Yoshihiro Fukushima

Correlations among growth law of small cracks, low-cycle fatigue law and applicability of miner's rule

Abstract The initiation and propagation process of a small crack in low-cycle fatigue of a medium carbon steel is observed in detail by the plastic replica method. It is found that the Manson-Coffin relation is virtually the same as the crack growth law of a small crack; more than 90% (almost 100%) of the fatigue life of plain specimens is occupied by the crack propagation process. From this viewpoint, the ambiguity in the definition of the initiation of “an engineering-size crack” can be removed. The effects of prior fatigue history on the growth rate of a small crack are investigated systematically using special specimens containing an artificial small hole with diameters of 40, 100 and 200 μ m. Prior fatigue history which may be accumulated at the region where a crack will later propagate hardly influences the crack growth rate, i.e. prior fatigue history itself should not be regarded as fatigue damage. This is an essential condition for Miners rule to apply.

Experimental investigation on statistics of extremes for three-dimensional distribution of non-metallic inclusions

Abstract A wide range of studies have shown that the lower bound of fatigue properties of high strength steels is determined by the maximum size of non-metallic inclusions that are present in a component. The maximum size of inclusions in a given component or material volume can be reasonably estimated using the statistics of extremes. However, as long as the estimation is based on microscope inspections of two-dimensional (2D) surfaces, there will be errors and uncertainties in estimating the maximum particle in a three-dimensional (3D) volume. In addition it has been recently found that in some steels the distribution of extreme defects is composed of a mixture of different particle types. The scope of this paper is to clarify the validity of 2D inspections on the basis of 3D distribution of inclusions in a modern super clean steel. The 3D distribution was obtained with a combination of inclusions detected with a repeated slicing procedure and of particles at fatigue fracture origin. The 3D distribution of inclusions is composed of a mixture of two types of particles having similar chemical compositions and different 3D morphological structures: one with a large population and another with few rare particles. The 3D large population can be accurately estimated from maximum inclusions on small polished sections, while in order to estimate the characteristic size of inclusions at fatigue fracture origin by 2D inspections it is necessary to adopt a minimum inspection area S crit. In the case of the material examined in this study (SCM435 steel) this minimum inspection area is ~ 10 000 mm2.

Highly sensitive detection of net hydrogen charged into austenitic stainless steel with secondary ion mass spectrometry.

Secondary ion mass spectrometry (SIMS) is used to detect local distributions of hydrogen in various materials. However, it has been well-known that it is extremely difficult to analyze net hydrogen (H(N)) in metals with SIMS. This was because hydrogen, which is originated from moisture (H(2)O), hydrocarbon (C(x)H(y)) or other organic materials (C(x)H(y)O(z)) existing on a sample surface or in the SIMS chamber, is simultaneously detected in the SIMS measurement of the H(N), and the H(N) and the background-originated hydrogen (H(BG)) cannot be distinguished in a SIMS profile. The effective method for reductions and determinations of the H(BG) in hydrogen measurements of metallic materials with the SIMS method has not been established. The present paper shows an effective method for reduction and estimation of H(BG) in SIMS analyses of hydrogen charged into type 316 L austenitic stainless steel, and an accurate estimation method of the net charged hydrogen. In this research, a silicon wafer is sputtered by a primary ion beam of a SIMS near an analyzed area (silicon sputtering method) to reduce H(BG). An uncharged type 316 L sample was prepared for estimation of H(BG) in SIMS measurements of the hydrogen-charged sample. The gross intensities of hydrogen between the hydrogen-charged sample and the uncharged sample were compared. The gross intensities of hydrogen of the uncharged sample (26.8-74.5 cps) were much lower than the minimal gross intensities of hydrogen of the hydrogen-charged sample (462-1140 cps). Thus, we could reduce the H(BG) enough to estimate the hydrogen charged into the type 316 L sample. Moreover, we developed a method to determine intensities of H(BG) in the measurement of the hydrogen-charged sample by estimating the time-variation of hydrogen intensities in the measurements of the uncharged sample. The intensities of the charged hydrogen can be obtained by subtracting the estimated intensities of the H(BG) from the gross intensities of hydrogen of the hydrogen-charged sample. The silicon sputtering method used to reduce H(BG) and the determination method for H(BG) in this research can be applied to the accurate hydrogen analysis for other various metallic materials.

Reconsideration of macroscopic low cycle fatigue laws through observation of microscopic fatigue process on a medium carbon steel

In order to correlate macroscopic low-cycle fatigue laws such as the Mansun-Coffin relation and Miners rule with the actual microscopic fatigue process, cyclic strain-controlled low-cycle fatigue tests were conducted on a medium carbon steel under constant-, two-step-, and random-strain amplitude conditions. Through continual observation of the microscopic fatigue process of plain specimens and holed specimens with a very small hole (40 μm diameter), it was found that the total fatigue life is mostly occupied by the microcrack propagation life. Both the Manson-Coffin relation and Miners rule are derived using the microcrack propagation law. Then, background for the applicability of Miners rule is discussed. The results of the random loading fatigue tests showed that microcrack propagation life can be predicted on the basis of the microcrack growth law in conjunction with the rain flow method for counting fatigue damage. Finally, the microcrack growth law-aided approach was applied to interpret anisotropy in fatigue strength of a thick forged steel plate.

Low-cycle fatigue of SUS 304 stainless steel under two-step loading (Effect of prefatigue damage on microcrack growth rate)

Two-step loading LCF tests were conducted on a cyclic hardening SUS 304 stainless steel to examine the effect of prefatigue damage on the subsequent crack growth rate after switching the strain level. The effect of prefatigue damage yielded overshooting and undershooting of stress amplitude in comparison with single strain cycling and subsequently caused acceleration and retardation of the microcrack growth rate. Those trends were reversed depending on the combination of the level of the first and the second strain amplitude. Martensitic transformation measured by the X-ray diffraction method revealed that increasing strain amplitude promotes the transformation and results in acceleration of microcrack growth.

Low-Cycle Fatigue of Cyclic Hardening and Softening Materials

The microcrack growth law-aided approach to LCF life evaluation, previously developed by the present authors, was applied to cyclic hardening and softening materials. A stainless steel (SUS304) and two kinds of high strength steels (WT80, SCM435H) were low-cycle-fatigued at ambient temperature under stress amplitude- and strain amplitude-controlled cycling. The LCF tests were conducted under either constant or two-step strain amplitude. Each material showed the same trend that the LCF life was covered by the crack propagation life and that cyclic hardening or softening occurred at an early stage of the fatigue life and it saturated during subsequent cycling. In the case of WT80 and SCM435H, a linear type of the microcrack growth law and its application to the derivation of the Miner rule, having been derived on a cyclically stable material of medium carbon steel were found to be applicable. On the contrary, in the case of SUS304, pre-fatigue damage and structure change during cycling affected the microcrack growth, which resulted in the break of linear type of the microcrack growth law.

Microcrack Growth Law-Aided Approach in Evaluating Low-Cycle Fatigue Strength and Its Application — Directionality Assessment of Thick Forged Steel Plates(A533b Cl 1) and Prediction of the Microcrack Propagation Life of a Medium Carbon Steel Under Variable Strain Amplitudes

The microcrack growth law-aided approach previously developed by the present authors is first applied to examine the directionality or anisotropy in fatigue strength of thick forged steel plates with a different forging ratio. Fatigue tests are conducted on specimens sliced in the transverse (T) and thickness (S) direction of the plates. Through continual observation of the microcrack initiation and propagation processes, the elongated manganese-sulphide (MnS) inclusions are found to hardly affect the fatigue processes in T direction, while they strongly influence upon them in S direction.