To PrentP:
The modulus calculation (Delta Stress / Delta Strain) is dependent upon BOTH load and Elongation. If you are using a clip-on extensometer you have to make sure BOTH the LOAD and Strain are calibrated.
If you are using the crosshead position as the elongation device you have to account for the materials slipping out of the grips. If it is a low elongation material, the slip can be a big portion of the total strain and will screw the Modulus.
For high elongation materials, you have to account for the distance between the grips, the problem is with the Dog-Bone cut specimens assume the only place it stretches is in the reduced section, this is wrong and should only be used as a reference. The tab section elongates quite a bit.
To accurately determine the Modulus you have to use a calibrated extensometer for strain measuring and a load cell.
The measurement of the sample is also important the thickness having the most effect on the results.
GFThelen