Masateru Ohnami

Nonproportional Low Cycle Fatigue Criterion for Type 304 Stainless Steel

This paper describes a multiaxial low cycle fatigue parameter for correlating lives under nonproportional loadings. Constant amplitude low cycle fatigue tests were carried out under 14 proportional and complex nonproportional cyclic strain paths using type 304 stainless steel hollow cylinder specimens at room temperature. In nonproportional loading tests, fatigue lives are decreased by as much as a factor of 10 in comparison with those in proportional loading tests with the same strain range. Reduction in fatigue life due to nonproportional loading is closely related to additional nonproportional cyclic hardening. The product of the maximum principal stress and strain ranges correlated the nonproportional fatigue data. A nonproportional cyclic hardening parameter computed from the strain path is also proposed that allows life estimates to be obtained directly from the strain history without the need for a cyclic plasticity model.

High Temperature Multiaxial Low Cycle Fatigue of Cruciform Specimen

This paper describes high temperature multiaxial low cycle fatigue lives of type SUS304 stainless steel and 1Cr-1Mo-1/4V steel cruciform specimens at 923 K and 823 K in air. Strain controlled multiaxial low cycle fatigue tests were carried out using cruciform specimens at the principal strain ratios between [minus]1 and 1. The principal strain ratio had a significant effect on low cycle fatigue lives. Fatigue lives drastically decreased as the principal strain ratio increased. Multiaxial low cycle fatigue strain parameters were applied to the experimental data and the applicability of the parameter was discussed. The equivalent strain based on crack opening displacement (COD strain) developed in the paper and [Gamma][sup *] -- plane parameter successfully predicted multiaxial low cycle fatigue lives. The crack morphology was also extensively discussed from not only the surface crack direction but also the crack inclination into the specimen.

Fracture Plane of Cruciform Specimen in Biaxial Low Cycle Fatigue—Estimate by Variance Method and Experimental Verification

This paper presents the variance method of determining the fracture plane under random multiaxial stress states. The fracture plane was estimated analytically by the variance method with the three fatigue criteria. The estimated fracture planes were compared with experimental result using type SUS 304 and 1Cr-1Mo-1/4V steel cruciform specimens. The variance method with the maximum normal strain criterion, by neglecting the strain in direction in which no external forces act, could estimate the actual fracture planes of cruciform specimens in high temperature biaxial low cycle fatigue

High Temperature Multiaxial Low Cycle Fatigue of CMSX-2 NI-Base Single Crystal Superalloy

This paper describes studies of the tension-torsion multiaxial low cycle fatigue of CMSX-2 Ni-base single crystal superalloy. Tension-torsion low cycle fatigue tests were carried out at 1,173 K using CMSX-2 hollow cylinder specimens aligned with the {l_brace}001{r_brace} axis. Several multiaxial strain and stress parameters were applied to the experimental data to examine the suitability of the parameters to life prediction. All the strain parameters proposed so far gave a large scatter of the data correlation. Discrepant data correlation with the strain parameters resulted from the anisotropic stress response due to the crystallographic texture. Larger Mises equivalent stress was applied in torsion tests than in tension tests at the same Mises strain. However, Mises stress and the equivalent stress based on crack opening displacement gave a satisfactory data correlation. This study developed a new equivalent strain, taking account of the anisotropy of the elastic constants, which correlates the multiaxial low cycle fatigue data with a small scatter.

High temperature biaxial low cycle fatigue using cruciform specimen.

This paper describes the low cycle fatigue behavior of 1Cr-1Mo-1/4V and SUS304 cruciform specimens in wide range biaxial stress conditions of -1.0≤φ≤1.0 at high temperatures, where φ denotes the ratio of the minimum principal strain to the maximum principal strain existing on the specimen surface. The detailed description of the newly developed biaxial fatigue testing machine was made, and the biaxial test results were discussed in connection with the biaxial low cycle fatigue parameter. The COD and ther Γ*-plane parameters were most appropriate to express the biaxial fatigue behavior. Also the X-ray diffraction was made in order to examine the damage in biaxial stress states.

Cyclic behavior of a type 304 stainless steel in biaxial stress states at elevated temperatures

Abstract The results of companion, incremental/decremental, and stepup fatigue experiments on austenitic stainless steel tube (type 304) are presented. The experiments include proportional and nonproportional loading conditions at ambient as well as elevated temperatures. Empirical relations are developed between von Mises effective stress and strain, and these relations are shown to describe the cyclic behavior during proportional companion as well as incremental/decremental tests. In case of nonproportional incremental/decremental experiments, the material behavior is not accurately modeled either by using the von Mises effective stress and strain, or by relating Trescas maximum shear stress and strain.

Tension-torsion multiaxial low cycle fatigue of Mar-M247LC directionally solidified superalloy at elevated temperature

This paper studies the high temperature multiaxial low cycle fatigue of Mar-M247LC directionally solidified superalloy. Tension-torsion multiaxial low cycle fatigue tests were carried out using Mar-M247LC tubular specimens at 1173K in air. Several multiaxial strain and stress parameters were applied to the experimental data to examine the suitability of the parameters. All the multiaxial strain parameters proposed so far could not successfully correlate the maltiaxial low cycle fatigue data of the directionality solidified superalloy, but the maximum principal stress and the equivalent stress based on crack opening displacement could correlate the data within a small scatter. This paper proposes a new multiaxial strain parameter which takes account of the anisotropy of elastic constant of directionally solidified superalloys. The proposed strain parameter correlates the multiaxial low cycle fatigue data within a factor of two scatter band. This paper also describes the crack mode and cyclic constitutive relation of the superalloy in connection with the anisotropy of the elastic constant.

Notch Effect on Low Cycle Fatigue in Creep-Fatigue at High Temperatures: Experiment and Finite Element Method Analysis

This paper describes the notch effect on low cycle fatigue in creep-fatigue conditions by experiments and finite element method (FEM) analysis. FEM analyses for cyclic and monotonic loadings were made, and the analytical results were compared with the measured strain at the notch root using a high temperature strain gage. The calculated strain by FEM agreed well with the measured strain if we used the proper FEM model and proper constitutive equation. In order to examine the accuracy of life prediction by FEM analysis, low cycle fatigue tests were also carried out using SUS 304 stainless steel for notched plate and notched round bar specimens. Life prediction by FEM analysis had sufficient accuracy; the applicability of other conventional life prediction methods was also discussed.

Crack propagation rate and failure life in biaxial low cycle fatigue at elevated temperatures

Abstract This paper describes the crack propagation rate and the failure life in the biaxial low cycle fatigue test for a type 304 stainless steel at elevated temperatures. Macro crack propagation rates were observed for the tubular specimen in the push-pull and the reversed torsion tests. The crack propagation rate in both the tests was discussed in connection with the crack opening displacement in experiments and FEM analyses. The new equivalent stress derived from the FEM analysis was proposed so as to correlate the biaxial crack propagation rate. The equivalent stress well correlated not only the crack propagation rate but also the biaxial low cycle fatigue failure data.

High Temperature Low-Cycle Fatigue of Friction Welded Joints-Type 304-304 Stainless Steel and Alloy 718-718 Nickel Base Superalloy

This paper assesses the high-temperature low-cycle fatigue of the Type 304 stainless steel and Alloy 718 superalloy friction-welded joints. Strain controlled low-cycle fatigue tests for 304-304 and 718-718 friction-welded specimens were carried out at 923 K in air to obtain the fatigue strength of the joints. These materials were selected as the cyclic hardening and softening materials, respectively. The 304-304 welded specimens showed inferior fatigue strength in comparison with the base metal while the 718-718 specimens exhibited fatigue strength equivalent to that of the base metal. The difference in the fatigue strength between the two materials is discussed from the viewpoint of the cyclic deformation behavior and strain reduction at weld interface.