choice of characteristic length for interfacial crack in Abaqus

Hi,

I am using Abaqus to calculate the stress intensity factor solutions (SIF) and J-integral for an interfacial crack between dissimilar materials. I knew that ABAQUS  emplyed the interaction integral to extract K1, K2 of the complex SIF K=K1+i K2, and K_III from the mixed mode. As discussed in Rice's paper [1988], the SIFs K1 and K2 depend on the selection of the choice of a characteristic length (for ex, the size of the process zone). I do not know how Abaqus selected such kind of characteristic length? Also, are the results reported by ABAQUS are reliable? How do we justify it?

Any comments and suggestions are more than welcome.

Thanks,

 Best regards,

Xuan