Nickel chromium molybdenum steels (SNCM439) were fatigued with the rotating bending fatigue testing machine and the ultrasonic fatigue-testing machine (20 kHz) in laboratory air. The duplex S-N curve could not be observed for the ultrasonic fatigue tests. The origins of crack initiation in the long life regime were internal inclusions. The fish-eyes were observed in the specimens broken from internal inclusions. When the number of stress cycles to failure was longer than 8.1×10⁶, the facet region was observed around inclusions. The fracture surface could be divided into three regions. The first region surrounding inclusion was a facet area. The crack propagation in the facet region was Stage A and the crack propagation outside the facet region was Stage B. When the fatigue cracks reached the specimen surface, the fatigue cracks propagated as surface cracks. The crack propagation rate in each region was estimated to be a power function of the stress intensity range. Fatigue life was calculated by integrating with the crack propagation laws determined from the experimental data. The predicted fatigue life agrees with the experimental result.

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In order to clarify effects of stress ratios (R=σmin/σmax) on surface- and interior-originating fatigue properties, uniaxial fatigue tests of high strength steel (SNCM 439) were carried out under three different stress ratios, R=0.3, 0.1 and -1. Based on tests results and fractographic analysis in initial crack propagation regions (Optically Dark Area (ODA) and Stage 2 a), the following results were obtained, (1) Surface-originating fractures occurred in the same fatigue life region regardless of the stress ratio. On the other hand, interior-originating fractures occurred at a longer fatigue life region, as the stress ratio increased. It is unsure to apply the modified Goodman diagram to high strength steels of which main fracture mode was the interior-originating fracture. (2) In initial crack propagation regions, the fracture surface of the interior-originating fracture was strongly affected by the stress ratio, even though that of the surface-originating fracture was scarcely affected. The formation of a fine bumpy pattern which was characteristic of the ODA's fracture surface was affected by a multiplier effect of the inner environment of material and the compression load.

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Giga-cycle fatigue tests were conducted for heats B and C of SUP 7 spring steels tempered at 430 and 500°C. Rotating bending, electromagnetic, high-speed servohydraulic and ultrasonic fatigue testing machines were used at 30 or 100 Hz, 120 Hz, 600 or 800 Hz and 20 kHz, respectively. The heat B developed fish-eye fractures only with an origin of an Al₂O₃ inclusion, while Al₂O₃ and TiN inclusions and matrix cracks appeared at the origins in case of the heat C. The Al₂O₃ inclusions in the heat B were larger than those in the heat C. Hence, the heat C revealed higher fatigue limit than the heat B. In case of fish-eye fracture, S-N curves under 100 Hz rotating bending, and 600 or 800 Hz and 20 kHz uniaxial loading were coincident each other in both heats, while those under 120 Hz uniaxial loading were lower. The frequency effects were negligible in case of fish-eye fracture and the difference between 120 Hz uniaxial tests and others were explained by the difference of the control volumes.

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Inclusion inspection conducting fatigue tests was applied to a JIS-SWOSC-V valve spring steel in which oxide type inclusions were composite and softened. In this inspection, inclusion sizes were measured on the fracture surfaces of fish-eye fracture after fatigue tests. The fracture surfaces after fatigue tests revealed a TiN inclusion at the origin in most cases, and in some cases, no inclusion was found at the origin. The TiN inclusion sizes at the fracture origin were precisely measured after etching of fracture surfaces. Even in cases of the origin without an inclusion, a TiN inclusion mostly appeared after the etching of fracture surfaces. The composite oxide inclusions have never caused fish-eye fracture, while the composite oxide inclusions were detected in a conventional inclusion inspection scanning polished surfaces with an optical microscope. These results meant that inclusion inspection conducting fatigue tests were effective for the inclusion-controlled steel. In addition, three tempering conditions, that were 200 and 430°C furnace tempering and 460°C induction tempering, were employed for the fatigue tests. These fatigue test results, then, showed little difference in spite of the difference in hardness and microstructure. Convenient tempering conditions could, therefore, be selected in the inclusion inspection conducting fatigue tests.

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We sometimes experience fretting fatigue fracture of industrial machines after a long period of operation. It has been a question whether the fatigue limit really exists in fretting fatigue or not. In this report, fretting fatigue tests in giga-cycle region up to 109 cycles were performed. There was a knee point in the S-N curve and it was verified that no fracture was observed up to 109 cycles. An electro-potential drop technique was developed to detect the micro-crack growth behavior of the fretting fatigue. It was shown that the micro-crack generated by fretting ceased to grow and became a non-propagating crack at fatigue limit. These results indicated that the fatigue limit can exist also-in fretting fatigue. The infinite endurance limit can be achieved by the mechanism that initiated fretting fatigue cracks become non-propagating cracks.

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Bearings used for automotive powertrains sometimes suffer flaking damage that unexpectedly shortens their life due to peculiar microstructural changes at their subsurface. In this paper, based on various experiments and calculations, we describe the cause of such flaking. The experimental results suggest that this flaking is a kind of hydrogen embrittlement caused by the diffusion of hydrogen into the steel due to the decomposition of the lubricant, and that in affected by the amount of relative slip ratio between ball and bearing race.

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Gigacycle fatigue properties of two martensitic stainless steels with different level of inclusion size were investigated by using ultrasonic fatigue tests at room temperature. As a result of the tests, the S-N curves for the both materials were straight on the downside. All of the fatigue fracture started from defects at the internal or at the surface. And almost fracture surfaces had facet regions around the defects. There were not facet regions under 2×10⁵ cycles for SUS-A with larger level of inclusion and under 5×10⁵ cycles for SUS-B with smaller level of inclusion. There were some specimens of SUS-B which didn't break in the regime from 10⁹ to 10¹⁰ cycles, and which had smaller facet regions than those of the broken specimens. This gives a suggestion that the fatigue limit for internal fatigue fracture exists. For SUS-A and SUS-B, the √area parameter model with √area_fac can explain whether a specimen breaks.

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This paper shows results of fatigue properties and fracture mechanism of modified Cr-Mo steels. In refinery or chemical plants, Cr-Mo steels are used under hot and high-pressure environments. Recently, a new fatigue problem about these facilities becomes important, because number of cycles to failure that should be taken into consideration for maintenance increases over 10⁷. For the ultra-high-cycle fatigue, interior inclusions in materials are dominant factor in fatigue life, but the fracture mechanism is not made clear completely. Results of the elevated temperature ultra-high-cycle fatigue properties are shown. The interior fracture takes place at ultra-high-cycle region, although many cases show no inclusion in origins of the interior fracture. The crack growth life is predicted using fracture mechanics, so that life prediction for Cr-Mo steels is mode possible.

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An ultralong life fatigue design method is presented. Five factors are considered for the fatigue design. Five factors are (1) Statistics of extreme data of inclusions, (2) the size of ODA and its formation mechanism, (3) hydrogen content, (4) the √area parameter model and (5) the assumed design fatigue life. In particular, the influence of hydrogen is investigated by using Cr-Mo alloy specimens with hydrogen charge. The hydrogen content of the order of 10 ppm reduce fatigue life by-1/10⁴.

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In reliability assessment of mechanical structures in the long period, it is important to clarify the fatigue characteristics in such a long sequence of cyclic loading. Thus, ultra-long life fatigue properties of SCM 435 steel were experimentally examined in rotating bending. In addition, high cycle fatigue data of the same kind of steels were extracted from the Database on Fatigue Strength of Metallic Materials published by JSMS. These results and original data obtained by the authors were put altogether, and the statistical fatigue properties of this kind of steels were analyzed as duplex P-S-N characteristics.

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In this study the thermo-elastic analysis of a line crack interacting with a cracked elliptic hole and subjected to a uniform heat flux is carried out. By applying the complex variable method and rational mapping function technique, the solution to the given problem is obtained. This is accomplished by dividing the problem into the following three basic problems : (a) the problem of a cracked elliptic hole subjected to uniform heat flux (b) the problem of a cracked elliptic hole subjected to a heat source couple (c) and the problem of a cracked elliptic hole subjected to a point dislocation. By superposing the three basic problems so that the boundary condition of heat flux and stress on a line crack are satisfied, the solution to the given problem is obtained. Stress intensity factors at the crack tips are obtained soluting integral equation numerically and plotted for various orientation of the line crack and heat flux.

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Uniaxial fatigue tests of (α+β) Ti-6Al-4V alloy were carried out at various stress ratios (R=σ_min/σ_max) up to very high cycle region. The effect of stress ratio on very high cycle fatigue behavior was investigated based on SEM fractography. The S-N curves under positive stress ratios showed a complex folding shape with a steep incline in a long life region where interior-originating fractures occurred. On the other hand, the S-N curves under negative stress ratios had a general feature ; a monotonic increase of the fatigue lives with decreasing stress. SEM observations revealed that an interior crack was formed by linking of small cracks propagated from many interior origins located closely. This type of interior crack was particularly observed in low stress under positive stress ratios. The positive stress ratios facilitated the initiations of cracks and subsequent linkage of the cracks. This tendency may relate with the steep inclination of S-N curves under positive stress ratios. A clear difference was observed on the fracture surfaces between surface-and interior-originating fractures. A few micro-meter size granular substances existed only on the fracture surfaces of interior-originating fractures. The formation of the granular region was accelerated by negative stress ratios. Compressive loads may play an important role in the phenomenon.

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The effect of frequency on giga-cycle fatigue properties was investigated for 900-MPa-class Ti-6 Al-4 V alloy of three heats (heat A-C) with smoothed specimen and with 0.3 mm-hole-notched specimens. Fatigue tests were carried out at frequencies of 120 Hz, 600 Hz and 20 kHz, using electromagnetic resonance, high-speed servohydraulic and ultrasonic fatigue testing machines, respectively. Heats A and B developed internal fractures, and in that case, frequency effects were negligible. On the other hands, heat C revealed only surface fracture. In this case, high frequency tests showed higher fatigue strength, i.e. frequency effects were not negligible. The tests using notched specimens hardly not revealed frequency effects regardless of the heats. The frequency effect observed in the cases of surface fracture were considered to relate to the delay of local plastic deformation reacting to the high frequency loading, since the temperature increase of specimens were successfully suppressed. The delay of plastic deformation was found to be reduced in the notched specimens, because of stress concentration and limitation of plastic deformation zone. In turn, the significant conclusion of this research was that the high frequency tests could be applicable not only to internal fracture but also to notched problem, while those were not to surface fracture of smoothed specimens.

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High-cycle fatigue properties at 4 K, 77 K and 293 K were investigated in forged-Inconel 718 nickel-based superalloy with a mean gamma (γ) grain size of 25 μm. In the present material, platelike delta phases precipitated at γ grain boundaries and niobium enriched-MC type carbides precipitated coarsely. The 0.2% proof stress and the ultimate tensile strength of this alloy increased with decreasing temperature, without decreasing elongation or reduction of area. Fatigue strength in longer-life region did not increase in response to increments in tensile strength, since fatigue cracks initiated internally irrespective of the test temperatures. Fatigue cracks formed crack initiation sites consisting of facet-like structures. These facet-like structures mainly corresponded to coarse carbides at higher stress amplitude and crystallographic facets formed through transgranular initiation and growth of micro cracks at lower stress amplitude.

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In order to investigate the influence of loading frequency on the fatigue properties of Al alloy, ultrasonic fatigue tests were carried out for an age-hardened Al alloy, 6061-T6. The results were compared with those of rotating bending fatigue in view points of initiation and propagation behavior of a crack and fracture mechanism. Fatigue life was longer in ultrasonic fatigue than in rotating bending fatigue, which was mainly caused by the suppression of crack initiation. In ultrasonic fatigue, many fatigue voids and cleavage cracks were observed on the fracture surface in addition to the shear mode cracks, striations and dimples, which were typical fracture modes in rotating bending.

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Rotating bending fatigue test was carried out using AZ31 extruded magnesium alloy in order to clarify the S-N curve, crack initiation and propagation behavior of the alloy. The mechanisms of both crack initiation and crack arrest at the grain boundary during the fatigue process were investigated in detail with a special attention to the microstructure of the material. The material is divided roughly into 2 kinds of banded texture, the phase A (white zone) and B (black zone). The phase B consists of the crystal grains (phase C). Near the endurance limit of the material, fatigue life is greatly affected by a slight difference in the stress amplitude. This feature in fatigue lives is responsible to the sharply curved S-N curve at the endurance limit, a general characteristic of the extruded magnesium alloy. The fatigue crack initiates at the upper and lower edges of the grain boundary of the phase C in the early stage of the fatigue life. At stress amplitude of 122.5 MPa which is slightly larger than the fatigue limit, the crack propagates to the next B-phase without being blocked by the grain boundary of the phase C, however at 120 MPa which is slightly less than the fatigue limit, the crack advance was blocked by the grain boundary. The sharp curved S-N curve is attributable to the crack arrest which is caused by the grain boundary of the phase C. By integrating the relationship between stress intensity factor and crack growth rate, the calculated S-N curve agrees well with the experimental result.

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