Stroke just refers to the length of extension (the distance the Arm on the tensile test machine moved). I assume you are interested in a stress - strain curve. Stress = Force / cross sectional Area. Strain = (Change in length) / original length of the test object. This is the length in the direction of conpression/tension. Hopefully you measured the cross sectional area of the object in class in order to calculate the Stress.
Sorry for the brief answer, I'm currently in a hurry, but I'll edit with a more detailed response later if nobody else has responded with an accurate answer.