If anyone can offer some advice on this issue i would be greatful.
It appears that the COMBIN39 element is sensitive to steep F-D curve data when KEYOPT 3 is used. I have tested KEYOPT 4 and the results have been unaffected.

The element description notes that segments tending towards vertical should be avoided, see snippet below. I have considered this and considerably softened the gradient of the curve but i am still getting incorrect force results in the spring. I include two tables below tutorial_09 is the simple object with four springs as shown previously and Alem_01 is my model of two vertically stacked modules. Please note that the force diverges significantly in my model and appears to be unpredictable, as similar F-D curves are used in both examples and in compression and tension (highlighted in red).

I would be grateful for any assistance in understanding what is happening or any alternative way of achieving what i want.
Thank you Michael