Meaning of a negative failure index

Hey,

 

I'm performing a dynamic analysis on a composite structure and i'm using the hoffman failure criterium to analyse the results.

However i'm finding negative values for this failure index and i can't find anywhere what the meaning of a negative failure index is.

Does this mean i can't draw any conclusions if the index becomes negative and if so, at what value do i need to look then?

 

Kind regards

Jeroen