Yasuharu Chuman

Prediction of Type IV creep failure of a seam-welded mod. 9Cr-1Mo elbow based on microscopic damage simulation

Abstract Utilising the random-fracture-resistance model of grain boundaries, micro-macro combined creep damage simulation was applied to the prediction of the distribution of small defects in the FGHAZ (fine-grained heat-affected zone) of longitudinal welds in an actual-size elbow of modified 9Cr-1Mo (9Cr-1MoVNb) steel subject to internal pressure at 923 K. Based on the simulation results, a prediction scheme for the final rupture life of welds was considered using the damage mechanics concept together with effective stress. The applicability of nonlinear fracture mechanics was also discussed, assuming the initial crack length determined from the microscopic simulation results. The results thus obtained are summarized as follows: As the simulation results showed, the peaks of small defect density in the subsurface could be predicted, corresponding well with the observed results. Final failure life prediction based on the damage mechanics concept was found to be applicable, by considering both the final failure surface connecting the weakest grain boundaries and the effective stress against this surface. The fracture mechanics approach was also found applicable when assuming the initial crack length from the high peaks of the simulated small defects in the last stage of creep life.

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

Creep rupture behaviour of circumferentially welded mod. 9Cr–1Mo steel pipe subject to internal pressure and axial load

The multi-axial creep strength of circumferential welds in power piping, including failure mode and failure life against a wide range of stress ratios and stress levels, was newly examined in this paper. The creep rupture behaviour of modified 9Cr–1Mo steel (9Cr–1Mo–VNb steel; ASME P91) pipe with a circumferential weld subject to combined internal pressure and axial load was experimentally investigated at 650 °C. The test results, with several kinds of stress ratios of macroscopic axial and hoop stress can be summarised as follows. Along with the increased stress ratio (axial/hoop), both failure location and failure mode changed from a base metal failure caused by hoop stress to an FGHAZ (fine-grained heat affected zone) failure due to axial stress. The stress ratio where the failure mode changed was ‘0.8’. Strength reduction in FGHAZ failure by axial stress should be considered in the structural design of circumferential welds. A series of FEM creep analysis was carried out to discuss the relation of the failure mode to both the local stress distribution and the damage. The failure mode variation along with the increase in the stress ratio is also discussed based on simplified ductile creep failure analysis.

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

Probabilistic Approach for Creep-Fatigue Evaluation in Liquid Metal Cooled Fast Reactor

A procedure to calculate crack initiation probabilities by creep-fatigue damage is explained in this paper with a calculation example. Material properties of 316FR are determined as probabilistic distributions from test data. As the result yield stress, fatigue property, cyclic stress-strain relation and creep property are input into a creep-fatigue evaluation as the probabilistic distributions. The crack initiation probability is calculated with the condition for the Japanese commercialized sodium cooled fast breeder reactor. As the result, the allowable thermal cycles in the present design becomes 5 × 10−9 cumulative crack initiation probability.Copyright