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In this paper the ratcheting behavior of four pairs of stainless steel elbows is studied under conditions of steady internal pressure and dynamic conditions that induced out-of-plane external moments at frequencies typical of seismic excitations. The finite element analysis with the nonlinear kinematic hardening model has been used to evaluate ratcheting behavior of the piping elbows under mentioned loading condition. Material parameters have been obtained from several stabilized cycles of specimens that are subjected to symmetric strain cycles. The direction of maximum strain is at about 45˚ between the hoop and axial directions. The results show that the direction of highest ratcheting is along the hoop direction rather than the direction of maximum principal strain. Also, the initial rate of ratcheting is large and then it decreases with the increasing cycles. Also, the FE method gives over estimated values compared with the experimental data.

In this paper, the effect of temperature and out-of-plane dynamic moments on ratcheting behavior of pressurized elbows have been investigated. The combined hardening model (Chaboche kinematic hardening model with isotropic hardening rule) is used for the plastic analysis of materials. Piping elbow specimens had temperatures of 50, 100, 150 and 200 °C. By using of many tension-compression-stabilized tests of specimens under symmetric strain-controlled, constant parameters for materials are obtained. The results obtained by the FE method of specimens show that the deformations occurred mainly because of creep at high temperature and ratcheting strain by out-of-plane moments. Also, ratcheting rate increases by increasing moment bending and temperatures. Initial, strain accumulation rate is low, and it increases with the increasing temperatures, which express softening phenomenon due to thermal creep. The strain accumulation is highest along the hoop direction at flanks, similar to results from experimental data without considering temperature effects. The results show that the increase in ratcheting due to high temperature may lead to incontrollable damages to pipework structures. Of course, the prediction of strain accumulation is improved in the presence of isotropic hardening rule. Keywords Ratcheting • Elbow pipe • Out-of-plane bending moment • Temperature • Strain hardening List of symbols b, Q ∞ Materials constants for isotropic hardening 0 Yield stress at zero plastic strain p Plastic stress tensor p

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