Hello,
The analytical methodologies used by Caesar II address the linearly elastic structural analysis of piping systems and associated structures.

The B31 Pressure Piping Codes have addressed the non-linear "relaxation" phenomenon by employing a "stress range" approach to thermal expansion/contraction loadings. These B31 rules are incorporated into the Caesar II software. If you can obtain a copy of the FIFTH EDITION of the "Piping Handbook" by Sabin Crocker and Reno C. King, you will find a good write-up on such topics there. This is also a topic that is addressed by the "Kellogg Book" entitled "Design of Piping Systems" (it is out of print). You will find that the non-linear "relaxation" phenomenon is sometimes called "self springing" to describe the residual strain condition that exists after the "relaxed" system returns to ambient temperature.

Regards, John.