hanger
The Effective mass of the Arm at ‘P’
Mass of the Weight Hanger
Mass of the Support Pin
Note: table of results of time and extension will be supplied from software.
ANALYSIS:
1. Calculate of the Stress on a Specimen
a. Calculate the total mass at the end of the Arm (point ‘P’). To do this, add together:
• The value of the weight you are to add to the Weight Hanger
• The Effective mass of the Arm at ‘P’
• Mass of the Weight Hanger
• Mass of the Support Pin
b. Multiply the total mass by 8 (the mechanical advantage of the Arm), and then by 9.81
(acceleration due to gravity). This will give the total force on the specimen in Newtons.
c. Calculate the cross-sectional area of the specimen (width x thickness) in m2.
d. Divide the total force (Newtons) by the cross-sectional area (m2) to find the stress on the
specimen (in N/m2).
2. Calculation of the Strain on a Specimen
Extension
Strain = Gage length =
∆l
L
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3. Plot the creep curve and show on curve the three stages and fracture point.
4. Find the slope of the second stage which is the creep rate.
5. Find the time required for each stage.
6. State the importance of creep and its application in real life
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