Study on Creep-Fatigue Damage Evaluation for Boiler Weldment Parts

2000 ◽  
Vol 123 (1) ◽  
pp. 105-111 ◽  
Author(s):  
Takashi Ogata ◽  
Masatsugu Yaguchi
Keyword(s):  
Weld Joint ◽  
Creep Damage ◽  
Fem Analysis ◽  
Fatigue Tests ◽  
Type Iv ◽  
Fine Grain ◽  
Creep Fatigue ◽  
Damage Accumulation Model ◽  
Type Iv Cracking ◽  
Failure Life

Creep-fatigue tests on the heat-affected zone(HAZ) simulated materials, base metal, weld metal and weld joint of 2.25Cr-1Mo steel, and elastic-plastic and creep FEM analysis for the weld joint were conducted. It was found from the comparison between experimental evidences and the analytical results that “Type IV” cracking was caused by two major reasons. One is accumulation of creep strain during strain hold in the fine-grain region is larger than that in other regions, suggesting progress of creep damage in the fine-grain region prior to other regions. The other is existence of triaxial tensile stress field within the fine-grain region caused reduction of failure ductility. Crack initiation portion and failure life under the creep-fatigue test could be well predicted by the nonlinear damage accumulation model based on the FEM analysis results.

Creep Damage Evaluation of Fine-Grained HAZ in Mod. 9Cr Ferritic Heat-Resistant Steel Weldments

2007 ◽  
Author(s):  
N. Yoneyama ◽  
K. Kubushiro ◽  
H. Yoshizawa
Keyword(s):  
Creep Damage ◽  
Life Time ◽  
Creep Life ◽  
Creep Tests ◽  
Boundary Density ◽  
Fine Grained ◽  
Type Iv ◽  
Fine Grain ◽  
Type Iv Cracking ◽  
Rupture Mechanism

9Cr steel weldments are concerned with evaluation of creep life time and creep rupture mechanism. In fine grain HAZ (FG-HAZ) of weldments, TYPE IV cracking and creep voids occurred at lower stress than rupture stress level of base metal. In the crept specimen, FG-HAZ sometime has large coarsening grains near creep voids. These recovery phenomena are localized in FG-HAZ, and recovered microstructures are dependent on heat input of welding. In this study, creep tests are examined in two types of weldments, and relations between creep life time and coarsened sub-grains or grains have been studied by microstructural changing with EBSP analysis. In crept specimens, boundaries are moved and boundary density is decreasing in the fine-grained HAZ. Maximum grain size and creep life time have linear function, and EBSP can evaluate creep life time of 9Cr weldments. These microstructural changing are considered by morphology of precipitates in the several crept specimens.

Accuracy and Margin in Creep-Fatigue Evaluation for Modified 9Cr-1Mo Steel

2007 ◽  
Author(s):  
Yukio Takahashi
Keyword(s):  
Stress Relaxation ◽  
Nuclear Power ◽  
Creep Damage ◽  
Assessment Method ◽  
Fatigue Tests ◽  
Simple Method ◽  
Component Design ◽  
Creep Fatigue ◽  
Grade 91 ◽  
Fatigue Evaluation

Modified 9Cr-1Mo steel (ASME SA-213, Grade 91) is regarded as a promising candidate for structural materials in some of the nuclear power generation plants considered in Generation-IV project. If it is used at high temperature conditions, consideration of creep-fatigue interaction in addition to simple creep rupture is needed in component design. The author has been conducting many creep-fatigue tests for the steel at temperatures between 550°C and 650°C in order to search for a suitable creep-fatigue assessment method. It was found that creep damage at failure estimated by applying the time fraction approach to measured stress relaxation data strongly depended on the test temperature and became quite small at 550°C. However, application of calculated stress relaxation brought about the increase of creep damage over the linear damage summation line. Furthermore, addition of design factors significantly increased the values of creep and fatigue damages, making margin against failure quite large. A new definition of creep damage as a ductility consumer in strain based approach gave a simple method to estimate creep damage more properly and stably with a much smaller sensitivity on the stress relaxation behavior.

Assessment of Failure Life Evaluation Methods for Structural Discontinuities with Fatigue and Creep-Fatigue Tests on Multi-Perforated Plate Made of Mod.9Cr-1Mo Steel

2021 ◽  
Author(s):  
Masanori Ando ◽  
Yuichi Hirose ◽  
Takano Masahito
Keyword(s):  
Fatigue Life ◽  
Perforated Plate ◽  
Estimation Method ◽  
Evaluation Methods ◽  
Fatigue Tests ◽  
Reduction Factor ◽  
Life Evaluation ◽  
Creep Fatigue ◽  
Number Of Cycles ◽  
Failure Life

Abstract This study compares and assesses the different fatgue and creep-fatigue life eveluation methods by performing tests of perforated plate made of Mod.9Cr-1Mo steel. Multi-perforated plate was subjected to mechanical cyclic loading at 550°C, and crack initiation and propagation on the surfaces of the holes were observed. A series of finite element analyses (FEA) were carried out to predict the number of cycles to failure by the several failure life evaluation methods, and these predictions were then compared with the test results. Several types of evaluation methods that use the elastic FEA were applied, namely the stress redistribution locus (SRL) method, simple elastic follow-up method. In addition to these, evaluation was also carried out using the results of inelastic FEA to compare these elastic FEA based estimation method. The comparisons indicate that, for all conditions tested, the SRL method provided a rational prediction of the fatigue and creep-fatigue life when ? = 1.6 was applied, where ? = 1.6 is the recommended reduction factor for this method in general use. A comparison of the SRL method and the results of the inelastic FEA indicated that the applicability of the value of factor ? in the SRL method depends on the elastic region remaining in the cross-section including the evaluated point and the spread in the plastically deformed region in the specimen.

The Creep-Fatigue Evaluation Method for Intermediate Hold Conditions: Proposal and Validation

2011 ◽  
Author(s):  
Satoshi Okajima ◽  
Nobuchika Kawasaki ◽  
Shoichi Kato ◽  
Naoto Kasahara
Keyword(s):  
Initial Stress ◽  
Fatigue Damage ◽  
Evaluation Method ◽  
Strain Range ◽  
Creep Damage ◽  
Fatigue Tests ◽  
Reactor Vessel ◽  
Liquid Sodium ◽  
Improved Method ◽  
Creep Fatigue

In this paper, for the application to the Japan Sodium-cooled Fast Reactor, JSFR, the creep-fatigue damage evaluation method is improved to consider the intermediate holding condition. The improved method is validated through both of the uni-axial and the structure model creep-fatigue tests. In these validations, the target material is 316FR steel, which is planned to use for the reactor vessel. The reactor vessel portion near the liquid sodium surface is one of the most probable points where the creep-fatigue damage is considerable. Because of the relaxation of the temperature gradient, the steady operation stress on the portion near the liquid sodium surface is less than the maximum stress in the transient. In the conventional method, in order to evaluate the creep damage conservatively, the maximum tensile value in the thermal stress transient cycle is used as the initial stress. The improved method evaluates the creep damage using the lower initial stress than the conventional method, while it has the rational margin. For the validation of the improved method, uni-axial creep-fatigue tests and structure model tests are carried out. A series of uni-axial creep-fatigue tests was carried out in the following conditions: 600 degree C testing temperature, 1% total strain range, 1 hour holding time, vacuum or air environments, and the various holding position. While the test environment affects the fatigue damage, it didn’t have significant effect on the creep damage. In the cases with high holding position, the creep damages were evaluated based on the given initial stress with high precision. In the other cases, by the assumption of the steady-stress existence, the rational margin is given for the evaluation. Furthermore, in the design stage, the evaluated creep-fatigue damage has enough margins derived from the conservative evaluation of the initial stress. The structural tests modeled the movement of the liquid sodium surface in the start-up and the shut-down stages, and the relaxation of the temperature gradient in the operation stage. In these tests, the temperature distribution was given by coolant water and an external high-frequency heating coil for the cylindrical specimen, and moved in the axial direction. In addition, the primary stress, which was caused by the weight of the reactor vessel, was given by the screw jack. As a result, using the strain range evaluated by the elastic analysis, the improved method evaluated the crack initiation life due to the creep-fatigue damage with the sufficient safety margin. In the case when the strain range was evaluated by the elastic-plastic analysis, the method predicted the crack initiation life with the good precision. While the evaluation of the crack penetration life was possible, further examination was desired for the precision improvement.

Effect of Prior Creep Strain on High Cycle Fatigue Life of Ti 834

2018 ◽  
Author(s):  
Xijia Wu ◽  
Dongyi Seo ◽  
Marc Head ◽  
Stephen Chan
Keyword(s):  
Fatigue Life ◽  
Creep Strain ◽  
Room Temperature ◽  
High Cycle Fatigue ◽  
Crack Nucleation ◽  
Failure Process ◽  
Creep Damage ◽  
Constant Load ◽  
Fatigue Tests ◽  
Creep Fatigue

Room-temperature fatigue tests were conducted on Ti 834 with prior creep strains accumulated under constant load at 550°C and 600°C, respectively. Microstructural and fractographic examinations on specimens with prior creep strain > 3% revealed the failure process consisting of multiple surface crack nucleation and internal void generation by creep, followed by fatigue crack propagation in coalescence with the internally distributed damage, leading to the final fracture. The amount of prior creep damage increased with creep strain. The fatigue life of Ti 834 was significantly reduced by prior creep straining. The behavior is rationalized with the integrated creep-fatigue theory.

Characteristics of Repair Welds on Aged 2.25Cr-1Mo Steel for Creep and Creep-Fatigue Applications

2004 ◽  
Author(s):  
Yoshio Takagi ◽  
Shigeru Otsuki ◽  
Takuya Ito ◽  
Isamu Nonaka
Keyword(s):  
Weld Metal ◽  
Base Metal ◽  
Weak Link ◽  
Fem Analysis ◽  
Fatigue Tests ◽  
Coarse Grained ◽  
Fatigue Properties ◽  
Strain Concentration ◽  
Fine Grained ◽  
Creep Fatigue

The creep and the creep-fatigue properties of full repair welds (FRW) and partial repair welds (PRW) were evaluated in this study. Since the PRW contained the service-aged girth weld which was the weak link of the cross weld, the PRW was a shorter creep strength than the FRW. Moreover, the PRW showed a remarkably shorter creep-fatigue life compared to that of the FRW. In order to consider the poor creep-fatigue properties of PRW, finite element (FEM) analysis was conducted with experimentally measured material constants using service-aged base metal, aged weld metal, simulated coarse-grained HAZ, simulated fine-grained HAZ and repair weld metal. The analysis revealed that the strain concentrated on the aged and softened base metal or girth weld metal of the repair-welded cross weld specimen and not on the virgin cross weld specimen. The failure locations in creep-fatigue tests were close to the strain concentrated zone. Thus, the strain concentration is considered to work as a significant role and dominate the creep-fatigue properties of repair welds. In addition, the ductility of the weld metal was much less than that of the base metal. Consequently, the interaction of the strain concentration and the lack of ductility induced the lesser creep-fatigue properties of the PRW.

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