Stress Redistribution Locus Method Research Articles

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

Abstract In the design of components for service at elevated temperatures, fatigue and creep-fatigue due to cyclic loading are some of the most important failure modes, and the assessment of failure life at structural discontinuities is a key issue within the evaluation of the integrity of the components. Since several fatigue and creep-fatigue life evaluation methods have been proposed, this study compares and assesses these different methods by performing fatigue and creep-fatigue tests of perforated plate made of Mod.9Cr-1Mo steel. Multiperforated plate was subjected to mechanical cyclic loading at 550 °C, and crack initiation and propagation at the surfaces of the holes were observed. The stress distribution was varied by changing the hole arrangement and loading level. 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, and the methods described in the design and construction code for fast reactors (FRs) published by the Japan Society of Mechanical Engineers (JSME FRs code). In addition to these, evaluation was also carried out using the results of inelastic FEA to compare these elastic FEA-based estimation methods. The comparisons indicate that, for all conditions tested, the SRL method provided a rational prediction of the fatigue and creep-fatigue life when κ = 1.6 was applied, where κ = 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 κ 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.

Comparison and Assessment of the Creep-Fatigue Evaluation Methods With Notched Specimen Made of Mod.9Cr-1Mo Steel

In components design at elevated temperature, creep-fatigue is one of the most important failure modes, and assessment of creep-fatigue life in structural discontinuities is an important issue in evaluating the integrity of components. Therefore, a lot of creep-fatigue life evaluation methods were proposed until now. To compare and assess the evaluation methods, a series of creep-fatigue test was carried out with notched specimens. All the specimens were made of Mod.9Cr-1Mo steel, which is a candidate material for primary and secondary heat transport system components of the Japan sodium-cooled fast reactor (JSFR). Mechanical creep-fatigue tests and thermal creep-fatigue test were performed by using a conventional uni-axial push–pull fatigue test machine and a thermal gradient generating system with an induction heating. The stress concentration levels were adjusted by varying the notch radius in the each test. The creep-fatigue lives, crack initiation, and propagation processes were monitored by a digital microscope and the replica method. A series of finite element analysis (FEA) was carried out to predict the number of cycles to failure by the several creep-fatigue life evaluation methods. Then, these predictions were compared with the test results. Several types of evaluation methods such are stress redistribution locus (SRL) method, simple elastic follow-up method and the methods described in the design and constriction code for fast reactor (FR) published by the Japan Society of Mechanical Engineers (JSME FRs code) were applied. Through the comparisons, it was appeared that SRL method gave rational conservative prediction of the creep-fatigue life when the factor of κ = 1.6 was applied for all conditions tested in this study. A comparison of SRL method and simple elastic follow-up method indicated that SRL method applied factor of κ = 1.6 gave the smallest creep-fatigue life in practicable stress range level. The JSME FRs code gave an evaluation 70–100 times conservative lives comparing with the test results.

Estimation of Inelastic Behavior for a Tapered Nozzle in Vessel due to Thermal Transient Load using Stress Redistribution Locus Method

Simplified inelastic design procedures have been required to reduce simulation cost and to shorten the development period for the modern high temperature machines and structures. Stress redistribution locus (SRL) method has been proposed to provide a reasonable solution employing both the elastic Finite Element (FE) analysis and a unique hyperbolic curve. In the SRL method, dimensionless stress and strain that were normalized using the inelastic stress/strain and the corresponding elastic stress/strain were introduced. This methodology is based on the fact that the stress distribution in well deformed or in high temperature components changes with deformation or time and that the dimensionless stress-strain relation traces a kind of the elastic follow-up locus in spite of the constitutive equation of the material. In this paper, FE analyses incorporating plasticity and creep were performed for a tapered nozzle of a reactor vessel under some thermal transient loads. The dimensionless stress-strain relation was compared with a conventional and newly proposed SRL curves. FE analysis results showed there to be a critical point in the tapered nozzle due to the thermal transient load dependant on a descending rate of temperature from the higher temperature in the operation cycle. Whenever a certain amount of inelastic strain in the nozzle is produced in a restricted area, the dimensionless stress-strain relation is depicted inside the presently proposed SRL curve. Thus the appropriate coefficient in the SRL method is found to be greater than the proposed one, and the present criterion guarantees robust structures for complicated components involving inelastic deformation.

Crack Initiation Process for Semicircular Notched Plate in Fatigue Test at Elevated Temperature

Crack initiation and propagation process of fatigue test in semicircular notched plates at elevated temperature were conducted under strain-controlled condition in order to study the crack initiation/propagation/coalescence process and load decrease in structure having stress concentration. Test specimens made of SUS304 stainless steel are tested at 550°C and the geometry of the semicircular notched plate specimens are changed by diameter size of the circular notch in both of single-notched specimens and double-notched specimens. Photographs in all cycles were recorded to investigate crack initiation/propagation/coalescence as a function of number of applied cycle. The typical crack process is predicted by using the simplified method, namely, stress redistribution locus method as well as Neuber’s formula.

Estimation of Crack Initiation Life in Low Cycle Fatigue by Stress Redistribution Locus Method for Circumferential Notched Bar of Low Carbon Steel (S20C)

The stress redistribution locus (SRL) method has been proposed to make a reasonable estimate of inelastic behavior under elevated temperatures employing both the elastic Finite Element (FE) analysis and a unique hyperbolic curve, where dimensionless equivalent strain and stress are normalized by elastic ones in the elastic FE analysis. In order to investigate applicability of the SRL method to components under the elastic-plastic state, low cycle fatigue tests were conducted employing a smooth specimen and four types of circumferentially notched bars. The number of cycles to crack initiation estimated by the SRL method was compared with the experiment. This showed that when an adequate elastic region remained inside the notch root, the crack initiation life could be well predicted by the SRL method. However when a ratio of the plastic area to the entire section of the notch root occupied over approximately 0.3%, an equivalent strain range on a surface of the notch root estimated by the SRL method became smaller than the one calculated by using the detailed inelastic FE analyses. On this occasion the crack initiation life estimated by the SRL method was overestimated.

Simplified Estimation of Inelastic Behavior in Local Section of Components Subjected to Membrane, Bending and Peak Stresses Using Stress Redistribution Locus Method

Stress redistribution locus (SRL) method has been proposed to estimate stress and strain behaviors at the specific point employing the elastic Finite Element Analysis, an inelastic constitutive equation, and a unique hyperbolic curve: ε= {1/σ + (κ - 1) σ}/κ , where ε and σ show dimensionless equivalent strain and stress that are normalized by the elastic FEA result. FE analyses incorporating plasticity and creep in were performed for plates with a single edge notch where membrane, bending, and peak stresses appeared at the same time. The normalized equivalent stress and strain behavior was compared with the proposed SRL curve. Both at smaller deformation under monotonic loadings and at the early time under creep, the normalized equivalent stress and strain state was depicted on the SRL curve of κ=1.6 in the case of the plane stress condition. With the larger deformation and as time went by, the stress and strain locus went away from the SRL curve of κ=1.6, and approached to the one of κ=1.0. However the inelastic behavior almost followed the SRL curve of κ=1.6 in the generalized plane strain condition. Whenever the inelastic zone in the local section was limited, the precise inelastic behavior would be described using the proposed SRL curve of κ=1.6.

応力再配分軌跡法による熱過渡容器ノズルの非弾性変形挙動推定

Simplified inelastic design procedures have been required to reduce simulation cost and to shorten the development period for the modern high temperature machines and structures. Stress redistribution locus (SRL) method has been proposed to provide a reasonable solution employing both the elastic Finite Element (FE) analysis and a unique hyperbolic curve. In the SRL method, dimensionless stress and strain that were normalized using the inelastic stress/strain and the corresponding elastic stress/strain were introduced. This methodology is based on the fact that the stress distribution in well deformed or in high temperature components changes with deformation or time and that the dimensionless stress-strain relation traces a kind of the elastic follow-up locus in spite of the constitutive equation of the material. In this paper, FE analyses incorporating plasticity and creep were performed for a tapered nozzle of a reactor vessel under some thermal transient loads. The dimensionless stress-strain relation was compared with a conventional and newly proposed SRL curves. FE analysis results showed there to be a critical point in the tapered nozzle due to the thermal transient load dependant on a descending rate of temperature from the higher temperature in the operation cycle. Whenever a certain amount of inelastic strain in the nozzle is produced in a restricted area, the dimensionless stress-strain relation is depicted inside the presently proposed SRL curve. Thus the appropriate coefficient in the SRL method is found to be greater than the proposed one, and the present criterion guarantees robust structures for complicated components involving inelastic deformation.

1609 応力再配分軌跡法による高温局所部位の非弾性挙動推定(OS16-3 核融合炉-多種多様な力学的環境における強度・設計問題)

1609 応力再配分軌跡法による高温局所部位の非弾性挙動推定(OS16-3 核融合炉-多種多様な力学的環境における強度・設計問題)

706 応力再配分軌跡(SRL)法による環状切欠き材(S20C)の低サイクル疲労き裂発生寿命推定(OS10-(2) 発電機器の材料・構造解析と評価,オーガナイズドセッション)

Stress redistribution locus (SRL) method has been proposed to provide an inelastic behavior reasonably employing both the elastic FEM analysis and a unique hyperbolic curve: ε={1/σ+(κ-1)σ}κ, where ε and σ show the dimensionless equivalent strain and stress normalized by the elastic FEM result. After low cycle fatigue tests are conducted using a smooth specimen and four kinds of circumferentially notched bars, the number of cycles to crack initiation got from the experiments is compared with the one estimated by the SRL method. When the adequate elastic region remais inside the notch root, the crack initiation life can be well predicted. However when the plastic region area occupies more than 30% in the notch root section, the equivalent strain range on the notch root surface calculated using the proposed SRL curve and the dynamic stress-strain curve become to be smaller than the one by FEM analyses, and then the crack initiation life will be estimated a bit critically.

21316 円孔板に対する荷重制御型高温低サイクル疲労・クリープ疲労寿命予測(材料強度と構造設(2),OS.5 材料強度と構造設計)

The purpose of this study is to obtain the fundamental feature of perforated plate under the load-controlled low-cycle fatigue and creep-fatigue condition. The hysteresis-loop and the crack propagation were measured during the test to examine the relation between crack propagation and the history of strain. And the Stress Redistribution Locus (SRL) method is applied for estimation of stress concentration in order to predict fatigue and creep-fatigue life for the specimens. It was found that the strain varies with increasing number of cycle. In the latter of cycle the strain increased sharply. It was shown that the life evaluation can be more accurate using the strain obtained by applying SRL method. Additionally, it was shown that the life evaluation using crack initiation cycle is more accurate than that using failure cycle.

OS0735 応力再配分軌跡法による熱過渡容器ノズルの非弾性変形挙動の推定(構造用材料の疲労挙動と寿命評価,オーガナイズドセッション)

OS0735 応力再配分軌跡法による熱過渡容器ノズルの非弾性変形挙動の推定(構造用材料の疲労挙動と寿命評価,オーガナイズドセッション)

2227 応力再配分モデルに基づく有孔板の高温疲労強度評価(S03-2 材料の塑性挙動の構成式のモデル化とシミュレーション(2),S03 材料の塑性挙動の構成式のモデル化とシミュレーション)

This paper is to evaluate the fatigue strength for the perforated plate at elevated temperature based on Stress Redistribution Locus (SRL) method. The test specimens are made of SUS304 stainless steel, and temperature is kept to be 550℃, and the geometry of the perforated plate specimens are changed by number of holes and diameter size of the hole. The SRL method is used to predict number of cycle of crack initiation (N_c), and the failure (N_f) for the perforated plate.

1922 有孔板のクリープ疲労強度のSRL法による予測(S19-2 構造材のクリープ・疲労・損傷解析,S19 材料の非弾性変形とそのモデル化)

This study investigates the effects of prediction method of Stress Redistribution Locus (SRL) method in creep relaxation process by comparing to the previous method using elastic follow up approach. The SRL method can predict the experimental results better than the previous method for the perforated platemodel.