Optically Dark Area Research Articles

Gigacycle fatigue properties of a modified-ausformed Si-Mn steel and effects of microstructure

Fatigue tests were conducted for modified-ausformed, oil-quenched, and water-quenched versions of a Si-Mn steel (JIS-SMn443) with a chemical composition of 0.42C-0.2Si-1.52Mn (in mass pct). The Si-Mn Steel has an advantage with respect to recycling, since it contains no Cr or Mo, although the steel has disadvantages with respect to tempering resistance and hardenability. Although the tempering resistance was not improved by modified ausforming, the problem of hardenability was minimized, since modified ausforming created fine and uniform martensite. The gigacycle fatigue properties of the Si-Mn steel were improved by the modified ausforming. The low-strength modified-ausformed steel (AF1400) was free of fish-eye fractures and had a fatigue limit of 770 MPa at 109 cycles. In spite of the occurrence of fish-eye fractures, the high-strength modified-ausformed steel (AF2000) achieved a fatigue limit of 830 MPa at 1010 cycles, higher than the 710 MPa limit of the water-quenched steel (QT2000W). The fatigue limits of the oil-quenched steels (QT1400 and QT2000), which contained numerous and large nonuniform structures, could not be determined because of the temperature increase of specimens during the 20 kHz tests. The fracture surfaces revealed optically dark areas (ODAs) even in the modified-ausformed steel when the specimens failed at over 107 cycles, since the steel was partially recrystallized during modified ausforming. The gap of fatigue limit between the AF2000 and QT2000W steels was not entirely explained by the difference of the ODA sizes. Also, the sizes of the nonuniform structures were likely to play an important role in the gigacycle fatigue properties, since the sizes in AF2000 were smaller than those in QT2000W.

Effects of Stress Ratio on Surface- and Interior-Originating Fatigue Properties of High Strength Steel

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.

Ultra-long cycle fatigue of high-strength carbon steels part II: estimation of fatigue limit for failure from internal inclusions

Ultra-long cycle fatigue fracture origins in high strength steels are mostly at non-metallic inclusions due to the influence of the trapped hydrogen. In the vicinity of a non-metallic inclusion at the fracture origin, an optically dark area (ODA) is often observed inside a fish-eye mark, which represents the particular morphology associated with mechanism of failure at an early stage: hydrogen assisted fatigue. From an analysis of several features involved in this mechanism of failure, as the development of the ODA, the threshold for pure fatigue propagation, as a function of crack length and inclusion size, an expression was deduced to estimate the internal fatigue limit, that is, the stress level below which fracture produced by cracks initiated from an internal inclusion is not found after 10 10 cycles: σ e Int =256 ΔK th R i max where Δ K th is the pure fatigue crack propagation threshold as a function of crack length estimated with the following expression: ΔK th =4×10 −3(H V +120)(3R i max ) 1/3 If ΔK th ≤10 MPa m 1/2 Otherwise ΔK th =10 MPa m 1/2 R i max is the maximum radius of the non-metallic inclusions from which the crack initiates, in μm, and H V is the Vickers hardness, in kgf mm −2. The internal fatigue limit was estimated for several steels and compared with experimental results obtained from the literature for internal fatigue lives in the range of 10 8–10 10 cycles. Differences between the estimated internal fatigue limit and the experimental one ranged from 4 to −29%.

Effect of Hydrogen on Ultralong Life Fatigue Failure of a High Strength Steel and Fracture Morphology of ODA

ODA (Optically Dark Area) in the vicinity of inclusions at fracture origin has a crucial importance in the mechanism of ultralong life fatigue failure. From the authors' previous studies, it has been presumed that ODA is formed by the fatigue due to cyclic stress coupled with hydrogen which is trapped by the inclusion at fracture origin. The fracture morphology of ODA was observed by a SEM with low resolution, a SEM with high resolution, an AFM and a Laser microscope. The ODA observed by the SEM with low resolution revealed a granular morphology. However, the high resolution SEM images obtained by tilting the fracture surface identified that the true morphology of ODA was not granular but was very sharp and irregularly rough. In order to reveal the effect of hydrogen on the formation of ODA, the microstructure around the inclusions at fracture origin was investigated. The hardness in the vicinity of inclusions in Specimens QT which contained 0.7-0.9 ppm hydrogen was much lower than that for Specimens VQ which contained∼0.01 ppm hydrogen.

超長寿命疲労破壊におけるＯＤＡ（Ｏｐｔｉｃａｌｌｙ Ｄａｒｋ Ａｒｅａ）の形成に及ぼす諸因子

The mechanism of fatigue failure in the ultra-high cycle regime was studied using a bearing steel, JIS SUJ2 (SAE52100). In the case of tension-compression fatigue testing, the S-N curve for high strength steels consisted of a single straight line in contrast to the dual S-N curve observed in rotating bending. The effect of internal hydrogen trapped by nonmetallic inclusions on high cycle fatigue behavior has been discussed by Murakami et al. In order to investigate further the influence of hydrogen trapped by various types of inclusions, four materials of different chemical compositions were prepared. Those specimens having a longer fatigue life had a particular morphology designated as ODA (Optically Dark Area) surrounding the inclusion at the fracture origin. In order to investigate the growth of ODA with cycling, fatigue tests involving multi-step loading were carried out, and the appearance of the ODA was compared with that of specimens tested under a constant amplitude loading. A specimen which endured N=2×108 under a stress was fractured by tensile test at -190°C. The fracture origin in this tensile test was at a subsurface, nonmetallic inclusion. If the fatigue test had not been interrupted, the potential fatigue life of the specimen was estimated from the size of the inclusion and the ODA using the √area parameter model. The model was also used to estimate the fatigue limit in the ultra-high cycle range. The condition for the critical size of ODA for the start of conventional fatigue crack growth was analyzed using fracture mechanics. The critical stress intensity factor range ΔKODA for the critical size of the ODA can be correlated with the threshold stress intensity factor range ΔKth expressed by the √area parameter model for small cracks. A fatigue design method for ultra-high fatigue life regime was proposed based upon the growth behavior of the ODA, the statistics of extremes of inclusions, and the √area parameter model.

718 SMn443 鋼改良オースフォーム材のギガサイクル疲労特性

This report reveals gigacycle fatigue properties for a modified-ausformed SMn443 steel. The steel has an advantage in recycling since the steel dose not contain Cr and Mo which are difficult to be removed in re-melting. Modified-ausformed, oil-quenched and water-quenched steels were prepared for fatigue test, preparing two different tensile strength specimens of 1400 and 2000MPa. The low-strength modified-ausformed steel (AF1400) was free from fish-eye fracture with a fatigue limit of 770MPa at (10)^9 cycles, while the low strength oil-quenched steel (QT1400) caused fish-eye fracture. Although the high-strength modified-ausformed steel (AF2000) was suffered from fish-eye fracture, the fatigue limit at (10)^<10> cycles was 830MPa that was higher than 710MPa of the water-quenched steel (QT2000W) tempered at 473K. The fatigue limit at (10)^9 or (10)^<10> cycles for both oil-quenched steels (QT1400 and QT2000) could not be determined because of temperature increase of specimens in 20kHz tests. The fracture surfaces revealed optically dark areas (ODA) even in modified-ausformed steel since the modified-ausforming was not perfect in this research.

Si-Mn鋼のギガサイクル疲労特性に及ぼす改良オースフォームの影響

This report reveals gigacycle fatigue properties for a modified-ausformed Si-Mn steel (SMn443) with the chemical composition of 0.42C-0.2Si-1.52Mn in mass%. The Si-Mn steel has an advantage in recycling since the steel does not contain Cr and Mo which are difficult to be removed in re-melting. On the other hand, disadvantages of the Si-Mn steel are low tempering resistance and low hardenablility.Modified-ausformed, oil-quenched and water-quenched steels were prepared for fatigue tests, followed by tempering at 473K and 673K to prepare two different strength specimens. The tensile strengths of steels tempered at 473K and 673K were nearly 1400 MPa and 2000 MPa, respectively. The problem of hardenablility was solved by modified-ausforming, while the tempering resistance was not improved. The low-strength modified-ausformed steel (AF1400) was free from fish-eye fracture with a fatigue limit of 770 MPa at 109 cycles, while the low strength oil-quenched steel (QT1400) caused fish-eye fracture. Although the high-strength modified-ausformed steel (AF2000) was suffered from fish-eye fracture, the fatigue limit at 1010 cycles was 830 MPa that was higher than 710 MPa of the water-quenched steel (QT2000W) tempered at 473K. The fatigue limit at 109 or 1010 cycles for both oil-quenched steels (QT1400 and QT2000) could not be determined because of temperature increase of specimens in 20 kHz tests. The fracture surfaces revealed optically dark areas (ODA) even in modified-ausformed steel since the modified-ausforming was not perfect in this research.

Improvement of gigacycle fatigue properties by modified ausforming in 1600 and 2000 MPA-class low-alloy steels

Fatigue tests were conducted for a modified ausformed and tempered JIS-SCM440 low-alloy (denoted as AF) steel with tensile strengths of 1600 and 2000 MPa, respectively, and were compared with normally quenched and tempered (QT) steel. The 1600 MPa-class modified ausformed steel (AF1600) developed no fish-eye fractures and a fatigue limit of 920 MPa, whereas normally quenched steel (QT1600) developed fish-eye fractures and showed a lower fatigue limit of 840 MPa. The 2000 MPa-class modified ausformed steel (AF2000) achieved a high fatigue limit of 1010 MPa, but developed fish-eye fractures. The fatigue limit of normally quenched steel (QT2000) was 840 MPa. Results of fatigue tests at 20 kHz up to 109 and 1010 cycles agreed with the fatigue limits for QT1600 and QT2000, respectively. The fracture sites of fish-eye fractures in QT1600 were Al2O3 and TiN inclusions, and those in AF2000 and QT2000 were Al2O3 and TiN inclusions and internal facets. Fracture surfaces of QT2000 that broke at over 107 cycles had large, clearly identifiable optically dark areas (ODAs). The AF2000 showed no fractures over 107 cycles, and the authors could not conclude whether the fracture surface of the specimens broken at around 5 × 106 cycles had ODAs. The fact that long-life fatigue failure over 107 cycles did not occur in AF2000 showed that ODAs did not form in this steel, since ODAs developed in proportion to cycle number and became large and clearly identifiable at over 107 cycles. Their incidence was quantitatively demonstrated by modified Stress amplitude vs Number of cycles to failure (S-N) diagram analysis. The modified S-N curve for AF2000, in which the specimens were assumed to have no ODAs, showed good agreement with that for QT2000, in which ODAs were assumed to be formed, especially at over 107 cycles.

高強度鋼の疲労破面に及ぼす大気／高真空／材料内部環境の影響

Uniaxial tension fatigue tests of high strength steel, SNCM439, were carried out in air and high vacuum environments. In order to clarify how different environments affect the fatigue fracture surfaces and interior fatigue crack propagations, fracture surfaces were compared through fractographic analysis under air, high vacuum, and inner environment of materials. The following results were obtained; (1) Compared with surface fractures in air, the fatigue life of surface fractures in high vacuum tended to be longer. (2) High vacuum environment strongly influenced the fatigue fracture surface in initial stage of crack propagation. (3) When the transition of crack propagation from Optically Dark Area (ODA) to Stage II a occurred, there was the possibility that some mechanical factors were involved in the transition. The formation of ODA was closely related to the mean stress. (4) Inner environment of materials gave more similar effects on crack propagation with high vacuum rather than air. (5) When fracture origins were non-metallic inclusions, interior fatigue cracks of materials in Fish eye were propagated in the following order; ODA → Stage II a → Stage II b → Stage II c.

736 高強度鋼の内部起点型疲労破面に及ぼす圧縮負荷の影響

In order to clarify how compression load affects interior-originating fatigue fracture surfaces of high strength steel, uniaxial tension fatigue tests were carried out under different stress rations (R=0.3,0 and -1). The results were as follows; Interior-originating fractures under R=-1 occurred in shorter fatigue lives than those under R=0.3. Optically Dark Area (ODA) was formed even in short fatigue life of (10)^5 cycles. The morphology of ODA was closely related to the stress ratio, especially compression stress. Even though the fracture origin of the specimen was not inclusion, ODA was formed on the fracture surface.

430℃と500℃で焼戻されたばね鋼SUP7の10&lt;SUP&gt;10&lt;/SUP&gt;サイクル疲労特性

Giga-cycle fatigue properties were investigated for SUP7 spring steels tempered at 430 and 500°C. Tensile strength levels were 1423 and 1730MPa. Rotating bending, electromagnetic, high-speed servohydraulic and ultrasonic testing machines were used for 30 or 100Hz, 120 Hz, 600Hz and 20kHz tests, respectively. 1010-cycles fatigue properties were obtained in 3 years under 100Hz rotating bending and in 7days under 20kHz uniaxial loading, using 6-and 3-mm-diameter hourglass specimens. 108-cycles fatigue properties were also obtained under 120 and 600 Hz uniaxial loading, using 6- and 3-mm-diameter hourglass specimens. Almost all specimens fish-eye-fractured at internal inclusions of Al2O3. Optically dark areas (ODAs) were formed around inclusions even at 20kHz. The data obtained was analyzed according to the predicted fatigue limit and control volume of specimen for fish-eye fracture proposed by Murakami and others. S-N curves for 430°C tempered steel under 100Hz rotating bending, and 600Hz and 20kHz uniaxial loading were coincident each other, because the control volume was almost the same of 34 and 33mm3. S-N curves under 120Hz unixial loading with the large control volume of 227mm3 shifted to the lower strength level. The inclusion size was nearly proportional to the control volume. Modified S-N curves, that is σa/σw' versus Nf curves, were independent on the test speed, loading type, specimen configuration and strength level. Here, σw' is the fatigue limite for fish-eye fracture predicted by Murakami and others. These results give us two findings. One is that fish-eye fracture is not the simple hydrogen embrittlement process. Another is that 20kHz ultrasonic machine is effective for accelerated testing method of giga-cycle fatigue.

疲労 軸受鋼の超長寿命疲労破壊に及ぼす介在物にトラップされた水素の影響

The mechanism for fatigue failure in extremely high cycle fatigue in the regime of N>107 is studied on a bearing steel, JIS SUJ2. Special focus was given to the fracture morphology in the vicinity of fracture origin (subsurface nonmetallic inclusion) of a heat treated bearing steel (Specimen QT). The particular morphology looks dark during optical microscopic observation. Specimens with short fatigue life of the order of Nf=105 did not have such a dark area, ODA (optically dark area). To investigate the influence of the hydrogen trapped by nonmetallic inclusions on fatigue properties, specimens heat treated in a vacuum followed by quenching and tempering (Specimen VQ) were prepared. Specimens VQ contained 0.07ppm hydrogen as compared to 0.80ppm hydrogen for conventional Specimens QT. Specimens VQ had a slightly smaller ODA than Specimens QT. Hydrogen was detected by a Secondary Ion Mass Spectrometer around the inclusion at fracture origin of Specimens QT and Specimens VQ. Thus, it can be concluded that the formation of ODA is closely related to hydrogen trapped by nonmetallic inclusions. Estimations of fatigue limit by the √area parameter model based on the original size of inclusions for fatigue limit defined for 107 cycles are -10% unconservative. Considering the size of ODA into fatigue limit estimation, the √area parameter model can predict the mechanical fatigue threshold for small cracks without influence of hydrogen. The mechanism of duplex S-N curve is also discussed.

307 高強度鋼の疲労破面に及ぼす大気/高真空/材料内部環境の影響(OS1-1 高強度材の超高サイクル疲労(1))(OS1 高強度材・表面改質材の疲労と破壊)

Uniaxial tension fatigue tests of high strength steel, SNCM439, were carried out in air and high vacuum environment. As a result, the following were obtained : (1) Compared with surface fractures in air, the fatigue life of surface fractures in high vacuum tended to be longer. (2) High vacuum environment strongly influenced the fatigue fracture surface in the initial stage of crack propagation. (3) High vacuum environment was similar to inner environment of materials. (4) The formation of Optically Dark Area (ODA) closely related to mean stress.

On the mechanism of fatigue failure in the superlong life regime (N&gt;107 cycles). Part 1: influence of hydrogen trapped by inclusions

The fracture surfaces of specimens of a heat‐treated hard steel, namely Cr–Mo steel SCM435, which failed in the regime of N = 105 to 5 × 108 cycles, were investigated by optical microscopy and scanning electron microscopy (SEM). Specimens having a longer fatigue life had a particular morphology beside the inclusion at the fracture origin. The particular morphology looked optically dark when observed by an optical microscope and it was named the optically dark area (ODA). The ODA looks a rough area when observed by SEM and atomic force microscope (AFM). The relative size of the ODA to the size of the inclusion at the fracture origin increases with increase in fatigue life. Thus, the ODA is considered to have a crucial role in the mechanism of superlong fatigue failure. It has been assumed that the ODA is made by the cyclic fatigue stress and the synergetic effect of the hydrogen which is trapped by the inclusion at the fracture origin. To verify this hypothesis, in addition to conventionally heat‐treated specimens (specimen QT, i.e. quenched and tempered), specimens annealed at 300 °C in a vacuum (specimen VA) and the specimens quenched in a vacuum (specimen VQ) were prepared to remove the hydrogen trapped by inclusions. The specimens VA and VQ, had a much smaller ODA than the specimen QT. Some other evidence of the influence of hydrogen on superlong fatigue failure are also presented. Thus, it is concluded that the hydrogen trapped by inclusions is a crucial factor which causes the superlong fatigue failure of high strength steels.

On the mechanism of fatigue failure in the superlong life regime (N&gt;107 cycles). Part II: influence of hydrogen trapped by inclusions

High cycle fatigue fracture surfaces of specimens in which failure was initiated at a subsurface inclusion were investigated by atomic force microscopy and by scanning electron microscopy. The surface roughness Ra increased with radial distance from the fracture origin (inclusion) under constant amplitude tension–compression fatigue, and the approximate relationship: Ra ≅ CΔK 2I holds. At the border of a fish‐eye there is a stretched zone. Dimple patterns and intergranular fracture morphologies are present outside the border of the fish‐eye. The height of the stretch zone is approximately a constant value around the periphery of the fish‐eye. If we assume that a fatigue crack grows cycle‐by‐cycle from the edge of the optically dark area (ODA) outside the inclusion at the fracture origin to the border of the fish‐eye, we can correlate the crack growth rate da/dN, stress intensity factor range ΔKI and Ra for SCM435 steel by the equation and by da/dN proportional to the parameter Ra .Integrating the crack growth rate equation, the crack propagation period Np2 consumed from the edge of the ODA to the border of the fish‐eye can be estimated for the specimens which failed at Nf &gt; 107. Values of Np2 were estimated to be ∼1.0 × 106 for the specimens which failed at Nf ≅ 5 × 108. It follows that the fatigue life in the regime of Nf &gt;107 is mostly spent in crack initiation and discrete crack growth inside the ODA.

Miscellaneous

Fatigue failure of high strength steels mostly originates at nonmetallic inclusions. An optically dark area (ODA) beside the inclusion can be observed in specimens fractured at very high cycle fatigue (VHCF) regime. The present paper proposes fatigue life prediction models from low to VHCF regime. The fatigue life prediction model inside ODA has been constructed in the VHCF regime based on the master curve of the growth of ODA where fatigue failure is caused by cyclic loading assisted by hydrogen trapped by inclusion. The fatigue crack growth law is proposed for a small crack outside ODA within the framework of the area parameter model where the concept of “continuously variable fatigue limit” for small crack is introduced. The life and scatter of fatigue life originating at inclusions can be well evaluated by the proposed models.