Dear Alin10,

These Stress Concentration factors for fillet-shell junction can be obtained from Appendix B of WRC-107.

In the program when the fillet radius is specified then the program automatically computes these factors and applies then to the calculated stresses to get the peak stresses.

This appendix from WRC-107 references the following texts:

Peterson, R.E., "Stress Concentration Design Factors" John Wiley and Sons, Inc., New York 1953.

and,

Heywood, R.B., "Designing by Photoelasticity," Chapman and Hall, London, 1952.