I was reading a paper which provided the following graph and described it as taking the form of a 'conic function':

I couldn't find anything about 'conic functions', though I could find something on 'conic sections' - which can be functions of a variety of different forms.

My question is this: is there a (simple) function which can describe the features in the graph above? The function would be described by:

- The initial stiffness (k);

- Ultimate y-value (y_{u});

- x-value at y_{u}(x_{u});

- A curvature value (n), the value of which varies between 0 and 1 to achieve the effect described by the graph.

Thanks in advance for any help with this. And apologies if this is not the right place for this sort of thread.