Biaxial Thermomechanical-Fatigue Life Property of a Directionally Solidified Ni-Base Superalloy

2008 ◽  
Vol 130 (6) ◽  
Author(s):  
Takashi Ogata
Keyword(s):  
Fatigue Life ◽  
Shear Strain ◽  
Hold Time ◽  
Strain Range ◽  
Strain Ratio ◽  
Life Assessment ◽  
Wave Form ◽  
Creep Fatigue ◽  
High Temperature Components ◽  
Failure Life

High-temperature components in thermal power plants are subjected to creep-fatigue loading where creep cavities initiate and grow on grain boundaries. Development of life assessment methods of high-temperature components in gas turbine for maintenance and operating cost reduction is strongly demanded by Japanese utilities. Especially, first row blades are subjected to complicated thermomechanical-fatigue (TMF) loading during start, steady state, stop cycles. Therefore it is important to clarify the TMF life property of blade materials to develop a life assessment procedure. In this study, tension-torsion biaxial TMF tests have been performed between 450°C and 870°C on a Ni-base directional solidified superalloy. Strain ratio ϕ was defined as shear strain range, Δγ, to normal strain range, Δε, and ϕ varied from 0 to infinity. The “Blade wave form,” which simulated temperature and strain condition of the blade surface, was employed. The biaxial TMF tests were also carried out on coated specimens with CoCrAlY. Fatigue life under the biaxial TMF loading showed strain ratio dependency giving shorter life with increasing ϕ. Considering biaxial stress effect on the failure life, an equivalent shear strain range was derived based on the Γ-plane theory, and the biaxial TMF life was well correlated with the equivalent shear strain range. The biaxial TMF life was reduced by introducing strain hold duration at the maximum temperature. The maximum stress increased by introducing the hold time due to increasing mean stress level in the Blade wave form. It was concluded that creep damage gradually accumulated during cycles resulting in reduction in the TMF life. The nonlinear creep-fatigue damage accumulation model was applied to predict failure life of the hold time tests. As a result, the failure lives were predicted within a factor of 1.5 on the observed life. It was found that the fatigue life of CoCrAlY coated material reduced 1∕2 to 1∕3 from that of the substrate. From observation of the longitudinal section of the coated specimens, many cracks started from the coating surface and penetrated into the substrate. It was concluded that the CoCrAlY coating reduced the biaxial TMF life due to acceleration of crack initiation period in the substrate.

Biaxial Thermo Mechanical Fatigue Life Propety of a Ni Base DS Super Alloy

2006 ◽  
Author(s):  
Takashi Ogata ◽  
Masato Yamamoto
Keyword(s):  
Fatigue Life ◽  
Shear Strain ◽  
Hold Time ◽  
Strain Range ◽  
Strain Ratio ◽  
Biaxial Stress ◽  
Life Assessment ◽  
Mechanical Fatigue ◽  
Coated Material ◽  
Failure Life

Development of life assessment methods of high temperature components in gas turbine for maintenance and operating cost reduction is strongly demanded by Japanese utilities. Especially, first row blades are subjected to complicated. Thermo-Mechanical-Fatigue (TMF) loading during start-steady state-stop cycles. Therefore it is important to clarify the TMF life property of blade materials to develop a life assessment procedure. In this study, tension-torsion biaxial TMF tests have been performed between 450C and 870C on a Ni base directional solidified (DS) supper alloy. Strain ratio, φ was defined as shear strain range, Δγ, to normal strain range, Δε, and φ varied from 0 to infinite. The “Blade waveform” which simulated surface temperature and strain loading condition of the blade, was employed. The biaxial TMF tests were also carried out on coated specimens with CoCrAlY. Fatigue life under biaxial TMF loading showed strain ratio dependency giving shorter life with increasing φ. Considering biaxial stress effect on failure life, an equivalent shear strain range was derived based on Γ -plane theory, and biaxial TMF life was well correlated with the equivalent shear strain range. The biaxial TMF life was reduced by introducing strain hold duration at the maximum temperature. The maximum stress increased by introducing the hold time due to increasing mean stress level in the Blade waveform. It was concluded that creep damage gradually accumulated during cycles resulting in TMF life reduction. The nonlinear creep-fatigue damage accumulation model was applied to predict failure life of the hold time tests. As a result, failure life could be predicted within factor of 1.5 on observed life. It was found that fatigue life of CoCrAlY coated material reduced 1/2 to 1/3 from that of without coated material. From observation of longitudinal section of the coated specimens, many cracks started from coating surface and penetrated into the substrate. It was concluded that CoCrAlY coating reduced the biaxial TMF life due to acceleration of crack initiation period in the substrate.

Creep-Fatigue Life Assessment Under Multiaxial Strain State

2006 ◽  
Author(s):  
Shengde Zhang ◽  
Masao Sakane ◽  
Takamoto Itoh
Keyword(s):  
Fatigue Life ◽  
Biaxial Tension ◽  
Creep Damage ◽  
Strain Ratio ◽  
Life Assessment ◽  
Principal Strain ◽  
304 Stainless Steel ◽  
Strain Wave ◽  
Element Analysis ◽  
Creep Fatigue

This paper studies the multiaxial creep-fatigue life for type 304 stainless steel at elevated temperature. Strain controlled biaxial tension-compression creep-fatigue tests were carried out using cruciform specimens under four strain waves at three principal strain ratios. The strain wave and the principal strain ratio had a significant effect on creep-fatigue life of the cruciform specimen. The creep-fatigue life ratio decreased as the principal strain ratio increased which indicates that larger creep damage occurred at larger principal strain ratio. The effects of the strain wave and principal strain ratio were discussed in relation to the observations of surface crack and void area density in the gage part of the specimen. Creep-fatigue lives were discussed in relation to the principal stress amplitude calculated by finite element analysis and creep-fatigue damage was evaluated by linear damage rule.

EVALUATION OF CREEP-FATIGUE LIFE BY FRACTION OF CAVITY AREA

2006 ◽  
Vol 20 (25n27) ◽  
pp. 4237-4242
Author(s):  
BUMJOON KIM ◽  
BYEONGSOO LIM
Keyword(s):  
Fatigue Life ◽  
Base Metal ◽  
Gas Turbines ◽  
Cyclic Load ◽  
Static Load ◽  
Hold Time ◽  
Fatigue Tests ◽  
Creep Fatigue ◽  
High Temperature Components ◽  
Main Steam

The components of power plant such as main steam pipe and gas turbines are operated under static and cyclic load conditions. As the period of static load increases, the service life of these components decreases. Generally, the increase of cyclic load results in fatigue damage and the increase of static load period results in the metallurgical degradation by the effect of creep. Under the creep-fatigue interaction, cavities cause rapid degradation of material and decreases the creep-fatigue life of high temperature components. In this paper, creep-fatigue tests were performed to investigate the relationship between the cavity and creep-fatigue life under various tensile hold times. Test materials were HAZ and base metal of P122 (12 Cr -2 W ) alloy weldment. The effect of hold times on the cavity damage was examined and the fraction of cavity area was analyzed. From the linear relationship of Fca (fraction of cavity area) and experimental life, a new parameter for life evaluation, Fca, was introduced and the creep-fatigue life was predicted by Fca. Good agreement was found between experimental and predicted life. Under the same hold time condition, the Fca of HAZ was greater than that of base metal while the creep-fatigue life of HAZ was shorter than that of base metal.

Creep and Creep-Fatigue Deformation and Life Assessment of Ni-Based Alloy 740H and Alloy 617

2018 ◽  
Author(s):  
Shengde Zhang ◽  
Yukio Takahashi
Keyword(s):  
Fatigue Life ◽  
Life Prediction ◽  
Hold Time ◽  
Fatigue Tests ◽  
Fatigue Life Prediction ◽  
Life Assessment ◽  
Prediction Methods ◽  
Creep Tests ◽  
Alloy 617 ◽  
Creep Fatigue

This paper presents creep and creep-fatigue deformations and lives of both Ni-based alloys, Alloy 740H and Alloy 617. Creep tests were performed using solid bar specimens at 650°C-800°C, and effect of cyclic loading on creep deformation and rupture was discussed. Strain controlled creep-fatigue tests were also performed under triangular and trapezoidal waveforms at 700°C. Alloy 740H showed stronger creep-fatigue resistance compared to Alloy 617. Creep-fatigue lives in trapezoidal waveform were smaller than those in the pure fatigue test and the creep-fatigue lives decreased as the hold time increased. Applicability of four representative creep-fatigue life prediction methods was discussed.

Creep and Creep-Fatigue Strength Property of a Long-Term Used Boiler Weldment Parts of 2.25Cr-1Mo Steel

2002 ◽  
Author(s):  
Takashi Ogata
Keyword(s):  
Weld Joint ◽  
Hold Time ◽  
Prediction Method ◽  
Strain Range ◽  
Fatigue Properties ◽  
Lower Strain ◽  
Creep Fatigue ◽  
Failure Life ◽  
Strain Hold

The purpose of this study is to make clear effect of material degradation on creep and creep-fatigue properties of a weldment. Materials used in this study were a weldment of a boiler super header operated for about 200,000 hours (U weld joint) and a simulated as-received weldment (S weld joint). In the U weld joint, although creep cavities were not observed at all, significant coarsening of carbide precipitation and low dense dislocation structure were observed at all locations in the weldment compared with microstructure of the S weld joint. Creep rupture time of the U weld joint is shorter than that of the S weld joint at higher stress region, while the discrepancy of the rupture strength tends to disappear as stress level becomes lower. Strain controlled creep-fatigue tests with 1 hour and 10 hours strain hold time at the maximum tensile strain were performed at strain range of 0.3% and 0.5% on S and U weld joints. No difference of failure life between materials was observed. Reduction in failure life by introducing strain hold period becomes larger as decreasing strain range level indicating that creep-fatigue interaction is enhanced under lower strain range. Based on the nonlinear damage accumulation model, a simplified creep-fatigue life prediction method of weldments under creep-fatigue loading conditions was proposed. It was demonstrated that failure lives under the long-term creep-fatigue conditions were well predicted by the simplified prediction method.

Fatigue Behavior of K447A Superalloy under Low Cycle Fatigue and Creep-Fatigue Interaction Conditions

2015 ◽  
Vol 750 ◽  
pp. 121-126 ◽  
Author(s):  
Hui Chen Yu ◽  
Cheng Li Dong ◽  
Ying Li
Keyword(s):  
Fatigue Life ◽  
Low Cycle Fatigue ◽  
Fatigue Behavior ◽  
Hold Time ◽  
Strain Ratio ◽  
Mean Stress ◽  
Cycle Fatigue ◽  
Creep Fatigue ◽  
Cyclic Damage ◽  
Mean Stresses

Strain-controlled low cycle fatigue (LCF) and creep-fatigue interaction (CFI) tests of K447A are conducted at 760oC in order to investigate the effects of different dwell times and strain ratios on the fatigue behavior and life. For the cases of stain ratio Rε=-1 with balanced hold time, the tensile and compressive mean stresses will generate. For the case of stain ratio Rε=-1 with compressive holding 60s, the tensile mean stress will produce. For the case of stain ratio Rε=-1 with tensile holding 60s, the compressive mean stress will produce. For the cases of stain ratio Rε=0.1 and Rε=-1with no hold time, the tensile mean stress will produce. The cyclic damage accumulation (CDA) method and modified CDA method were employed to predict the fatigue life for K447A, respectively. The fatigue life predicted by CDA method is within the scatter band of 18.2X. The fatigue life predicted by the modified CDA method agrees very well with the experimental life and the predicted life is well within the scatter band of 3.1X, which means that the modified CDA method is able to consider the influences of dwell time and strain ratio on the fatigue life of K447A.

scholarly journals Characterization of Austenitic Stainless Steels with Regard to Environmentally Assisted Fatigue in Simulated Light Water Reactor Conditions

Metals ◽  
2021 ◽  
Vol 11 (2) ◽  
pp. 307
Author(s):  
Matthias Bruchhausen ◽  
Gintautas Dundulis ◽  
Alec McLennan ◽  
Sergio Arrieta ◽  
Tim Austin ◽  
...  
Keyword(s):  
Surface Roughness ◽  
Fatigue Life ◽  
Elevated Temperature ◽  
Strain Rate ◽  
Power Plants ◽  
Research Effort ◽  
Hold Time ◽  
Strain Range ◽  
Austenitic Steels ◽  
Mean Strain

A substantial amount of research effort has been applied to the field of environmentally assisted fatigue (EAF) due to the requirement to account for the EAF behaviour of metals for existing and new build nuclear power plants. We present the results of the European project INcreasing Safety in NPPs by Covering Gaps in Environmental Fatigue Assessment (INCEFA-PLUS), during which the sensitivities of strain range, environment, surface roughness, mean strain and hold times, as well as their interactions on the fatigue life of austenitic steels has been characterized. The project included a test campaign, during which more than 250 fatigue tests were performed. The tests did not reveal a significant effect of mean strain or hold time on fatigue life. An empirical model describing the fatigue life as a function of strain rate, environment and surface roughness is developed. There is evidence for statistically significant interaction effects between surface roughness and the environment, as well as between surface roughness and strain range. However, their impact on fatigue life is so small that they are not practically relevant and can in most cases be neglected. Reducing the environmental impact on fatigue life by modifying the temperature or strain rate leads to an increase of the fatigue life in agreement with predictions based on NUREG/CR-6909. A limited sub-programme on the sensitivity of hold times at elevated temperature at zero force conditions and at elevated temperature did not show the beneficial effect on fatigue life found in another study.

scholarly journals Creep Fatigue Life Assessment of a Pipe Intersection with Dissimilar Material Joint by Linear Matching Method

2016 ◽  
Vol 853 ◽  
pp. 366-371
Author(s):  
Daniele Barbera ◽  
Hao Feng Chen ◽  
Ying Hua Liu
Keyword(s):  
High Temperature ◽  
Fatigue Life ◽  
Direct Method ◽  
Life Assessment ◽  
Dissimilar Material ◽  
Matching Method ◽  
Fatigue Life Assessment ◽  
Creep Fatigue ◽  
The Impact ◽  
Linear Matching Method

As the energy demand increases the power industry has to enhance both efficiency and environmental sustainability of power plants by increasing the operating temperature. The accurate creep fatigue life assessment is important for the safe operation and design of current and future power plant stations. This paper proposes a practical creep fatigue life assessment case of study by the Linear Matching Method (LMM) framework. The LMM for extended Direct Steady Cycle Analysis (eDSCA) has been adopted to calculate the creep fatigue responses due to the cyclic loading under high temperature conditions. A pipe intersection with dissimilar material joint, subjected to high cycling temperature and constant pressure steam, is used as an example. The closed end condition is considered at both ends of main and branch pipes. The impact of the material mismatch, transitional thermal load, and creep dwell on the failure mechanism and location within the intersection is investigated. All the results demonstrate the capability of the method, and how a direct method is able to support engineers in the assessment and design of high temperature component in a complex loading scenario.

Proposal for a Total Fatigue Life Assessment Methodology That Predicts Fatigue Life From Defect Initiation to Through Wall Leak

2020 ◽  
Author(s):  
Joseph Batten ◽  
Chris Currie ◽  
Jonathan Mann ◽  
Andrew Morley
Keyword(s):  
Fatigue Life ◽  
Effective Strain ◽  
Strain Range ◽  
Assessment Methods ◽  
Life Assessment ◽  
Assessment Methodology ◽  
Traditional Assessment ◽  
Fatigue Life Assessment ◽  
Definition Of

Abstract Even with improvements to remove excessive conservatisms, current fatigue assessment approaches can result in high Cumulative Usage Factors (CUFs) for some analyses. In order to improve plant availability from these assessments and mitigate future changes to design codes, an improvement in understanding in this area is desirable. Hence the proposal for a Life Assessment Methodology (LAM) was created. The LAM is a concept for an approach based on modelling each stage of fatigue life to predict total fatigue life, as a means of minimising conservatism in an assessment, where necessary. It should also be capable of incorporating statistical methods to assign reliability figures to calculated plant lives. This paper describes the proposed definition of the LAM and how a proof of concept version of the LAM was developed to assess the Bettis Bechtel Stepped Pipe (BBSP) test. The results were presented with two seeded cases (fixed inputs) and a range of lives corresponding to associated Target Reliabilities (TRs). The Best Estimate (BE) and TR associated lives produced were based on using the latest methods available for calculating Fatigue Initiation (FI) and Fatigue Crack Growth (FCG), whereas the seeded Effective Strain Range (ESR) comparison case used current deterministic assessment methods. The results for the case study concluded that there is a benefit to pursuing the development of the LAM when compared to traditional assessment methods. It highlighted and quantified the conservatism present in traditional assessment methods for these cases as well as the need to understand the required TR for a specific component as this can have a large effect on the predicted life. With further refinements to the method, a more realistic and robust output of the total fatigue life distribution (for specific cases) would be obtained, which in turn would allow us to better quantify the conservatism associated with a TR.

A Study on Fatigue and Creep-Fatigue Life Assessment Using Cyclic Thermal Tests With Mod.9Cr-1Mo Steel Structures

2012 ◽  
Author(s):  
Masanori Ando ◽  
Hiroshi Kanasaki ◽  
Shingo Date ◽  
Koichi Kikuchi ◽  
Kenichiro Satoh ◽  
...  
Keyword(s):  
Fatigue Life ◽  
Temperature Distribution ◽  
Crack Initiation ◽  
Thermal Loading ◽  
Life Assessment ◽  
Estimation Methods ◽  
Short Term ◽  
Cyclic Thermal Loading ◽  
Fatigue Life Assessment ◽  
Creep Fatigue

In a component design at elevated temperature, fatigue and creep-fatigue is one of the most important failure modes, and fatigue and creep-fatigue life assessment in structural discontinuities is important issue to evaluate structural integrity of the components. Therefore, to assess the failure estimation methods, cyclic thermal loading tests with two kinds of cylindrical models with thick part were performed by using an induction heating coil and pressurized cooling air. In the tests, crack initiation and propagation processes at stress concentration area were observed by replica method. Besides those, finite element analysis (FEA) was carried out to estimate the number of cycles to failure. In the first test, a shorter life than predicted based on axisymmetric analysis. Through the 3 dimensional FEA, Vickers hardness test and deformation measurements after the test, it was suggested that inhomogeneous temperature distribution in hoop direction resulted in such precocious failure. Then, the second test was performed after improvement of temperature distribution. As a result, the crack initiation life was in a good agreement with the FEA result by considering the short term compressive holding. Through these test and FEA results, fatigue and creep-fatigue life assessment methods of Mod.9Cr-1Mo steel including evaluation of cyclic thermal loading, short term compressive holding and failure criterion, were discussed. In addition it was pointed out that the temperature condition should be carefully controlled and measured in the structural test with Mod.9Cr-1Mo steel structure.

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