Improving Simulations for Low Cycle Fatigue and Ratcheting Responses of Elbows

Abstract

A rate-independent constitutive model is developed incorporating various uniaxial and multiaxial modeling features for improving the simulations of elbow low-cycle fatigue and ratcheting responses. The model development is motivated by the fact that the Chaboche model in ANSYS is unable to simulate the strain ratcheting responses of elbows subjected to internal pressure and opening-closing displacement-controlled cycles. This drawback of the existing model is traced to the isotropic and kinematic hardening modeling features. The isotropic hardening in the Chaboche model can reasonably simulate the material test stress peaks but fails to simulate the hysteresis loop shapes. Incorporation of a strain range dependent modeling feature in evolving the isotropic and kinematic hardening rule parameters improved the simulation of the hysteresis loops both at the material and component levels. The axial and circumferential strain ratcheting simulation of elbow is improved by incorporating a biaxial ratcheting parameter. A modeling feature for nonproportional loading developed by Tanaka is also incorporated in order to simulate the additional cyclic hardening under multiaxial loading. The performance of modified model developed is validated against simulating a broad set of cyclic responses both at the material and component levels. Finally, a numerical technique is developed to simulate the initial and welding residual stresses in elbows, and thereby analytically demonstrate the influence of initial residual stresses on elbow responses.