Using Nonlinear Kinematic Hardening Material Models for Elastic-Plastic Ratcheting Analysis

Abstract

Applicable design codes for power plant components and pressure vessels demand for a design check against progressive plastic deformation. In the simplest case, this demand is satisfied by compliance with shakedown rules in connection with elastic analyses. The possible non-compliance implicates the requirement of ratcheting analyses on elastic-plastic basis. In this case, criteria are specified on maximum allowable accumulated growth strain without clear guidance on what material models for cyclic plasticity are to be used. This is a considerable gap and a challenge for the practicing CAE (Computer Aided Engineering) engineer. As a follow-up to two independent previous papers PVP2013-98150 ASME [1] and PVP2014-28772 [2] it is the aim of this paper to close this gap by giving further detailed recommendation on the appropriate application of the nonlinear kinematic material model of Chaboche on an engineering scale and based on implementations already available within commercial finite element codes such as ANSYS® and ABAQUS®. Consistency of temperature-dependent runs in ANSYS® and ABAQUS® is to be checked. All three papers together constitute a comprehensive guideline for elasto-plastic ratcheting analysis. The following issues are examined and/or referenced: • Application of monotonic or cyclic material data for ratcheting analysis based on the Chaboche material model • Discussion of using monotonic and cyclic data for assessment of the (non-stabilized) cyclic deformation behavior • Number of backstress terms to be applied for consistent ratcheting results • Consideration of the temperature dependency of the relevant material parameters • Consistency of temperature-dependent runs in ANSYS® and ABAQUS® • Identification of material parameters dependent on the number of backstress terms • Identification of material data for different types of material (carbon steel, austenitic stainless steel) including the appropriate determination of the elastic limit • Quantification of conservatism of simple elastic-perfectly plastic behavior • Application of engineering versus true stress-strain data • Visual checks of data input consistency • Appropriate type of allowable accumulated growth strain. This way, a more accurate inelastic analysis methodology for direct practical application to real world examples in the framework of the design code conforming elasto-plastic ratcheting check is proposed.

Chaboche-based cyclic material hardening models for 316 SS–316 SS weld under in-air and pressurized water reactor water conditions

“…Due to the convergence issues associated with the parameter optimization scheme, the stress-strain curves were limited to 5% when 0.05%-offset strain stress was selected as the yield stress. Note that according to ASME code Section III, a vessel design is considered acceptable if the maximum accumulated local strain does not exceed 5% [2,9]. Also, the estimates of the reported parameters considered the true stress-strain curves since a typical FE code requires that the stress-strain or equivalent parameters be represented in true stress-strain form.…”

“…Most of the literature on fatigue modeling has focused on improving the stress-life data and related empirical fatigue design curves [4][5][6] for estimating fatigue life given the stress/strain state of a reactor component. A few studies [7][8][9][10][11][12][13] A previous Argonne report [14] presented a baseline FE model for cyclic stress analysis of PWR components. In this earlier work, elastic analysis approaches were used.…”

Tensile and Fatigue Testing and Material Hardening Model Development for 508 LAS Base Metal and 316 SS Similar Metal Weld under In-air and PWR Primary Loop Water Conditions

“…Note that due to convergence issues associated with the parameter optimization scheme, we limited the stress-strain curves to 2% when considering condition 1 as the yield stress and to 5% when considering conditions 2-4 as the yield stress. According to ASME code Section III, a vessel design is considered acceptable if the maximum accumulated local strain does not exceed 5% [2,10]. Also, while estimating the reported parameters, we considered the true stress-strain curves since a typical FE code requires that the stress-strain or equivalent parameters be represented in true stress-strain form.…”

“…Most of the presently available fatigue modeling literature has focused on improving the stress-life data set and related empirical fatigue design curves [5][6][7] for estimating fatigue life given the stress/strain state of a component. A few studies [8][9][10][11][12][13][14] have given emphasis to the more mechanistic aspects of fatigue life prediction, such as through-ratcheting or shakedown analysis of reactor component by means of FE models based on nonlinear kinematic hardening. Most of the models discussed in these studies are based on the monotonic stress-strain curve obtained from a tension specimen.…”

Study the Cyclic Plasticity Behavior of 508 LAS under Constant, Variable and Grid-Load-Following Loading Cycles for Fatigue Evaluation of PWR Components

“…Hence, for better accuracy, it is essential to estimate the fatigue and ratcheting damage of reactor components based on the results of elastic-plastic stress analysis rather than pure elastic stress analysis alone. Recently, elasticplastic analysis and fatigue life estimation based on flaw tolerance [17][18][19][20][21][22] have increasingly become an active research area by infusing more mechanics-based prediction capability into the overall fatigue evaluation methodology. Since elastic-plastic ratcheting is a phenomenon closely related to the transient plastic deformation behavior, its accurate description requires the calculation of material hardening stressstrain states as a function of fatigue cycles or time.…”