Takamoto Itoh

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.

A Simple Model for Stable Cyclic Stress-Strain Relationship of Type 304 Stainless Steel Under Nonproportional Loading

This paper proposes a simple two-surface model for cyclic incremental plasticity based on combined Mroz and Ziegler kinematic hardening rules under nonproportional loading. The model has only seven material constants and a nonproportional factor which describes the degree of additional hardening. Cyclic loading experiments with fourteen strain paths were conducted using Type 304 stainless steel. The simulation has shown that the model was precise enough to calculate the stable cyclic stress-strain relationship under nonproportional loadings.

A damage model for estimating low cycle fatigue lives under nonproportional multiaxial loading

Abstract This paper develops a damage model for the evaluation of low cycle fatigue lives under complex cyclic multiaxial loadings. One of the authors has proposed the equivalent strain parameter for the life prediction of the nonproportional low cycle fatigue. This strain parameter can evaluate the dependence of fatigue lives on strain history and material, and correlates the fatigue lives within a small scatter band for 15 kinds of proportional and nonproportional strain paths. However, the parameter was only applicable to the life prediction under a limited nonproportional strain history, so that some modifications were required. In this study, a simple damage model for the life prediction is proposed by combining the equivalent strain parameter with Miners law in order to apply it to the life prediction under more complex nonproportional loadings. The applicability of the proposed model is examined for the life evaluation of nonproportional low cycle fatigue for different materials; type 304 stainless steels, copper, aluminum alloys, chromium-molybdenum and carbon steels, which were obtained from different research institutes. The model can correlate most of the fatigue data within a factor of two scatter band and has a potential to become a good damage model for nonproportional low cycle fatigue.

Study on Fatigue Life Evaluation Using Small Specimens for Testing Neutron-Irradiated Materials

The effect of specimen size and shape on the fatigue life of reduced activation ferritic/martensitic steel “F82H IEA-heat” was investigated to develop a fatigue life evaluation method using a small specimen for testing neutron-irradiated materials. The fatigue test was carried out at room temperature in air using three kinds of round-bar specimen (minimum diameter cross sections of 1, 4, and 7 mm) and one hourglass specimen (minimum diameter cross section of 1.25 mm). The effect of specimen size on fatigue life from a level of 103 to 104 cycles was almost negligible for the round-bar specimens. The shorter fatigue life at the total strain range below 0.6% and the slightly longer fatigue life at the total strain range above 0.8% of the hourglass specimen relative to that of the standard specimen were observed from a level of 102 to 104 cycles.

J-integral estimate for biaxially stressed Mode I cracks based on COD strain

Abstract This paper proposes a simple equation for evaluating J-integral values for biaxially stressed Mode I cracks. Finite element analyses were carried out to obtain the relationship between J-integral and crack opening displacement by varying the inelastic constitutive relationship, crack length and stress/strain biaxiality. A quantitative relationship was derived between the J-integral and crack opening displacement for various kinds of stress–strain relationships. A new equation for estimating the J-integral was proposed based on the crack opening displacement. The equation evaluated J-integral values for biaxially stressed Mode I cracks within a 25% error if the yield stress, strain hardening coefficient and exponent were known.

Evaluation and visualization of multiaxial stress and strain states under non-proportional loading

This paper presents a simple method of determining stresses and strains and a severity of loading history under non-proportional loadings with defining the rotation angles of the maximum principal stress and strain in a three dimensional stress and strain space. Based on the method, non-proportional stress and strain ranges are derived and applicability of the range to the life evaluation for low carbon steel under non-proportional random strain paths are discussed. The strain range taking account of intensity of loading path reducing life can be suitable parameter for multiaxial fatigue life evaluation under multiaxial non-proportional loading. This paper also shows a method of visually presenting the principal stress and strain that assists designers to understand the loading mode whether proportional or non-proportional under 3 dimensional (6 stress/strain components) multiaxial loading.

Creep-Fatigue Life Assessment Under Multiaxial Strain State

This paper studies the multiaxial creep-fatigue life for type 304 stainless steel at elevated temperature. Strain controlled biaxial tension-compression creep-fatigue tests were carried out using cruciform specimens under four strain waves at three principal strain ratios. The strain wave and the principal strain ratio had a significant effect on creep-fatigue life of the cruciform specimen. The creep-fatigue life ratio decreased as the principal strain ratio increased which indicates that larger creep damage occurred at larger principal strain ratio. The effects of the strain wave and principal strain ratio were discussed in relation to the observations of surface crack and void area density in the gage part of the specimen. Creep-fatigue lives were discussed in relation to the principal stress amplitude calculated by finite element analysis and creep-fatigue damage was evaluated by linear damage rule.Copyright

A Simple Inelastic Constitutive Model for Evaluation of Stable Cyclic Stress Response under Nonproportional Straining

This paper proposes a simple two-surface model for cyclic incremental plasticity based on combined Mroz and Ziegler kinematic hardening rules under nonproportional loading. The model has only seven material constants and a nonproportional factor which describes the degree of additional hardening. Cyclic loading experiments with fourteen strain paths were conducted using Type 304 stainless steel. The simulation has shown that the model was precise enough to calculate the stable cyclic stress-strain relationship under nonproportional loadings.Copyright

Creep-fatigue life evaluation of high chromium ferritic steel under non-proportional loading

T Multiaxial creep-fatigue tests under non-proportional loading conditions with various strain rates were carried out using a hollow cylinder specimen of a high chromium ferritic steel at 823K in air to discuss the influence of non-proportional loading on failure life. Strain paths employed were a push-pull loading and a circle loading. The push-pull loading test is proportional strain path test. The circle loading test is non-proportional strain path test in which sinusoidal waveforms of axial and shear strains have 90 degree phase difference. The failure life is affected largely by the strain rate and the non-proportional loading. This paper presents a modified strain range for life evaluation considering the strain rate based on a non-proportional strain parameter proposed by authors. The strain range is a suitable parameter for life evaluation of tested material under non-proportional loading at high temperature. KEYWORDS. Life evaluation; Creep-fatigue; Multiaxial strain; Nonproportional loading; High chromium ferritic steel.