I am running something called cell periodicity study in comsol, which finds the material properties of a model. It is done by applying a uniaxial compression load, and performing a nonlinear buckling analysis - this gives the stress strain curve you see below. You can think of the material being for example epoxy, and I asked colleague how we define the stiffness, and was told it is when the material stiffness becomes 30% of the initial one, and they told me it has nothing to do with strain??
So how can I tell when the material has lost 70% of its stiffness? Especially when it has nothing to do with strain.