Daigo Watanabe

Application of Gurson Model to Ni-Based Alloy Weld Joint Pipe With an Axial or a Circumferential Surface Flaw: Phase II

The previous paper ASME PVP2012[1] reported application of Gurson model to the fracture test results using pipe models with part-through wall cracks on the dissimilar metal (DM) welds. The predicted maximum loads and the crack behaviors after penetration did not agree well. These results may originate from improper parameter values of Gurson model. This paper revised these parameters and improved the estimated fracture behaviors of the pipe models. A suitable fitting process of Gurson parameters was also proposed.Copyright

Study on Minimum Wall Thickness Requirement for Seismic Buckling of Reactor Vessel Based on System Based Code Concept

The minimum wall thickness required to prevent seismic buckling of a reactor vessel (RV) in a fast reactor is derived using the system based code (SBC) concept. One of the key features of SBC concept is margin optimization; to implement this concept, the reliability design method is employed, and the target reliability for seismic buckling of the RV is derived from nuclear plant safety goals. Input data for reliability evaluation, such as distribution type, mean value, and standard deviation of random variables, are also prepared. Seismic hazard is considered to evaluate uncertainty of seismic load. Minimum wall thickness required to achieve the target reliability is evaluated, and is found to be less than that determined from a conventional deterministic design method. Furthermore, the influence of each random variable on the evaluation is investigated, and it is found that the seismic load has a significant impact.

Application of Gurson Model to Different Constraint Specimens

This paper introduces the application of Gurson model to simulate the ultimate ductile failure of three types of specimens with different constraints. Fracture tests were conducted using three kinds of notched round bar tensile specimens with different notch radii, a flat plate tensile specimen with a centered semielliptical surface flaw and a 1/2TCT specimen. Using the test results of the notched round bar tensile specimens, the Gurson model parameters were determined from the literatures, fracture observation and experimental design calculations. After fixing the Gurson parameters, they were applied to the flat plate tensile specimen and the 1/2TCT specimen model. As a. result Gurson model could simulate the fracture behavior of the flat plate with good accuracy, on the other hand there was a large difference with the test result of the CT specimen. In order to determine the Gurson model parameters universally and quickly, the blind optimization calculations were performed without arbitrariness. Three of five parameters were in the same order of the parameters by the 1st consideration and the others were in the different order. However the blind optimization parameters could show the similar simulation results of the fracture behaviors of the flat plate and 1TCT specimen as the 1st consideration parameters. The further investigation suggested the reason of the different behavior of the CT specimen model may be the different stress distribution, bending component dominant in the cracked ligament of the CT specimen from those of the other models. Additionally the accuracy of the local strain criteria according to ASME Sec. VIII Division 2 was confirmed.© 2014 ASME

Development of Limit State Design for Fast Reactor by System Based Code

Limit state design was newly developed in order to apply the margin exchange which is one of the innovative concepts of the System Based Code (SBC). It was shown that limit state design method is applicable to plant design instead of current design criteria. In this report, working example of a reactor vessel of a Fast Reactor subject to thermal load is conducted to demonstrate this concept. As the result allowable stress was increased by changing the acceptance criteria from current design criteria to limit state design criteria.Copyright

Elaboration of the System Based Code Concept — Activities in JSME and ASME: (2) Development of Evaluation Tools Based on LRFD

In the System Based Code (SBC), a reliability target is defined according to the importance of components (risk and/or failure probability), and the grade of material, design, manufacture and maintenance are chosen to satisfy the reliability target. Therefore, a reliability assessment of the components plays an important role in the concept of SBC. In the reliability assessment guidelines of SBC under development in JSME, the Load and Resistance Factor Design (LRFD) method and the Monte Carlo method will be applied to reliability assessment. Along with a development plan of the SBC, examination of the target reliability according to the importance of components and preparation of the LRFD methods and assessment of partial safety factor for some failure modes are underway. Until now, the LRFD methods have been developed for burst due to internal pressure, plastic collapse due to membrane and bending stress, fatigue, plastic collapse of a pipe having a circumferential crack, and buckling of a thin wall cylinder. This paper describes the action plans for the development of the reliability assessment methods and examination results till date.Copyright

Ductile Crack Propagation Simulation of a Cylinder With a Through-Wall Circumferential Flaw Under Excessive Cyclic Torsion Loading

A lot of applications of elastic plastic FE analysis to flawed structural fracture behaviors of mode I have been investigated. On the other hand the analysis method has not been established for the case of the excessive cyclic torsion loading with mode II or III fracture. The authors tried simulating the fracture behavior of a cylinder-shaped specimen with a through-walled circumferential flaw subjected to excessive monotonic or cyclic loading by using elastic plastic FE analysis. Chaboche constitutive equation of the used FE code Abaqus was applied to estimate the elastic plastic cyclic behavior. As a result in the case of monotonic loading without crack extension, the relation of torque-rotation angle of the experiment was estimated well by the simulation. Also J-integral by the Abaqus’ function agreed with a simplified J-equation using the calculated torque-rotation angle relation. On the other hand under load controlled cyclic loading associated with ductile crack growth, the calculated torque-rotation angle relation did not agree with the experimental one because of high sensitivity of the used stress-strain curve. J-integral from Abaqus code did not increase regardless of the accumulated crack growth and plastic zone. Several simplified ΔJ calculations tried to explain the experimental ductile crack growth and it seemed that da/dN-ΔJ relation follows the Paris’ law. From these examinations an estimation procedure of the structures under excessive cyclic loading was proposed.Copyright

Structural Integrity for Nuclear Power Plant Against Excessive Load on Design Extension Condition

This paper introduces an example of structural integrity evaluation for Light Water Reactor (LWR) against excessive loads on the Design Extension Condition (DEC). In order to assess the design acceptance level of DEC, three acceptance criteria which are the stress basis limit of the current design code, the strain basis limit of the current design code and the strain basis limit by using Load and Resistance Factor Design (LRFD) method were applied. As a result the allowable stress was increased by changing the acceptance criteria from the stress basis limit to the strain basis limit. It is shown that the practical margin of the LWR’s components still keeps even on DEC by introducing an appropriate criterion for integrity assessment and safety factors.Copyright

Study on Minimum Wall Thickness Requirement of Reactor Vessel of Fast Reactor for Seismic Buckling by System Based Code

In this paper, minimum wall thickness requirement of reactor vessel of fast reactor for seismic buckling is discussed on the basis of the System Based Code (SBC) concept. One of key concepts of SBC is the margin optimization. To implement this concept, reliability design method is employed, and the target reliability for seismic buckling of reactor vessel is derived from nuclear plant safety goals. Input data for reliability evaluation such as distribution type, mean value and standard deviation of random variable are prepared. Seismic hazard is considered to evaluate uncertainty of seismic load. Wall thickness needed to achieve the target reliability is evaluated, and as a result, it is shown that the minimum wall thickness can be reduced from that required by a deterministic design method.Copyright

Measurement of Thermal Ratcheting Strain on the Structures by the Laser Speckle Method

Prevention of thermal ratcheting is an important problem for high temperature components of fast breeder reactors that are subjected to cyclic thermal loads. To clarify ratcheting behaviors, structural model tests were planned. Strain measurement is important for understanding the thermal ratcheting phenomenon The conventional measurement by strain gauge is difficult at high temperature. Then, Laser speckle strain measurement system using the dual-beam set-up was developed to apply to high temperature structural model tests. This system was applied to the thermal ratcheting tests, which demonstrated the actual operative conditions of reactor vessels. Through comparison with uniaxial test results obtained by extensometers, the laser speckle method was verified. Measured data of structural model tests were utilized to certify the guidelines of inelastic analysis for design, which provide prediction method of strain in components of fast reactor.Copyright