ENMAT101 Engineering Materials and Processes
Laboratory 2: Tensile Testing
AIM
For a selection of engineering materials:
 To draw / generate a Load-Extension curve to fracture.
 To find and compare the following properties
a) Yield Stress (or elastic limit where there is no yield point)
b) UTS
c) Ductility as % elongation, and as % reduction in area
d) Modulus of Elasticity E (Young’s Modulus)
PROCEDURE
Before starting the lab, you must calculate the maximum load that can be applied to the
specimen, assuming a sensible UTS of the material. Note that in practice, the UTS is nearly
always higher than the published values. This is because the published data is usually a
'guaranteed' value, or the lowest value you would rarely, if ever, encounter.
Part 1: Test to Destruction:
1. Take measurements of the specimen. Diameter, Length etc. Ascertain the
material as best you can.
2. Load the specimen into the machine and set the strain rate
3. Run the test while recording force values at a certain time interval. This data
should be logged to a file and then opened up in Excel.
Part 2: Measuring Yield (and Proof Stress):
1. Take measurements of the specimen. Diameter, Length etc. Ascertain the
material as best you can.
2. Increase the load until you find the yield point. Check the length for Proof Stress
set.
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THE LABORATORY REPORT
In this lab we are measuring the UTS - Ultimate Tensile Strength. This is the highest stress a
material can take before it breaks. We will also analyse the test to determine the material
stiffness E, called Modulus of Elasticity. (Also called Young's Modulus)
1. Determine the following:


Ultimate Tensile Stress (UTS), MPa
Yield Stress (YS), MPa
2. Draw a Graph:


Plot FORCE (Y axis) against extension (X axis).
Plot STRESS (Y axis) against STRAIN (X axis)
3. Determine;



Modulus of Elasticity (E)
Quantify resilience and toughness. (Note: Toughness is energy related – where
W=Fs, so the graph must be in N and m to obtain Joules. Convert this to J/mm2
Ductility as % elongation, and as % reduction in area
4. Explain why it can be difficult to measure Elastic Limit for materials that do not
exhibit a definite Yield Point. What do we use instead and describe how this
procedure is carried out (Proof Stress)
5. Explain curvature: What is happening to cause the pulling and relaxing actions
to trace non-straight lines? What is the name of this phenomenon? What factor
influences the degree of curvature?
6. Describe the fracture surface: Define as ductile/brittle/semi. Make a quick
sketch/photo of the break.
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NOTES: TENSILE TESTING
Finding YS, UTS, E and Toughness
In this lab we are recording force and
time during the destructive testing of
the specimen. We are after the
following: UTS - Ultimate Tensile
Strength, YS - Yield Strength.
Later, we can try to interpret the data
with the assumption that E is known
(Modulus of Elasticity. or Young's
Modulus), we can also determine
strain, toughness, resilience and %
elongation.
Axial Stress Theory
Axial stress acts along an axis, which is really just a shorthand way of saying Tensile or
Compressive Stress.
 = F /A
Axial Strain:  = x / L
Modulus of Elasticity: E =  / 
 = Axial Stress (MPa)
A = Cross-sectional Area (mm2)
F = Force (N)
Axial Stress:
s
 = Axial strain (no units)
s
L = Gauge length - originally (mm)
x = extension (mm)
E = Modulus of Elasticity (MPa)
By recording the force as we increase the load on the specimen, the highest force is used
to determine UTS.
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Legacy Data
Non-extensometer data from the Schenck tensometer:


Test 20101025a: 3pm test: Diam 6 x 80mm long, Mild steel, 0.4%C, normalised at
900oC. Extension 10.5mm
Test 20101025b: 7pm class: Diam 6 x 80mm long, Mild steel, 0.4%C, normalised
at 900oC. Extension 12.6mm
Example data loaded in Excel and a graph drawn (below).
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Yield Point and Proof Stress:
1: True elastic limit
2: Proportionality limit
3: Elastic limit
4: Offset yield strength
The yield strength or yield point of a material is defined in engineering and materials
science as the stress at which a material begins to deform plastically. Prior to the yield
point the material will deform elastically and will return to its original shape when the
applied stress is removed. Once the yield point is passed some fraction of the
deformation will be permanent and non-reversible.
Proof stress
When a yield point is not easily defined based on the shape of the stress-strain curve an
offset yield point is arbitrarily defined. The value for this is commonly set at 0.1 or 0.2%
of the strain. High strength steel and aluminium alloys do not exhibit an obvious yield
point, so this offset yield point is used on these materials.
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Shenck Tensometer Searching for yield, work hardening effect.
Shenck Tensometer: Determining Young’s Modulus
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