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 curveThe horizontal axis should also pass through the bottom of the cast iron curveLabel 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 250:1This 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 wayalong 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