Kazuo Kuwabara

High-Temperature Low Cycle Fatigue Crack Propagation and Life Laws of Smooth Specimens Derived from the Crack Propagation Laws

The objectives of this work are (1) to make clear the behavior of high-temperature low-cycle fatigue (LCF) crack propagation, (2) to verify the applicability of the J-integral to the fracture mechanics equations of crack propagation rates, (3) to derive two types of LCF life laws of smooth specimens based on the two types of crack propagation equations (i.e., cycle-dependent and time-dependent), (4) to show a resemblance to or a difference from the Manson-Coffin equation and the strain-range partitioning equations, and (5) to characterize the cycle-dependent and time-dependent fatigue laws. The effectiveness of the proposed failure-life equations was examined using experimental data on isothermal and thermal fatigue of smooth specimens.

Relationship between fracture mode and fatigue life under biaxial loading at 550.DEG.C in SUS 304 stainless steel.

In order to investigate the properties of biaxial low-cycle fatigue in SUS304 stainless steel at elevated temperature, strain controlled, tension-compression-torsion fatigue tests were carried out under in-phase and out-of-phase conditions between axial and torsional strain cyclings. The fracture mode under the in-phase cycling was found to be classified into two types; i.e. Mode I and Mode II. It was also found that the fracture mode changed from Mode I to Mode II with an increase in the strain ratio, Δγ/Δe, and the transition occured at about 1.7 of Δγ/Δe. On the other hand, under the out-of-phase cycling, the mixed failure of Mode I and Mode II was found from SEM fractographic observation. The in-phase fatigue life with the Mode I type failure was correlated well with the tensile strain energy and that with Mode II type failure the torsional strain energy. By using these correlations, the out-of-phase fatigue life could be predicted by the linear damage rule.

Crack initiation life at notch root under the transition of creep condition

Abstract Acceleration of creep fracture is posible in a high-strength material such as a superalloy under the transition from the small scale creep (SSC) condition to the large scale creep (LSC) condition. In this study, an analytical method of predicting the creep crack initiation life for a notched body was presented. In order to assess the validity of this method, the crack initiation at a notch root was also experimentally observed on a high-strength Ni-base superalloy through load-controlled creep-fatigue tests at 923 K. As a result, this method was found to be sufficiently applicable to the crack initiation life prediction for a notched body under the transition from SSC to LSC.

Fatigue strength of dissimilar welded joints for the main vessel of an LMFBR

Abstract The employment of welded joints composed of dissimilar metals is one simple and inexpensive way to connect a main vessel made of austenitic stainless steel and a roof slab constructed of ferritic steel in the design of liquid metal fast reactors. Since dissimilar-metal welded joints have not been used for such large structures so far in Japan, the structural integrity of this type of joint should be carefully examined for such a design option to be selected. Here various kinds of tests were conducted for eleven types of welded joints of 50 mm thickness to obtain this fundamental strength characteristics. Type 304 stainless steel was used as one of the parent metals in all the joints. They differ from each other in regard to the type of ferritic steel, welding metal and welding procedure. Low-cycle fatigue tests were conducted for round-bar specimens made from these welded joints at room temperature. Fatigue crack-propagation tests were also conducted for some of the joints. Tests after manufacturing a large-scale shell model were also conducted. The results of these tests demonstrated that the present manufacturing technique can, produce welded joints of high quality and reliability. A trial calculation for actual design conditions showed the existence of large margins against fatigue failure or fatigue crack-propagation of a significant amount.

THE EFFECT OF STRAINING PHASE ON BIAXIAL FATIGUE LIFE AT 555° IN TYPE 304 STAINLESS STEEL

ABSTRACT To investigate the effects of axial-torsional straining on the elevated-temperature, biaxial low-cycle fatigue life of Type 304 stainless steel, a series of axial-torsional strain-controlled fatigue tests were conducted at 550°C. These were performed under in-phase and out-of-phase conditions between axial and torsional strain states. Based on the experimental results, a criterion for biaxial, low-cycle fatigue failure was discussed in this study. As a result, the fatigue life in the out-of-phase strain condition was found to be shorter than that in the in-phase strain condition at a given value of the von Mises equivalent strain range. From an observation of microcracks on the surfaces of the specimens and by fractography, this seemed to be due to a difference in the fracture mode related to a macroscopic strain condition between the in-phase and out-of-phase conditions. Accordingly, a unified correlation for both the in-phase and out-of-phase biaxial fatigue lives could be obtained by extending the equivalent shear strain theory which was proposed by Brown and Miller as a factor controlling fatigue-crack initiation and propagation under biaxial loading.