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MCEN30017 Laboratory - Tensile Testing
1
MCEN30017 Mechanics and Materials
LABORATORY PRAC 1 - TENSILE TESTING
This manual covers the lab Tensile Testing and can be downloaded from Canvas.
This assignment is worth 7.5% of the final grade for this subject.
Laboratory - Tensile Testing
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Laboratory report instructions
Please read the following instructions carefully.
1. The data will be made available on Canvas.
2. Students must submit individual reports. Students must digitally attach a cover sheet to
their completed laboratory reports clearly stating the name of the experiment (Tensile
testing), their full name and student number.
3. Reports should contain between 7 and 12 pages. Pages after page 12 will NOT be
marked.
4. Use a font size of 11 or 12. Reports with smaller font sizes will not be marked.
5. Read the section “Content of report and marking scheme” of this manual carefully before
writing your report.
6. The report must be submitted electronically via Canvas. Type your report using Word,
Google Docs or Latex and plot the graphs using MATLAB or Microsoft Excel. Hand-
written reports (including text and equations) and hand-drawn figures will NOT be
marked.
7. The submission must be as a single .pdf or MS word file. Do NOT include MATLAB codes
and Excel tables that you use for plotting figures or calculating parameters. Check the file
legibility, fonts and orientation of pages before submitting. Reports submitted in other file
formats will NOT be marked.
8. Trying to upload an overly large file may result in an upload failure. Your final submission
should be under 10MB to avoid uploading errors.
9. It is advisable to use Internet Explorer to submit files, as Firefox or Chrome occasionally
returns errors when trying to upload files.
10. Extensions will not be granted for not knowing how to submit electronically via Canvas.
11. You have unlimited attempts but only your last attempt will be graded, even if it is
submitted past the due date (see penalty for late submissions below).
12. The report submission is due by 1 week (by 11.59 PM on the same day on next week)
after your zoom session. Submissions made after this time will be deemed to be late and
will incur a penalty.
13. Late submission penalty: (i) All reports submitted after the due date will incur a penalty of
10% per day or part thereof, for each day that the report is late. (ii) No marks will be
given for any submission made after seven days of the due date.
Laboratory - Tensile Testing
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TENSILE TESTING OF DUCTILE MATERIALS
INTRODUCTION
In service, we usually expect materials to behave elastically, i.e. to deflect under load but to
return to the original size and shape when the load is removed. However, for any component there
will be a limit to the load that it can sustain while still behaving elastically.
Some materials are brittle - when the elastic limit is exceeded, they break without further
deformation, either suddenly or slowly as the load is increased. Most engineering metals and
plastics are not brittle; when the elastic limit is exceeded, they undergo permanent deformation.
In this experiment we are going to examine at this permanent, or plastic, deformation and learn
something about how it happens and what effect it has on the material.
Plastic Deformation
It is usually not desirable to have large amounts of plastic deformation in service, of course, but
many of the materials we use can undergo a great deal of deformation before they break. This is
useful because it means that mechanical shaping processes like rolling, drawing, extrusion, and
forging can be used to produce the shapes that we want. In addition, products made from ductile
materials can absorb energy and deform during a collision and are more tolerant of cracks and
other defects. The materials that are capable of such large deformations are metals and polymers,
although not all metals or all polymers can deform in this way.
The mechanisms of deformation are different in metals and polymers. Metals deform by slip, i.e.
sliding of crystal planes over one another, although other mechanisms are also possible,
particularly at high temperatures (above 0.3–0.4 TM, where TM is the melting temperature in
Kelvin). Polymers, in general, deform by sliding of polymer chains past one another.
Deformation of Metals
Metals are commonly polycrystalline, consisting of large numbers of crystals, often referred to as
grains, usually with the crystal axes randomly oriented in space. Each grain has a specific number
of crystallographic systems were slip is preferred, each slip system consisting of a combination of
a crystal plane and a direction within that plane. The preferred plane is usually the most closely-
packed plane and the preferred direction is nearly always the most closely- packed direction in
that plane. Thus, for example, a crystal of a metal with the FCC crystal structure has four close-
packed planes and in each of these planes there are three close-packed directions; so there are
twelve preferred slip systems. When the load on such a crystal is increased beyond the elastic
limit, it starts to deform on the preferred slip system which has the highest resolved shear stress.
This slip occurs by the glide of dislocations.
In a metal containing large numbers of randomly-oriented grains the deformation of each
individual crystal has to conform to those of all its neighbours, and so slip on a single system is
rapidly replaced by slip on several systems. This has important consequences for the way that
the metal behaves, but we can leave that for discussion during lectures.
Laboratory - Tensile Testing
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Deformation of Polymers
Many polymeric materials are amorphous in structure (i.e. not crystalline) and deformation occurs
by sliding of the irregularly-arranged chains past one another. However other polymers,
particularly those that are used in load-bearing applications, are crystalline to a greater or lesser
extent, being composed of crystalline and amorphous regions in varying proportions and
distributions. In such cases it is more difficult to describe how the crystalline regions behave during
plastic deformation. By comparison with metals, polymers are more sensitive to the rate of
deformation (the strain rate) and to temperatures around room temperature.
Experiment
In this experiment, tensile tests will be carried out on six materials: four metals and two polymers.
The tensile test is a convenient way to measure the strength of materials and it is widely used to
obtain design data and for quality control during manufacturing.
Your demonstrator will show you how to use the Instron testing machine.
A clip-on extensometer is used to provide accurate strain measurements. This is especially
important when determining the elastic modulus. Note that the gauge length of the extensometer
is 25 mm with a travel distance of ±2.5 mm (i.e. approximately ±10%).
The raw data are saved in .CSV files. In the .CSV files the column, “Displacement” corresponds
to the displacement (mm) of the cross-head and the column “Strain 1” corresponds to the strain
(%) as measured by the extensometer.
Content of report and marking scheme
Your report should contain the following points:
1. The objectives of the experiments (5%)
2. Experimental setup, method and procedure (incorporate Table 1 in this section) (5%)
3. Results: (20%)
a. Six stress-strain figures are required (one for each material). Each figure should
include two curves, one showing strain from the extensometer and the other from
the cross-head. As the extensometer is not used for the LDPE test, the LDPE
figure should only have one curve.
b. Do NOT include the load-extension plots to your report.
c. Clearly display five important features of the strain-stress curves on all of the
plots. If an important feature (e.g. a parameter) is not applicable for a material it
should be stated in the caption of the figure.
4. The methods of calculation, equations or definition for all the parameters listed in Table 2
should be clearly represented. (5%)
5. Complete Table 2 and include it in the report. (10%)
6. Discussion. (Each 10% - total 50%)
a. Answer all five questions below, in order and separately.
b. Include the question or the question number before your answer.
7. Conclusions. (5%)
Laboratory - Tensile Testing
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Late submission rules: (i) All reports submitted after the due date will incur a penalty of
10% per day or part thereof, for each day that the report is late. (ii) No marks will be given
for any submission made after seven days of the due date.
QUESTIONS
1. Calculate the elastic moduli of one of the metals using both the extensometer and the
displacement of the cross-head. Why is there a large difference between the modulus
measured using the extensometer and the cross-head displacement and which one is more
reliable?
2. Why is there such a large difference between the elastic moduli of metals and polymers?
3. What feature of the load-elongation curves for metals shows that work-hardening is
occurring? Explain what mechanisms are responsible for work hardening in metallic
materials.
4. The polymeric material undergoes stable "necking" and then shows something similar to
work hardening in metals. What are the physical processes and how are they responsible
for this behaviour? How does this explain the difference between the behaviour of the cold-
drawn LDPE compared with that of the "as-received" LDPE?
5. Identify the difference(s) and explain the reason(s) between the tensile curves of the low
carbon steel and Cu or Al around the yield point. What term(s) are used to describe yielding
in low carbon steel, in comparison to that used to describe yielding in Cu or Al?