Calculation of the axial stress in UCS test simulation

I am using PFC to simulate UCS test. I used three methods to monitor the axial stress (see attached): (1) based on the stress on the loading wall; (2) measurement circles that cover the whole specimen; (3) measurement circles that cover the center area of the specimen.
I obtained three significant different results of UCS (see attached). I wondering which one should be chosen for UCS? What is the best way to monitor the axial stress in UCS test in PFC? Thanks.

I suggest using the wall-based stress measurements, because these can be compared directly with what is measured in a real physical test. Note that all three measurement schemes are giving the same axial stress during the portion of the test where the specimen is not experiencing any damage — i.e., during the portion in which the specimen is behaving elastically. As the peak load is approached in a UCS test, the damage begins to localize, with some regions unloading, and it is this localization that may be causing the difference in peak values.
David Potyondy


Hello Dr. Potyondy
Thank you for your kind help.