Ratcheting assessment in the tubesheets of heat exchangers using the nonlinear isotropic/kinematic hardening model

In this paper, the ratcheting phenomena in the tubesheet of a heat exchanger, which made of modified 9Cr-1Mo steel, has been investigated. The nonlinear isotropic/kinematic hardening model (combined model) is used to investigate the progressive strains due to the cyclic pressure and temperature loading. Therefore, three series of tests were carried out at different temperatures (room temperature, 300 degrees C, 520 degrees C) to gain stable hysteresis cycles and to obtain the hardening parameters of the desired material. Via least-squares nonlinear regression, the constitutive coefficients are obtained. In addition the effect of tube number on the progressive strains has been investigated. Therefore, three models of tubesheets with different tube numbers are considered. The results show that the progressive strains initially are significant at each cycle and gradually decrease from their initial amount, they reach a stable state, as the number of the tubes increases, and the more progressive strain accumulates.