Properties of Materials Lab Instructions

1. Context
Aircraft structural integrity deals with the ability of an aircraft structure to retain its strength, function
and shape within acceptable limits without failure when subjected to the loads imposed throughout
the aircraft’s service life. Strut/bracket assembly is widely used on many aircraft for several reasons
(e.g. coping with relative movements, decoupling of loads, ability to cope with tolerances, ease of
manufacture, maintainability and operability…). Figure 1 shows some examples of their use in
aircraft.
(a) (b)
(c) (d)
Figure 1: Examples of strut/bracket assembly uses in aircraft: (a) attaching parts together in the cabin (e.g. galley, toilets,
storage), (b) attaching the ceiling sub-structure, (c) inside the wing box, and (d) cargo struts.
These parts are heavily loaded during the aircraft’s life and some failures may occur, as in the
examples shown in Figure 2. These cracks have been caused by fatigue, which occurs when a
material is subjected to repeated loading and unloading. To avoid such damage, the nominal
maximum stress values are normally much less than the strength of the material (yield stress limit).
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(a) (b)
Figure 2: Examples of (a) failure of a rod, and (b) failure of pins.
Temperature is also an important parameter to consider in aircraft applications, as temperatures
experienced by parts of the aircraft may vary from −50℃ (on the ground or in flight) to +85℃ (on the
ground). Temperatures could be even higher in areas close to the engines.
2. Objectives
The objective of this lab is to investigate the modes of failure of various materials, compare material
properties by performing standardised tests, and discuss the most appropriate materials for the rod
and the pin shown in Figure 3. Finally, you will need to define the new dimensions of these parts (pin
diameter, ? and rod diameter, ?) to avoid any plastic deformation and material fracture.
In this case, it is assumed that the pin is subjected to shear and the rod to tension. The applied load
is ? = 5 × 104 ?, the rod thickness ? = 10 ??, and in aeronautical applications a safety factor of
?? = 1.3 is commonly used.
(a) (b)
Figure 3: (a) Assembly view showing applied force, and (b) section view of the rod and pin assembly under investigation.
The standardised tests that are used in this lab allow investigation of material properties using a
small specimen instead of testing the real larger scale structure or component. Small scale tests are
more economical and enable us to repeat the same test a large number of times. All experimental
results have an uncertainty, and typically a minimum sample of 30 is required to determine the mean
result to a high level of confidence.
In this lab, you will experimentally determine crucial material properties such as yield strength,
ultimate tensile strength, breaking stress and toughness, and consider the physics that govern these
properties. You will carry out three different tests on multiple materials, which will allow you to
determine the mechanical properties which govern the materials’ behaviour in response to an applied
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load, and to suggest which material(s) should be used in the given application (the rod and pin
assembly in an aircraft structure). The three tests are:
• Tensile test: This involves pulling a specimen apart and is used to produce force-extension
(subequently stress-strain) curves and also to ascertain several mechanical properties that
are useful in design. You will perform this test on aluminium, high carbon steel, and low
carbon mild steel specimens.
• Torsion test: This is a variation of pure shear tests, and involves twisting a specimen such
that the torsional force produces a rotational motion. Stress-strain curves obtained from
tension tests allow us to determine what the modulus of elasticity and the breaking stress of
a material are, but also determine how ductile/brittle a material is. You will perform this test
on aluminium, cast iron, and low carbon mild steel specimens.
• Charpy impact test: This focuses on the effects of shear but still enables us to calculate the
toughness of a material, and understand how fracture occurs in terms of energy. You will
perform this test on aluminium, high carbon steel, and low carbon mild steel specimens
at 2 different temperatures for each material.
In sub-groups of 3 students, you will test all 3 different materials on each test machine. Each
student will take the lead for testing 1 material, and will share their results with others in the
sub-group. It is your responsibility to ensure you have shared data with all 3 members of your
sub-group.
3. Tensile testing
3.1. Introduction
Mild steel is a ductile material at room temperature. This means we can expect a very specific type
of behaviour, and obtain a very distinct stress-strain curve (Figure 4(b)). Initially, the sample deforms
elastically, thus if the load is removed at this moment, there will be full elastic recovery (i.e. the
specimen will return to its original size). If the load is increased past its limit of elasticity (yield
strength), the sample will start to deform plastically. This means that if the load is to be removed, the
sample will not return to its original shape, and will have been permanently deformed. You will use
a machine similar to that shown in Figure 4(a).
(a) (b)
Figure 4: Examples of (a) a tensile testing machine, and (b) stress-strain curve obtained from a tension test (taken from Dr
Peel’s lecture notes)
?0.2% 0.2% proof stress (i.e. ?/?0 at a permanent strain of 0.2%). This is useful for characterising
the yield of materials that yield gradually.
???? Ultimate tensile strength, (?/?0 at onset of necking) . The maximum engineering stress, in
tension, that may be sustained without fracture.
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?? Plastic strain after fracture. The broken pieces are reassembled and
?? is calculated from (? − ?0)/?0, where ? is the length of the reassembled pieces and ?0
is the original length.
3.2. Experimental Procedure
• Measure and record the diameter, ?0, of the specimen using a micrometer.
• Measure the gauge length, ?0, of the specimen.
• Place the specimen in the grips.
• Attach the extensometer to the specimen.
• Increase the load until yielding can be observed.
• Remove the extensometer.
• Increase the load until failure.
• Ensure data is saved.
You will measure both the extension over the gauge length (using the extensometer) and the overall
extension. The extensometer data should be used to determine both the yield strength and young’s
modulus. The overall extension data is used to plot the complete stress-strain profile, and determine
the ultimate tensile strength.
4. Torsion testing
4.1. Introduction
This test allows you to compare the materials by twisting them until failure. Torsional forces produce
a rotational motion about the longitudinal axis of one end of the specimen relative to the other end
(Figure 5(a) and (b)). Not only will they fracture at different torques, but you will notice that they
behave completely differently (Figure 5(c)). From the this test, you will also become familiar with the
concept of toughness which represents a measure of the amount of energy absorbed by a material
as it fractures.
(a) (b)
(c)
Figure 5: (a) Photo and (b) schematic of the torsion machine to be used in the lab, and (c) example of specimens after
testing.
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4.2. Experimental procedure
• Measure and record the diameter, ?0, of the specimen using a micrometer.
• Measure the length, ?, of the specimen.
• Place the specimen in between the hexagonal grips.
• Replace cover on top.
• Zero both the load and rotation angle displays.
• Rotate the handwheel clockwise until failure in steps as shown in the suggested table for
each material, and record the equivalent torque at each step.
• Make sure to keep rotating the handwheel at a constant rate.
• Once the specimen has broken, remove the cover and feel the temperature of the specimen.
5. Charpy impact testing
5.1. Introduction
The Charpy impact test is used to measure the impact toughness of a material (test machine shown
in Figure 6(a)). A notch is cut into the sample to act as a stress concentration and a starting point for
the fracture. A swinging pendulum hammer is held in place with an electromagnet. When the current
in the electromagnet is turned off, the pendulum swings down and hits the sample. The amount of
energy absorbed by the sample is calculated from the maximum swing angles of the pendulum
before and after impact.
This test is an useful tool to compare materials and to show how the toughness of materials changes
with temperature. When the pendulum is released, it accelerates downwards, exchanging its
potential energy for kinetic energy. As it strikes the specimen, some of its kinetic energy is transferred
to the specimen in order to break it. The pendulum then continues with a reduced amount of kinetic
energy, which is exchanged for potential energy as the pendulum rises to a height lower than the
starting height (Figure 6(b)). By knowing the difference in starting and finishing height, the amount
of energy absorbed at fracture can be determined.
The initial potential energy of the pendulum is ??ℎ1, where ℎ1 = ?(1 + sin(? − 90)).
The final potential energy of the pendulum is ??ℎ2, where ℎ2 = ?(1 + sin(? − 90)).
The difference in potential energy is therefore ???(sin(? − 90) − sin(? − 90)).
(a) (b)
Figure 6: (a) Charpy impact testing machine to be used and (b) example of pendulum swing during test.
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5.2. Experimental procedure
5.2.1. Making specimens
• Clamp each new specimen into the specimen block, using a hexagon tool to tighten the grub
screw in the side of the block. (Note that the ‘pip’ on the specimen block is on the top of the
block. Be careful not to tighten it too tightly.)
• Put the black cutting block over the specimen and hold it to the specimen block.
• Put the assembly into a suitable vice and use the hacksaw (supplied) to cut a notch into the
specimen. Cut the specimen with a flat action, across the dowels.
• Remove the black cutting block. The notch in the specimen should accurately face into the
direction of impact.
The specimen and block are now ready for testing.
5.2.2. Tests at room temperature
• Measure and record the temperature, material and exact diameter of each specimen before
use.
• Allow the pendulum to come to a complete rest just above the specimen shear block – a
square black indicator should illuminate in the top right hand corner of the display, next to
‘Ready to Arm’.
• Use the lifting control on the front of the apparatus to move the pendulum up to its starting
position at the top left. An electromagnet will energise and hold the pendulum in place. The
Instrumentation Box will display the words ‘Load Specimen Press Release’.
• Insert the specimen and block into its place under the shear block at the base of the
apparatus, making sure it is fully inserted.
• Make sure everyone is standing clear of the apparatus and press the release button.
• Record the readings displayed by the Instrumentation Box.
• Wait for the pendulum to stop swinging before preparing the next specimen.
• On completion of the test, try one more swing without a specimen in place to determine how
much energy is lost to friction and air resistance.
5.2.3. Tests at low temperature
• Repeat the procedure as described previously in the Making Specimens section.
• Wear suitable gloves to handle the specimen and block when cooling them.
• Place the specimen and its block into the cooling source (acetone and dry ice).
• Record the temperature of the cooling source.
• Quickly insert the specimen block into the shearing block.
• Conduct the test as quickly as possible before the specimen’s temperature increases.
6. Appendix
You may find some of the following equations useful in your analysis of the experimental results.
? =
?
?0
[1]
Where ? is engineering stress, ? is load, ?0 is original cross-sectional area.
?? =
?
??
= ?(1 + ?) [2]
Where ? is true stress, ? is load, ??
is cross-sectional area, ? is engineering strain.
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? =
?
?0
[3]
Where ? is engineering strain, ? is extension, ?0 is original length.
?? = ln (
?
?0
) = ln(1 + ?) [4]
Where ?? is true strain, ? is length, ?0 is original length, ? is engineering strain.
? =
?
?
[5]
Where ? is Young’s modulus, ? is stress, ? is strain.
? =
??0
?
[6]
Where ? is shear stress, ? is torque, ?0 is initial specimen radius, ? is polar moment of inertia.
? =
??0
4
2
[7]
Where ? is polar moment of inertia, ?0 is initial specimen radius.
? =
?0?
?
[8]
Where ? is shear strain, ?0 is initial specimen radius, ? is rotational angle (in radians), ? is specimen
length.
? =
?
?
=
??
?? [9]
Where ? is shear modulus, and all other symbols are as previously defined