Jim Flaherty
Stress-Strain Lab
In this lab we tested the modulus of elasticity and the elongation of a piece of steel using
a universal testing machine. We placed the piece of steel into the machine and made two pin
sized holes 8 inches apart. We then ran the machine which slowly pulled the steel apart. The
machine pulled with increasing force and we read the stretch of the steel using an extensometer.
Then we had to create a stress-strain graph to display the data we recorded. The piece of steel
we used was 1020 HRS with a diameter of 0.750 inches.
We then began to run the machine. As the load went up by 1000 lbs. from 1000 to 21,000
lbs. the steel bar began to stretch and the data was recorded. Once the load was at 21,000 our
professor removed the
extensometer and began to give
the elongation of the bar starting
at .1 inches going up by .1
inches using venire calipers until
the bar snapped. And we had a
student reading the amount of
load that the machine was
pulling by. We recorded all the
data. Once the entire test was
done we plotted a stress-strain
graph using the data we
collected. We did this by
converting load into stress and
elongation into strain. The
equation for stress is
,
where
.
The equation for strain
is
, where the
gage length was 8 in. This
graph is shown above. The
graph of the stress-strain
diagram shows us the ultimate
strength, which is around
70,000 psi, and the rupture
strength, which is around
50,000 psi. The area
underneath the curve is called
the modulus of toughness. This
graph also shows the ultimate
strength, which is the maximum
stress a material can withstand
without breaking. In addition it
shows us the modulus of
resilience, which is the area
Jim Flaherty
until the peak of yield. The yield strength is the area of the graph which rises strait up.
Then we plotted a graph using only the set of data where the elongation only went up to .1
inches or up to the yield strength. This is the graph of the modulus of elasticity and is shown
above. The modulus of elasticity also known as young’s modulus is the slop of the line. For
1020 HRS the modulus of elasticity is 30E6. The area underneath the curve is called the
modulus of resilience. This line is linear because according to Hooke’s law a stress strain
diagram the relationship is linear up to the yield point. The experiment was a success because the
trend line of the data fits the data very well and the slope of the line is 30E6, which is the
modulus of elasticity for 1020 HRS. We know this because we looked the information up to
compare the data of out lab. The proportional limit is when the highest stress is still directly
proportional to strain. The yield range is the strain that takes place up to the yield point.