Toshimitsu Yokobori

Results of an intercomparison of creep crack growth tests made in Japan

This paper concerns the results and analysis of Japanese round-robin tests on the VAMAS TWA 11 Creep Crack Growth Project. A high-temperature ductile material, CrMoV steel, was used. For high strain rates, creep crack growth rate is correlated by the parameter C* within less than one order of magnitude of scatter in the temperature range between 538 and 594°C and in the load range tested. However, at lower strain rates the scatter becomes larger (i.e. greater than one order of magnitude). In the early stages of crack growth in particular, it was found that a dual value of da/df appears with respect to the same value of C*. This peculiarity has been explained by the relation between the characteristics of creep crack length and creep strain with respect to time. By comparison, if the parameter Q* is used such a peculiar pattern does not appear at an early stage.

The proposal of Q* parameter and derivation of the law of creep crack growth life for a round bar specimen with a circular notch for Cr–Mo–V steel

Abstract Using a round bar circular notched specimen for Cr–Mo–V steel and the proposed equation predicting crack length on the basis of electric potential drop method, the creep crack growth tests were previously conducted to investigate the effect of multi-axial stress on creep crack growth rate(CCGR). In this paper, more detailed experiments and analyses on the creep crack growth were conducted and the Q* parameter which characterizes CCGR for this circular notched specimen was derived. Furthermore, using the Q* parameter, the prediction law of creep crack growth life was also derived.

Evaluation of Creep Crack Growth Rate of P92 Welds Using Fracture Mechanics Parameters

High Cr ferritic heat resisting steels have been widely used for boiler components in ultrasupercritical thermal power plants operated at about 600°C. In the welded joint of these steels, type-N crack initates in the fine-grained heat affected zone during long-term use at high temperatures and their creep strength decreases. In this paper, creep properties and creep crack growth (CCG) properties of P92 welds are presented. The CCG tests are carried out using cross-welded compact tension C(T) specimens at several temperatures. The crack front was located within the fine-grained HAZ region to simulate type-IV cracking. Finite element analysis was conducted to simulate multiaxiality in welded joints and to compare experimental results. The constitutive behavior for these materials is described by a power-law creep model. C* and Q* parameters are used to evaluate CCG rate of P92 welds for comparison. C* parameters can characterize approximately 20% of the total life of CCG in P92 welds, and Q* parameters can characterize approximately 80% of the total life. Q * parameter is one of the useful parameters to predict CCG life in P92 welds,

Creep Crack Growth of P92 Welds

This paper represents creep properties and creep crack growth properties for P92 welds. The CCG tests were carried out using cross-welded compact tension (C (T) specimens at several temperatures. The crack front was located at HAZ region to simulate Type IV crack. Finite element analysis was conducted to simulate multiaxiality in welded joints and compare the experimental results. The constitutive behaviour for these materials is described by a power law creep model.Copyright

Stress Rate and Grain Size Dependence of Dynamic Stress Intensity Factor by Dynamical Piling-up of Dislocations Emitted

The study concerns computer simulation of dynamic piling-up behaviour of dislocations emitted from the source against the grain boundary. As the number of dislocations emitted increases, the stress singularity near the pile-up becomes to reveal \(1/\sqrt {\rm{r}}\) characteristics, where r=the distance from the pile-up outward. The dynamic stress intensity facter by the dislocation piling-up was obtained in analytical equation in terms of stress application rate, grain size and materials constants. The comparison of the results was made with the static (equilibrium) result. Finally, the result was applied to the dynamic yielding strength of steel, and it is in good agreement with the experimental data in literatures.

The Analysis of Hydrogen Diffusion Behaviors Coupled With the Analysis of Heat Transfer Around the Heat Affected Zone of Welding

Hydrogen penetrates into the metal and causes Hydrogen embrittlement due to the increase in hydrogen concentration. This is caused by the local stress fields such as residual stress field at the site of welding or local stress field around a crack tip. It accompanied with incubation time of several hours since the components were exposed to hydrogen atmospheric condition. This incubation time is time lag of hydrogen diffusion and concentration at the site where the hydrogen embrittlement occurs. Therefore, clarification of the hydrogen diffusion behavior is important to prevent from fracture of hydrogen embrittlement.In this paper, the numerical analyses of hydrogen diffusion around weld part including HAZ (Heat Affected Zone) under residual stress coupled with that of heat transfer during the cooling process before and after weld were conducted and the behaviors of hydrogen concentration were analyzed. On the basis of these analyses, the method of heat treatment to prevent from hydrogen concentration at the weld part was investigated. Results obtained by these analyses showed that pre weld heat treatment is effective in the prevention of hydrogen concentration and combined pre weld heat treatment with post weld heat treatment was found to be the most effective treatment.Copyright

1307 THE EFFECT OF HYDROGEN DIFFUSION BEHAVIOR ON THE EMBRITTLEMENT OF STEEL UNDER HYDROGEN ATMOSPHERIC CONDITION

Much hydrogen penetrates from a container wall surface when a petroleum-refining reactor operates under high temperature and pressurized hydrogen environmental condition. Major part of the hydrogen remains in the wall for a long time during cooling process when the reactor is out of operation, which results in the inducement of peculiar delayed fracture. In this paper, the proposed analytical method on hydrogen diffusion and concentration around a crack tip was applied to this phenomenon and the mechanism of this behavior was clarified.

The Effect of Damage on Fatigue Crack Growth Rate under High-Temperature Creep-Fatigue Multiplication.

The characterization for the crack growth rate and the progressive damage region around the crack tip of SUS 304 stainless steel were investigated with various holding times at the maximum stress in a trapozoidal wave in a vacuum at 680°C. From these experimental results, the crack growth rate approaches the creep crack growth rate, and the geometry of the progressive damage region around the crack tip becomes similar with the increase of stress holding time. The fracture surface shows intergranular features dominated by creep in the case of with stress holding time, but does not show without stress holding time. This is distinguished from a triangular wave with no stress holding time, where the fracture implicitly involves fatigue and creep effects.

Relative notch opening displacement concept for crack initiation in high temperature time dependent fracture of stainless steel.

ステンレス鋼切欠きからのき裂発生時の相対切欠開口変位(RNOD)は,高温クリープ,疲労およびクリープ疲労の重量のすべてに共通して,切欠形状,試片幅,空気中ないし真空中によらず,公称応力および温度にのみ若干依存し,ほとんど一定の値を取る.定常クリープを示す応力・温度のもとでは,RNODの値はほぼ0.25～0.38の範囲をとる.切欠形状や試片幅によって,き裂の発生や進展は,安定,不安定挙動といった異なる挙動を示すにもかかわらず,RNODのほうはこれに無関係に唯一性を示す.

Stress analysis of core support post in a very high-temperature reactor

Abstract A very high-temperature gas-cooled reactor (VHTR) has been developed by Japan Atomic Energy Research Institute and its core consists of so-called fuel blocks, removable reflector blocks and so on. These graphite blocks are supported by thin cylindrical graphite bars called support posts. These posts are in contact with the blocks through hemispherical seats to absorb the relative displacement of blocks by small inclination or rotation of the posts. Stress distributions and stress concentration coefficients of the support post under the reactor core weight are analysed by means of photoelastic experiment and compared with two-dimensional calculation by using the finite element method. The following are the conclusions we have derived: 1. (1) Stress concentration coefficients of the post are expressed uniquely as a function of Ed( 1 r p − 1 r p ) σ g regardless of the shapes of model and material properties. 2. (2) Inclination of the post caused by small rotation has little effect on the stress concentration coefficients.