Trace and Product of a Diagonal use in Composite Analysis

[kirani]

I am analyzing the bending stiffness matrix (also called D matrix) to determine properties of laminates and it was suggested I use the Trace (sum of the diagonal) and product of the diagonal of D to analyze the critical load. I generally found that there is a somewhat linear relationship between the two, but am unclear on what this actually proves. Any ideas on what these numbers or trends represent?