Examine the MCT failure criterion.
Perhaps the greatest benefit derived from the availability of constituent average stress and strain states is the potential to make significant improvements in the prediction of damage evolution and failure in composite materials. Typical fiber reinforced composite materials contain substantial differences in the strengths of the individual constituent materials. Therefore, the concept of formulating separate failure criteria for the fibers and the matrix is a widely accepted idea. However, efforts to base constituent failure criteria on the composite average stress state have been met with limited success. The reason for this limited success is that the composite average stress state is not strictly relevant to the fiber or matrix constituent materials. Instead, the composite average stress state represents the stress that would be present in a fictitious, statically equivalent, smeared material. It is much more physically correct to base the constituent failure criteria on the constituent average stress state. Constituent average stresses have the additional advantage that they do not filter out the self-equilibrating normal stresses resulting from Poisson or thermal interactions between constituent materials. Given these compelling arguments, the approach taken in Helius PFA is to utilize separate failure criteria for each constituent material and to base the constituent failure criteria on the constituent average stress state.