The Influence of Creep Strain Rate on Creep Damage Formation in Austenitic Stainless Steel

2018 ◽  
Author(s):  
Edward Hares ◽  
Mahmoud Mostafavi ◽  
Richard Bradford ◽  
Chris Truman
Keyword(s):  
Stainless Steel ◽  
Stress Relaxation ◽  
Creep Strain ◽  
Stress Triaxiality ◽  
Creep Damage ◽  
Operating Conditions ◽  
Element Analysis ◽  
Creep Ductility ◽  
User Subroutine ◽  
Lower Test

Motivated by the need to more accurately account for real, in-service, operating conditions, this paper aims to investigate whether creep strain accumulated at different strain rates is equally damaging. Previous research has suggested that creep strain is more damaging when accumulated more slowly in creep of notched bars. The research presented here seeks to address this question by considering the accumulation of creep strain during stress relaxation of notched bars. Repeat stress relaxation tests with varying dwell lengths were conducted so that the relative damaging effects of the early, rapid accumulation and later, slow accumulation of creep strains could be compared. Another aim was to determine how a lower test temperature affects this creep strain accumulation. In repeat relaxation tests the load is reestablished repeatedly after relaxation dwells of equal duration, until rupture of the specimen occurs. The material used was an ex-service powerplant stainless steel Type 316H. Notched bar specimens were used to introduce stress triaxiality at the notch tip to imitate the multiaxial loads plant components are subjected to during in-service operation. The stresses and strains in the specimens were then assessed using finite element analysis; a user subroutine was implemented so the onset and propagation of creep damage could be simulated throughout the specimens’ creep life. The research found that the material in question had a lower creep ductility at 515°C than at 550°C. The research also showed that creep strain accumulated rapidly at the start of a dwell is significantly less damaging than creep strain accumulated more slowly towards the end of the dwell.

Numerical Simulation of Stress Relaxation and Creep Response of X20 Steam Piping under Diverse Operating Conditions

2021 ◽  
Vol 57 ◽  
pp. 19-32
Author(s):  
Smith Salifu ◽  
Dawood A. Desai ◽  
Schalk Kok
Keyword(s):  
Steady State ◽  
Stress Relaxation ◽  
Damage Accumulation ◽  
Creep Behaviour ◽  
Creep Damage ◽  
Operating Conditions ◽  
Creep Model ◽  
Element Analysis ◽  
Peak Period ◽  
Creep Response

The creep response and stress relaxation of X20 CrMoV12-1 steam piping under diverse operating conditions were simulated using finite element analysis (FEA) code, Abaqus alongside fe-safe/Turbolife software. In the study, steady-state creep and creep analysis characterized by 24 hours daily cycle consisting of a total of 6 hours peak, 4 hours transient and 14 hours off-peak period was considered. Modified hyperbolic sine creep model used in the analysis was implemented in Abaqus via a special creep user-subroutine to compute the stress relaxation and creep behaviour, while the useful service life and creep damage was estimated using fe-safe/Turbolife. The optimum creep strain, stress, damage, and worst life were found at the intrados of the piping, with the steady-state analysis having a higher useful creep life and slower creep damage accumulation. Furthermore, slower stress relaxation with faster damage accumulation was observed in the analysis involving cycles. Finally, a good agreement was obtained between the analytical calculated and simulated rates of the piping.

Revised Weld Residual Stress and Creep Damage Assessments

2009 ◽  
Author(s):  
A. Price ◽  
M. C. Smith ◽  
R. Dennis ◽  
M. W. Spindler
Keyword(s):  
Stainless Steel ◽  
Kinematic Hardening ◽  
Stress Triaxiality ◽  
Creep Damage ◽  
Relaxation Processes ◽  
Two Phase ◽  
Creep Ductility ◽  
Reheat Cracking ◽  
Steel Welds ◽  
Creep Strains

Reheat cracking is a brittle inter-granular type of cracking that occurs in some types of stainless steel welds that have not been post-weld heat treated. It is caused by the accumulation of small creep strains that accompany the relaxation of the weld induced residual stresses. Because relaxation processes produce small creep strains, reheat cracking can occur only if creep ductility is low. Sufficiently low ductilities arise if there is a combination of three factors, susceptible material, operation in the susceptible temperature range and stress triaxiality. The current method used to predict the extent of the zone where reheat cracking can initiate in Type 316H steel weldments is based on continuum damage mechanisms using specially developed material models. In this work more advanced methods have been developed. These methods have been applied to welded specimens manufactured from Esshete 1250. These specimens are described as ‘Borland’ specimens and have been used to study reheat cracking in stainless steel welds. The advanced methods include an improved plasticity model that incorporates combined isotropic and kinematic hardening as well as a two phase annealing function. A number of simulated thermal soak conditions were performed, with a reheat cracking initiation model employed to predict the development of creep damage. This paper details the methodology of the improved welding simulation, its influence on the prediction of reheat cracking and a comparison to previous simulations using existing methods.

Creep Crack Initiation in a Weld Steel: Effects of Residual Stress

2005 ◽  
Author(s):  
Noel P. O’Dowd ◽  
Kamran M. Nikbin ◽  
Farid R. Biglari
Keyword(s):  
Residual Stress ◽  
Finite Element ◽  
Crack Initiation ◽  
Equivalent Stress ◽  
Stress Triaxiality ◽  
Compact Tension ◽  
Residual Stress Field ◽  
Element Analysis ◽  
Creep Ductility ◽  
Uniaxial Test

In this paper, the effect of residual stress on the initiation of a crack at high temperature in a Type 347 austenitic steel weld is examined using the finite element method. Both two and three dimensional analyses have been carried out. Residual stresses have been introduced by prior mechanical deformation, using a previously developed notched compact tension specimen. It has been found that for the 347 weld material, peak stresses in the vicinity of the notch are approximately three times the yield strength at room temperature and the level of stress triaxiality (ratio between hydrostatic and equivalent stress) is approximately 1 (considerably higher than that for a uniaxial test). The finite element analysis includes the effects of stress redistribution and damage accumulation under creep conditions. For the case examined the analysis predicts that crack initiation will occur under conditions of stress relaxation if the uniaxial creep ductility of the material is less than 2.5%. Furthermore, the predicted life of the component under constant load (creep conditions) is significantly reduced due to the presence of the residual stress field.

Development of the RCC-MR Creep Deformation Model for the Prediction of Creep and Stress Relaxation in Type 321 Stainless Steel

2014 ◽  
Author(s):  
A. J. Moffat ◽  
J. P. Douglas ◽  
M. White ◽  
M. W. Spindler ◽  
C. Austin ◽  
...  
Keyword(s):  
Stainless Steel ◽  
Stress Relaxation ◽  
Creep Strain ◽  
Creep Deformation ◽  
Constant Load ◽  
Fatigue Tests ◽  
Creep Model ◽  
Deformation Model ◽  
Relaxation Data ◽  
Creep And Stress Relaxation

In this paper a creep deformation model has been developed for Type 321 stainless steel which has been based on a modified version of the creep model that is used in the French fast reactor design code RCC-MR. The model has been evaluated using: 1) constant load creep data covering the temperature range from 550°C to 650°C and 2) constant displacement, stress relaxation data obtained from creep-fatigue tests at 650°C. Samples in the heat-treatment conditions of solution-treated, aged, and simulated ‘heat affected zone’ have been assessed. The standard RCC-MR model was fitted to the constant load data and provided good predictions of forward creep. However, when this model was used to predict stress relaxation it was observed that the model significantly over predicted creep strain rates and therefore the level of stress drop during each cycle. During constant load tests the stress remains relatively constant (noting that true stress does increase a small amount prior to rupture). However, in relaxation tests the stress varies significantly over the dwell. Due to the poor predictions of stress relaxation it was hypothesised that the fitted model did not capture the stress dependence of creep appropriately. The RCC-MR model was therefore modified to include a primary and secondary threshold stress term that is a function of the accumulated creep strain. This work indicates that the RCC-MR model, modified to include threshold stresses, can be used to provide good predictions of both forward creep and stress relaxation in Type 321 stainless steel. Further work is required to validate this model on stress relaxation data at additional temperatures and lower start of dwell stresses.

Prediction of Welding Residual Stresses, Crack Initiation and Creep Crack Driving Forces C(t) Within a Continuous Finite Element Solution

2010 ◽  
Author(s):  
D. P. Bray ◽  
R. J. Dennis ◽  
M. C. Smith
Keyword(s):  
Finite Element ◽  
Residual Stresses ◽  
Driving Forces ◽  
Welding Process ◽  
Creep Damage ◽  
Operating Conditions ◽  
Finite Element Solution ◽  
Element Solution ◽  
Element Analysis ◽  
Creep Crack

The work reported in this paper investigates the manufacture, through-life operation and cracked behaviour of an attachment weld in a UK AGR boiler. A structural assessment of the attachment weld was performed to demonstrate its integrity. This assessment made use of complex finite element analysis of both the welding process and postulated defects. A simulation of the welding process was performed in order to predict the residual stresses and hardened material state throughout the attachment weld. The welding simulation was performed in two stages since a butter weld was deposited prior to the attachment weld itself. The accumulation of creep damage was predicted during steady normal operating conditions for the lifetime of the component. A contour map of creep damage was used to postulate the location and size of hypothetical single and double edge surface cracks within the weld. These postulated cracks were then explicitly introduced into the finite element model. The crack tip stress parameter C(t) was evaluated in order to predict the creep crack driving forces. The results from a cracked body simulation suggested that the creep crack driving force C(t) reduces as the crack grows, due to relief of the dominant welding residual stresses. The residual stress, creep damage and cracked body simulations have been brought together into a novel continuous finite element solution. The results can be used to support a safety case for continued operation of existing plant.

Identification of Stress Multiaxiallity Effect on Creep Damage by a Combination of Experiments and Numerical Analyses

2009 ◽  
Author(s):  
Yukio Takahashi
Keyword(s):  
Finite Element ◽  
Synergetic Effect ◽  
Stress Triaxiality ◽  
Creep Damage ◽  
Compact Tension ◽  
Simple Expression ◽  
Creep Rupture ◽  
Creep Tests ◽  
Element Analysis ◽  
Rupture Behavior

Structural materials experience various stress states and their integrity under a wide variety of stress multiaxility needs to be evaluated in design and life management for various components. Especially creep rupture behavior is known to be quite sensitive to the stress multiaxiality. To systematically evaluate the multiaxial effect on creep rupture behavior of modified 9Cr-1Mo steel, a number of creep tests were conducted on round-bar specimens with circumferential notches. Strong effects of temperature and deformation rate on the reduction of area were observed and their synergetic effect was modeled by a simple expression. Then crack growth in compact tension specimens was simulated by finite element analysis to derive ductility under higher stress triaxiality. Finally, true rupture strain was expressed as a function of temperature, inelastic strain rate and triaxiality factor and its validity was demonstrated through finite element analyses on notched bar and compact tension specimens employing it as a local fracture criterion.

Creep Damage Analysis of a Lattice Truss Panel Structure

2017 ◽  
Vol 36 (1) ◽  
pp. 89-96 ◽  
Author(s):  
Wenchun Jiang ◽  
Shaohua Li ◽  
Yun Luo ◽  
Shugen Xu
Keyword(s):  
Creep Strain ◽  
Mechanical Load ◽  
Compressive Load ◽  
Stress Triaxiality ◽  
Tensile Load ◽  
Creep Damage ◽  
Good Prediction ◽  
Creep Failure ◽  
Damage Rate ◽  
Compressive Loads

AbstractThe creep failure for a lattice truss sandwich panel structure has been predicted by finite element method (FEM). The creep damage is calculated by three kinds of stresses: as-brazed residual stress, operating thermal stress and mechanical load. The creep damage at tensile and compressive loads have been calculated and compared. The creep rate calculated by FEM, Gibson–Ashby and Hodge–Dunand models have been compared. The results show that the creep failure is located at the fillet at both tensile and creep loads. The damage rate at the fillet at tensile load is 50 times as much as that at compressive load. The lattice truss panel structure has a better creep resistance to compressive load than tensile load, because the creep and stress triaxiality at the fillet has been decreased at compressive load. The maximum creep strain at the fillet and the equivalent creep strain of the panel structure increase with the increase of applied load. Compared with Gibson–Ashby model and Hodge–Dunand models, the modified Gibson–Ashby model has a good prediction result compared with FEM. However, a more accurate model considering the size effect of the structure still needs to be developed.

A Microstructural Sensitivity Study of 316H Austenitic Stainless Steel to Inter-Granular Creep Fracture

2011 ◽  
Vol 488-489 ◽  
pp. 658-661
Author(s):  
B. Chen ◽  
Peter E.J. Flewitt ◽  
David John Smith ◽  
C.M. Younes
Keyword(s):  
Stainless Steel ◽  
High Temperature ◽  
Damage Model ◽  
Creep Damage ◽  
Longitudinal Section ◽  
Sensitivity Study ◽  
Assessment Procedure ◽  
Creep Failure ◽  
Creep Ductility ◽  
Damage Prediction

A preliminary sensitivity examination of the ductility exhaustion based creep damage prediction model, currently used in the R5 high temperature assessment procedure, showed that material property inputs had significant effects on damage prediction. In the present work, the link between the microstructural factors and the susceptibility to inter-granular high temperature creep failure is considered. The latter was judged to be associated with the low creep ductility. Here, the longitudinal section of a creep specimen and the fracture surface were examined. Auger electron spectroscopy was used to investigate the grain boundary composition in this specimen, which failed after a creep test of 1038h at 550°C under a triaxial stress state. The present results demonstrate that there is a possibility to correlate the susceptibility to high temperature inter-granular fracture from the low temperature fracture investigations. Finally, the susceptibility of the pre-treated 316H stainless steel to inter-granular high temperature failure and the contribution to the creep damage model are briefly discussed.

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

2009 ◽  
Vol 131 (6) ◽  
Author(s):  
Stefan Holmström ◽  
Juhani Rantala ◽  
Anssi Laukkanen ◽  
Kari Kolari ◽  
Heikki Keinänen ◽  
...  
Keyword(s):  
Creep Strain ◽  
Three Dimensional ◽  
Creep Damage ◽  
Tertiary Creep ◽  
Dimensional Structure ◽  
Rigid Plastic ◽  
Element Analysis ◽  
Filtering Technique ◽  
Long Term Behavior ◽  
Creep Regime

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.”

A Comparison of Measurement and Modelling of Plastically Induced Residual Stresses in a 316H and a Weld 347 Stainless Steel

2007 ◽  
Author(s):  
M. Turski ◽  
R. C. Wimpory ◽  
N. P. O’Dowd ◽  
P. J. Withers ◽  
K. N. Nikbin
Keyword(s):  
Stainless Steel ◽  
Elevated Temperature ◽  
Neutron Diffraction ◽  
Residual Stresses ◽  
Notch Root ◽  
Creep Behaviour ◽  
Parent Material ◽  
Operating Conditions ◽  
Residual Stress Field ◽  
Creep Ductility

Neutron diffraction measurements on two types of stainless steel have been carried out on Compact Tension (CT) specimens containing plastically induced residual stresses at the blunt notch root. The materials were a type 316H stainless steel parent material and a type 347 stainless steel weld material. The former exhibited a high creep ductility of ∼25% and the latter exhibited brittle behaviour under operating conditions with less than 10% creep ductility. The work is based in part on an ongoing collaborative effort by the Versailles Agreement on Materials and Standards, Technical Working Area, VAMAS TWA 31 Committee working on ‘Crack Growth of Components Containing Residual Stresses’. The objective of this paper is to examine how residual stresses and/or prior straining and subsequent relaxation at high temperature (550 °C for 316H and 650 °C for 347 weld) contribute to creep crack initiation and growth in the two steels. Elastic/plastic/creep finite-element results and neutron diffraction measurements are presented for the CT specimens before and after elevated temperature exposure. The results suggest that the mechanical induced normalised stresses and strains profiles ahead of the crack tip are insensitive to material, however the relaxation response of the materials appear to be dependent on the creep behaviour and ductility. Localised cracking in the plastically deformed material has been observed in both materials due to the redistribution of the residual stress field and associated creep deformation at elevated temperature.

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