Tension & Compression Labs
SMTL 246
These are the results from one of the tension labs
These are the results from one of the tension labs
The top

left curve is for the mild steel and the bottom

right curve is for the cast iron
These are the results from one of the tension labs
The top

left curve is for the mild steel and the bottom

right curve is for the cast iron
Draw horizontal and vertical axes through the bottom end
of the mild steel curve
The horizontal axis should also pass through the bottom
of the cast iron curve
Label the axes
Now, we add the units to the axes. The graph paper has mm
and cm markings.
For the vertical load axis:
For the horizontal axis, the scale is 250:1
This means that 250 cm on the paper is equal to 1 cm
deformation in the sample … so 1 cm on the paper is equal to
0.004 cm in the sample … or 0.04 mm
Place units all the way up the vertical axis and a little way
along the horizontal axis. Don’t go as far as the cast

iron
curve; we will need to start at 0 again there
Draw a line along the proportional part of the curve,
approximating the slope as accurately as possible
Now, find the slope by measuring the rise (change in
load) over the run (change in deformation). Make a large
triangle by adding vertical and horizontal lines (a small
triangle is less accurate since any error in the reading is
magnified).
My (rough) measurements from the scan of the graph give me
a rise of 15.5 cm ( = 73.2 kN) and a run of 3.35 cm ( = 0.134
mm).
We want to calculate
E
, the modulus of elasticity.
To find the yield strength, use the 0.2% offset method (this
value is for steel
–
use 0.05% for cast iron)
Draw a line parallel to the proportional part of the curve through
the point on the horizontal axis representing 0.2% strain. This
line intersects the curve at the yield point.
Where is the point that represents 0.2% strain?
Now, calculate the stress at the yield point based on the
load and the original cross

sectional area
Repeat the process for the cast

iron. Reset the origin to
the point where the curve crosses the horizontal axis.
Do the same process for the compression test. The
horizontal scale is 1000:1
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