Modeling and Verification of Creep Strain and Exhaustion in a Welded Steam Mixer

Abstract

Structures operating in the creep regime will consume their creep life at a greater rate in locations where the stress state is aggravated by triaxiality constraints. Many structures, such as the welded steam mixer studied here, also have multiple material zones differing in microstructure and material properties. The three-dimensional structure as such, in addition to interacting material zones, is a great challenge for finite element analysis (FEA), even to accurately pinpoint the critical locations where damage will be found. The studied steam mixer, made of 10CrMo 9-10 steel (P22), has after 100,000 h of service developed severe creep damage in several saddle point positions adjacent to nozzle welds. FE-simulation of long term behavior of this structure has been performed taking developing triaxiality constraints, material zones, and primary to tertiary creep regimes into account. The creep strain rate formulation is based on the logistic creep strain prediction model implemented to ABAQUS, including primary, secondary, and tertiary creep. The results are presented using a filtering technique utilizing the formulation of rigid plastic deformation for describing and quantifying the developing “creep exhaustion.”

Modeling of the High Temperature Creep and Rupture under the Complex Stress State

“…where ij ε is a creep strain rate tensor; σ vM is von Mises equivalent stress σ vM =(3S ij S ij /2) 1/2 ; S ij is a deviatory stress S ij =σ ij -δ ij σ kk /3 (δ is Kronecker symbol); is multi-axial ductility factor, σ ef is effective stress. Multi-axial ductility factor is used to take into account dependence of damage variable from the stiffness of the stress state [1,4,13,14]:…”

Crystal Plasticity with Micromorphic Regularization in Assessing Scale Dependent Deformation of Polycrystalline Doped Copper Alloys

“…Typically, the respective works have not explicitly treated material plasticity and creep at the scale of the microstructure, but rather presented "top-down"-like approaches to assess the behavior of bulk material and discontinuities such as welds and focused on extrapolation of the experimental results towards long-term operational periods. Simultaneously, strategies have been developed to better integrate more complex creep models to integrity analyses [4,5] to account for different stages of creep (such as extensive primary creep exhibited by several relevant copper alloys [3]) and effects of joints and welds via concepts such as the weld strength factor (WSF). However, none of these nor other earlier works have performed direct modeling of plasticity at the scale of the microstructure, but rather have identified the possible need to further the respective capabilities to improve upon the respective predictive nature of the used material models and approach them in a somewhat more physicsbased manner.…”