Offsetting Data Though Curve Fitting

If I have a matrix of stress and strain values, how would I shift (add an offset value to) the strain data so that at strain=0 stress=preload. I have different samples of data but I'm not sure how to write a function that would add the offset value. It was suggested that we might use curve fitting or interpolation of multiple samples and then fzero but I'm still confused.

[Matt J]

What is the equation relating stress&strain that you are using? If none, you need to propose one.

[Claire]

Right now I have the following where diameter is set to 40 and height is 15:

function[stress strain] = convert1(displacement,force,height,diameter);
stess = force/(400*pi);
strain = extension/15;
end

Answer 1

[Matt J]

If the stress-strain data is all linear elastic (or if you can extract a portion that is), then you could use polyfit:

p=polyfit(strain, stress,1);

youngsModulus=p(1);
offset=p(2);

stress=stress-offset+preload;

[Claire]

Okay that makes sense, I guess I'm also confused on where the preload value is coming from?

[Matt J]

It's coming from you.