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Small guide about thermodynamics
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SCHOOL OF MECHANICAL ENGINEERING
MECH 1280 - Engineering Materials Semester 1 Laboratory
Elastic/Plastic Tensile Properties Materials
Objectives:-
To complete this class you need to:-
Conduct tensile tests until fracture on three materials, Material A, B, and C.
Plot the engineering stress versus engineering strain graphs in the low strain (<1%) region to
determine Young’s Modulus (E), 0.1% proof stress and strain at yield. Note that a linear regression
line can be created easily in Excel for the elastic region by creating an additional column of strain
values and adding the offset (0.001). By clicking on the new line and selecting the linear fit option
this will produce a regression line. The option to display the regression equation will provide you
with the slope (E).
Plot the engineering stress versus engineering strain and true stress versus engineering strain graphs
from zero strain up to failure.
Determine the ultimate tensile strength, fracture stress, ductility and work to fracture.
Determine the relationship between Vickers hardness and Yield Strength = x (VHN) g.
Examine the fracture surfaces and describe the fracture process.
Use CES Edupack (available in the Mechanical Engineering 5th floor cluster under
Programs/Mechanical/ CES Edupack ) to determine which material you have tested for A, B, C and
discuss your results vs what was expected from the database.
Deliverables:-
Each student should produce a brief but concise laboratory report containing an abstract, introduction,
materials and methods, results, discussion and conclusion. Your report should include results of using
the Materials Selector Database (CES Edupack) as a means to determine which materials were used in
the study (A, B, C) based on their appearance, fracture behaviour, and mechanical properties of your
experimental laboratory. Introduce all background material initially in the Introduction of your report.
The materials and methods should be as brief as possible. You can include this laboratory procedure in
an appendix and refer to it in your materials and methods to keep this section brief. Also include a
brief description of the on-line Materials Selector Database (CES Edupack).
The two required graphs for each material in the Results section are:
(a) Engineering stress versus engineering strain up to yield.
(b) Engineering stress versus engineering strain and true stress versus engineering strain from the
start up to fracture.
These may be presented together in a single figure.
The laboratory report accounts for 10 % of the overall module mark.
EXPERIMENTAL METHOD
Note : The Load Applied is Different for Each Material
Important Safety Notice:
The guards on the tensile test machine must be closed whilst applying a load, and you must keep
your hands clear of the working area once the extensometer has been removed.
Tensile test protocol:
1. Release the hydraulic pressure by opening the valve (turn it anti-clockwise).
2. Mount specimen into machine, do not use the spanner to tighten the clamps since these are for
removal of the specimen ONLY.
3. Measure the original specimen diameter (Øo) using the vernier callipers provided. Record this on
results sheet and calculate cross sectional area (Ao).
4. Attach extensometer securely to the specimen. Ensure the starting gauge length (Lo) is set correctly
and the counter is on zero. Set the digital load read-out to zero.
5. At this time you will need to get the demonstrators to check your Apparatus.
6. Close the hydraulic valve (clockwise) and apply load using the hand pump. Initially you will need
to take any slack out of the system then the load will increase RAPIDLY.
7. Apply load in the following increments for each material (A - 10kN ), (B - 5kN), (C - 1kN) .
8. At each loading increment record the load (F) and extension ( L) on the results sheet. Remember
that you will have to convert Load into Stress and that the Strain measurement you record has a
scale factor associated with the particular type of measurement device used (screw on, hand held).
9. Continue to apply the correct load increments for each material, recording the load and extension
each time until the specimen yields and extends rapidly. You will notice that the strain
measurement has increased a proportionally greater amount than previously where there was a
linear increase in strain with increasing load. If you are unsure get the demonstrators to advice.
10. Once yielded, remove the extensometer and set the points on the hand extensometer to match the
indentations left on the specimen. Set the dial on the hand extensometer to zero.
11. Continue to extend the specimen by pumping the handle in increments for each material (one pump
for Material A), (two pumps for Material B and C). After each pumping increment record the
load, the extension and the specimen diameter at the smallest section (measuring at the site of
necking once apparent).
12. Continue applying pumping increments, recording the load, extension and specimen diameter until
the specimen fractures. If you find that you have 6 measurements in the plastic region without
fracture you may double the number of pumps before each subsequent measure of load, diameter
and strain.
13. Convert the load and extension data into stress and strain on the results sheet and plot this on the
graphs. Remember that the different strain gages have different scaling factors associated with the
measure of strain.
14. Calculate the Young’s modulus from the gradient of the linear part of the graph. To do this you
will need to project a line from 0.1 % strain (0.001 strain) parallel to the stress/strain curve until it
crosses the experimental stress/strain line, read the stress and strain values at this point for the 0.1
% proof stress and strain at yield.
15. In the plastic region calculate the true stress by dividing the load by the actual cross sectional area.
16. Plot graphs of engineering stress versus strain and true stress versus strain from start to fracture.
17. Measure the ultimate tensile strength (UTS) as the highest engineering stress achieved, the
engineering stress at fracture and the total strain to fracture (ductility) from the engineering
stress/strain graph. Estimate the percentage reduction in area from the broken specimen.
18. Estimate the toughness of each material (area under the stress vs. strain curve), the work to fracture
can also be calculated (area under the load vs. deformation curve).
19. Record the Rockwell hardness of each material as demonstrated by the Laboratory Technician and
convert to Vickers Hardness as instructed within the lab.
20. Summarise all in an individual report written in a standard engineering lab report format as
outlined in the MECH 1280 handbook and the Transferable Skills document (available on-line).
21. A tutorial for using the CES Edupack software is available on-line via the module website on the
VLE which will guide you step by step through the use of the software. The software is installed on
the 5th floor cluster in Mechanical Engineering under Programs/Mechanical/CES Edupack. As you
go through the tutorial you can change the material to determine the ones you tested in the
laboratory.
22. All reports must be typed with your student number clearly shown on the front page of the report.
A marking scheme is available online on the module website.
23. Individual lab reports are to be handed in to the Lab Report Box located within the Solid
Mechanics Laboratory by Friday 5pm, Week 7.
Figure 1. Sample engineering stress/strain graph for low strains.
Figure 2. Sample engineering stress/strain and true stress/strain graph up to fracture.
Engineering Stress = force / original cross-sectional area 0A
F
True Stress = force / current cross-sectional area 1A
Ft
Engineering strain = change in length / original length 0L
L
True strain = change in length / current length 0
1lnL
Lt
Elastic/plastic tensile test results for Material A
Elastic Region
Names:-____________________________________________________________________
Date:-_____________________ Lab group number(s):-_______________
Original specimen diameter:-_____________ Original specimen area:-_____________
Load (kN) Engineering stress (MPa) Extensometer
reading
Engineering strain
Continue this table on a separate sheet if necessary.
Young’s modulus (E) =
0.1% Proof stress ( 0.1) =
Strain at yield ( y) =
- 7 -
Elastic/plastic tensile test results for Material A
Plastic Region
Load (kN) Engineering stress
(MPa)
Extension Additional
strain
Total
strain
Specimen
diameter (mm)
Specimen
area (m2)
True stress
(MPa)
Continue this table on a separate sheet if necessary.
Ductility = Final diameter =
Percentage reduction in area = Ultimate tensile strength (UTS) = Fracture stress =
- 8 -
Elastic/plastic tensile test results for Material B
Elastic Region
Names:-____________________________________________________________________
Date:-_____________________ Lab group number(s):-_______________
Original specimen diameter:-_____________ Original specimen area:-_____________
Load (kN) Engineering stress (MPa) Extensometer
reading
Engineering strain
Continue this table on a separate sheet if necessary.
Young’s modulus (E) =
0.1% Proof stress ( 0.1) =
Strain at yield ( y) =
- 9 -
Elastic/plastic tensile test results for Material B
Plastic Region
Load (kN) Engineering stress
(MPa)
Extension Additional
strain
Total
strain
Specimen
diameter (mm)
Specimen
area (m2)
True stress
(MPa)
Continue this table on a separate sheet if necessary.
Ductility =
Final diameter = Percentage reduction in area =
Ultimate tensile strength (UTS) = Fracture stress =
- 11 -
Elastic/plastic tensile test results for Material C
Elastic Region
Names:-____________________________________________________________________
Date:-_____________________ Lab group number(s):-_______________
Original specimen diameter:-_____________ Original specimen area:-_____________
Load (kN) Engineering stress (MPa) Extensometer
reading
Engineering strain
Continue this table on a separate sheet if necessary.
Young’s modulus (E) =
0.1% Proof stress ( 0.1) =
Strain at yield ( y) =
- 12 -
Elastic/plastic tensile test results for Material C
Plastic Region
Load (kN) Engineering stress
(MPa)
Extension Additional
strain
Total
strain
Specimen
diameter (mm)
Specimen
area (m2)
True stress
(MPa)
Continue this table on a separate sheet if necessary.
Ductility =
Final diameter = Percentage reduction in area =
Ultimate tensile strength (UTS) = Fracture stress =