Using Stress/Strain Curve to Find Yield Strength and Modulus of Elasticity

Hey guys,

I recently did a compression lab with different materials (wood and pvc pipe) and I have to plot the stress/strain curves given the data collected, as well as find yield strength (0.2% offset), ultimate compressive strength, and modulus of elasticity. I've already calculated stress and strain and plotted the data for one of them. I just wanted to make sure I was doing everything right. So:

Yield Strength @ 0.2% Offset: from what I understood, I take the slope of the curve (before it peaks) and draw a line with the same slope, but starting at 0.2 on the y axis instead of 0, and wherever it intersects with the original graph is my yield strength

Ultimate Compressive Strength: I'm not too sure about this, but I assume that the highest peak on the graph is the ultimate compressive strength, because it is the most stress it can endure before failing. Could someone confirm this/explain the theory if I'm wrong about this? And if I'm wrong, is there a way to calculate this?

Modulus of Elasticity: This is equal to E = (F)(L1)/(A)(L2) where:
F = the force applied to the material
A = the cross-section area through which the force was applied to the material
L2 = amount the length of the material changes when the force is applied
L1 = original length of the material (before the force was applied)

For Modulus of Elasticity,is L2 the final length that it changed when force was applied?

Sorry that these are kinda dumb questions, I was out of the province when the material was covered in my lectures. Thanks for looking!

rock.freak667
Homework Helper
The modulus of elasticity is defined as the ratio of the stress to strain, so from your stress-strain curve, the gradient of the linear region would give you the value for E of the material.

rock.freak667
Homework Helper
The modulus of elasticity is defined as the ratio of the stress to strain, so from your stress-strain curve, the gradient of the linear region would give you the value for E of the material.

So then...the slope of the linear portion of the curve?

rock.freak667
Homework Helper
So then...the slope of the linear portion of the curve?

Yes.

Awesome, thanks! :)

Hey guys,
Yield Strength @ 0.2% Offset: from what I understood, I take the slope of the curve (before it peaks) and draw a line with the same slope, but starting at 0.2 on the y axis instead of 0, and wherever it intersects with the original graph is my yield strength

The 0.2% offset is done by taking the slope of the linear component of the plot and offsetting it 0.2% on the x-axis. See here for pictorial description: http://en.wikipedia.org/w/index.php?title=File:Metal_yield.svg&page=1. You can then read the stress value at the intersection point.

Cheers!