Akito Nitta

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.

Propagation behavior of small cracks in 304 stainless steel under biaxial low-cycle fatigue at elevated temperature

To investigate the propagation behavior of small cracks and the relationship between crack propagation and fatigue life, strain- controlled in-phase and 90{degrees}-out-of-phase axial-torsional fatigue tests were conducted at 550{degrees}C using tubular specimens of 304 stainless steel with and without surface notches. During the tests, observations of crack initiation and propagation were frequently made using replica films and an optical microscope. Most of the fatigue life was spent in the propagation of small cracks less than 2 mm long. Regardless of loading conditions, the crack propagation rate correlated well with the equivalent shear strain range defined as a function of maximum shear strain range and normal strain range on the plane of the maximum shear strain range. The biaxial low-cycle fatigue life could be estimated from the relationship between the crack propagation rate and the equivalent shear strain range. 10 refs., 10 figs.

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.

Application of Hot Hardness Test to the Evaluation of Creep Damage in Cr-Mo-V Turbine Rotor of Fossil Power Plant

In residual life evaluation of fossil power plants, improvement of accuracy of creep damage evaluation is extremely important. One of the powerful non-destructive evaluation methods is hardness test. This method is effective because it enables us the on-site evaluation and the results are obtained without laborious work. However, in order to make it more powerful method, improvement of accuracy is inevitable. In this paper, the Vickers hardness test not at ambient but at high temperature is applied to the deteriorated rotor material and the method for the residual life evaluation with high accuracy is newly developed. In the method, creep constitutive law is determined from the results of hot hardness test first. Next, the variation of constitutive law with the creep damage is investigated. Finally, the dependency of exponent of constitutive law on the creep damage is shown and the method to evaluate the creep damage from this dependency will be proposed.© 2004 ASME

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.