Optically Dark Area Research Articles

Study of the Crack Propagation Property in Very-long Life Regime in Vacuum Environment of High Strength Steel

In general, high-strength steel is known to occur an internally initiated fatigue fracture called "Fish-eye failure" in the very long life region. In addition, the fatigue life of a fish-eye failure is dominated by the ODA (Optically Dark Area) that forms around the fracture origin. Therefore, it is necessary to elucidate the formative mechanism of ODA in order to predict the fatigue life of fish eye failure. Besides, it has been reported that small surface cracks propagate at extremely low speeds below the lattice spacing in a vacuum simulating the interior of high-strength steel. However, it has not been confirmed that the growth of these small cracks forms ODA. In this study, fatigue tests were conducted from a very short crack to a long crack, which corresponds to internal failure in the very long life region, instead of the fatigue tests with a relatively long crack of around 150 μm that have been conducted so far. The results of intermittent observation of the growth behavior and the relationship between the crack growth rate, stress intensity factor, and maximum stress suggest that small cracks grow and form ODAs.

Effects of MnS on Gigacycle Fatigue Properties of SCM440 Steel

Gigacycle fatigue tests were conducted on high-strength steels whose Mn and S contents were so high as to be close to upper limits of the JIS standard. Three types of materials were prepared with different working ratios. The fatigue strengths were in order of the working ratios, indicating that the high working ratios improved the fatigue strength. All specimens ended in internal fractures mostly originating from MnS. The MnS observed on the fracture surface was largely elongated in the cases of the materials with high wording ratios. ODAs (Optically Dark Areas) were then observed at around the center of the elongated MnS, meaning that small internal cracks were formed there. Although the shapes of the MnS were very complicated, the sizes were approximated by using ellipses which covered the whole MnS. When the MnS was largely elongated, the length l was cut off at four times of the width W, i.e., l = 4W, based on the results of Makino’s fracture mechanics analysis. The measured MnS sizes were applied to analysis by using a previously derived prediction, comparing the effects of MnS with those of oxides. As the result, it was suggested that the MnS was more harmful to the fatigue strength than the oxides. This was probably attributable to the properties of the MnS, while too large number of the MnS possibly reduced the fatigue strength.

Fatigue limit in very high cycle fatigue of high-strength steel

1011 cycles fatigue tests were conducted on high-strength steel to clarify a new fatigue limit in very high cycle regions. The new fatigue limit had not been confirmed by fatigue tests up to 1010 cycles, while our previous study suggested that the new fatigue limit was probably confirmed by those up to 1011 cycles. However, the 1011 cycles fatigue testing was challenging since it took 2 months even by using ultrasonic fatigue testing at 20 kHz. In this study, 3 specimens were tested beyond 1010 cycles. Although a test on a specimen was terminated at around 5 x 1010 cycles, 2 specimens reached 1011 cycles without failure. In other word, no specimen failed above 1010 cycles. These results demonstrated the existence of the new fatigue limit. The fractured specimens below 1010 cycles revealed internal fractures originating from oxidetype inclusions. When the specimens failed in very high cycle regions, clear ODAs (Optically Dark Areas) were observed on the fracture surfaces, while the ODAs were obscure in case of failure in conventional life regions. The runout specimens up to 1011 cycles were forcibly fatigue-fractured at higher stress amplitudes in the conventional life regions. As the result, the ODA was observed on the forcibly fatigue-fractured surface. This meant that small internal cracks existed in the runout specimens since the ODA was a trace of small internal crack growth. Namely, non-propagating cracks were the mechanism of the appearance of the new fatigue limit.

The influence of hydrogen on the very high cycle fatigue property and crack growth characteristics of bearing steels

In the present study, the growth rate of optically dark area (ODA) was investigated for two types of bearing steels using a fracture mechanics approach. Ultrasonic fatigue tests were performed for the specimens with hydrogen pre-charging followed by exposing in the atmosphere for a week in order to investigate the effect of hydrogen trapped by inclusions on the growth of ODA. Together with the constant amplitude loading test, two-step and repeated two-step variable amplitude loading tests were performed in order to determine the fatigue crack growth rate by beach marks. The results revealed that fatigue strengths of the both materials were reduced by the hydrogen pre-charging. The size of ODA increased with decreasing the stress amplitude at fracture so that the stress intensity factor range at the end of the ODA growth became a constant value ranging between 4.5 and 5.5 MPa √m independent of the materials and the fatigue lives. ODA formation behavior is enhanced by trapped hydrogen by inclusions, which reduces the fatigue strength in the very high cycle fatigue regime.

Small crack growth model from low to very high cycle fatigue regime for internal fatigue failure of high strength steel

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.

G0301305 高強度鋼の極低速き裂進展下におけるODA形成に及ぼす応力比の影響

It's well known High Strength Steel shows internal origin type fatigue failure called Fish eye failure in low stress and high cycle region. In this failure, ODA(Optically Dark Area) is formed and this ODA occupies the great portion of fatigue life of Fish eye failure. Although the explication of the mechanism of ODA formation is needed for the explication of the Fish eye failure mechanism, the explication is not yet accomplished. It's reported that internal fatigue crack in high strength steel is propagating extremely slowly by recent investigations. However, it is not clear the relationship between the extremely slow fatigue crack growth behavior and the mechanism of ODA formation, and whether it is a peculiar phenomenon of high strength steel. In study, fatigue test was done in vacuum environment. As a result, fractography suggest that more obvious ODA is formed by smaller stress ratio when internal crack is extremely slowly propagating.

Prediction of Life and Scatter of Fatigue Failure Originated at Nonmetallic Inclusions

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 hypothetical cycle-by-cycle fatigue crack growth model inside ODA has been developed in the VHCF regime based on the master curve 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 originated at inclusions can be well evaluated by the proposed models.

A fracture mechanics approach to interior fatigue crack growth in the very high cycle regime

Growth rates of optically dark areas (ODA) and fish-eyes (FE) were quantified in kHz-ultrasonic fatigue tests on SUJ2 and 17-4PH steels at constant and repeated two-step amplitudes. Sizes of ODAs and FEs depended on the stress intensity factor (SIF) range, and interior fatigue crack growth rates (FCGR) were slower than those of “long” cracks in air, suggesting vacuum as ODA growth environment. Repeated two-step tests on SUJ2 steel served to form beach marks so that, a quantification of ODA sizes, interior FCGRs and SIFs became possible. Additional FCGR measurements of long cracks in vacuum and comparable fracture morphologies allowed estimating the growth rates of ODAs and FEs in 17-4PH steel.

Fatigue Strength and Internal Crack Growth Behavior of High Strength Steel under Variable Amplitude Stressing in Very High Cycle Regime

Ultrasonic fatigue tests were carried out for high carbon chromium bearing steel, SUJ2, under two-step variable amplitude stressing at a frequency of 20 kHz in order to investigate the growth of an interior crack. Since it is important to clarify the damage mechanisms under variable amplitude stressing for practical applications, fatigue tests were conducted under repeated two-step variable amplitude stressing. An inclusion was observed at the initiation site of fracture. Optically dark area (ODA) and fish-eye were observed around the inclusion. Sizes of ODA and fish-eye depended on the stress intensity factor range. Interior crack growth rate was slower than the surface one due to vacuum environment. Cumulative damage values calculated by Miner's rule were relevant to the size of inclusions under repeated two-step variable amplitude stressing. Characteristics of interior fatigue crack growth were determined by the fracture surface observations for beach marks and the ODA sizes. Based on the results, the formation mechanism of ODA was discussed in conjunction with the reports by the other researchers.

The fatigue limit of bearing steels – Part I: A pragmatic approach to predict very high cycle fatigue strength

Bearing steels and other high strength steels exhibit complex fatigue behavior in excess of 10 7 cycles due to their sensitivity to defects like inclusions. Failure occurring in the very high cycle fatigue regime and the lack of an asymptote in the measured S–N data raise the questions as to the existence of fatigue limit and prediction of the fatigue strength of the high strength steel components. A series of two papers are written to discuss on the characteristics of the very high cycle fatigue and their implication for engineering applications. In the present paper (Part I) a deterministic defect model is developed to describe the fatigue crack growth from de-bonded hard inclusions. The model is shown to provide a unified prediction of fatigue behavior in different regimes, i.e. low cycle fatigue regime dictated by the tensile strength, high cycle fatigue regime obeying Basquin’s law and the very high cycle fatigue regime featured by the fish-eye and ODA (optically dark area) surrounding an interior fatigue-initiating inclusion on the fracture surface. The model predictions agree well with experiments. A combination of the deterministic model with a stochastic model that describes the inclusion size distribution allows prediction of fatigue strength and fatigue limit associated with certain reliability of a steel component. It is found that very high cycle fatigue, associated with interior inclusions, is attributed to the very slow crack propagation in vacuum condition, and that an asymptote for fatigue limit observed for mild steels also exists for high strength steels such as bearing steels, but extends beyond the very high cycle fatigue regime normally measured to-date. Monte Carlo simulation shows that such a fatigue limit asymptote becomes clearly visible in excess of 10 12 cycles, which is difficult to measure with today’s testing devices. Furthermore, the effects of steel cleanliness and specimen type and shape are studied by means of Monte Carlo simulations.

OS0609 真空環境下における高強度鋼のき裂進展挙動と開口応力の関係(OS6-2 内部破壊とそのメカニズム,OS-6 金属材料の超高サイクル疲労と信頼性評価)

Fish eye failure that is unique fatigue fracture occurs in very long life region of high strength steel. The fatigue life of Fish eye failure dominated ODA (Optically Dark Area) around the fracture origin. A granular looking area of ODA is visible close to the crack initiation, Fatigue life of Fish eye failure is almost exclusively spent propagating the fatigue clack in ODA. Therefore, to predict fatigue life of Fish-eye failure is important to know ODA of formative mechanism. In this study, fatigue and tensile test was performed in vacuum that simulated environment of subsurface failure and compared the result by the test condition. The observation of Process when ODA is formed on the precrack tip was examined in order to make clear a mechanism of ODA formation. Crack growth rate has depended on maximum stress by range of ΔK &le; 4MPa√<m>. Under the fixed maximum stress of value, initiated fatigue cracks became non-propagating cracks. When the tensile stress of 800MPa or less was given, the remarkable growth of the ODA crack was not seen.

Influential Factors on the Formation of Initial Crack Propagation Region in Sub-surface Fracture of Ti-6Al-4V Alloy

To determine the influential factors on the formation of initial crack propagation region in the very high cycle fatigue (VHCF), uniaxial fatigue tests and K-decreasing tests were conducted with Ti-6Al-4V alloy. The test data indicated that the crack growth rate in vacuum was much slower than that in air, which corresponded to longer fatigue lives of sub-surface fractures. The fracture surface of sub-surface fracture and that obtained in the crack propagation test in vacuum showed similar features that are characterized by a unique granular concavo-convex microasperity. Since the granular fracture surface was never observed in surface-initiated cracks, the granular concavo-convex pattern was considered as clues to an elucidation of the initial crack propagation mechanism in the sub-surface fracture. The granular features also resembled so-called ODA (Optically Dark Area) in high strength steels. Based on this observation, a common mechanism of sub-surface crack growth regardless of materials was discussed for the VHCF phenomena.

Characteristics and Formative Factors of Fracture Surfaces on Subsurface Initial Crack Growth in Very High Cycle Fatigue

In the very high cycle regime over 107 cycles, a fatigue crack often initiates from subsurface of materials. In subsurface fractures, a unique fracture surface with a fine concavo-convex pattern called Optically Dark Area (ODA) is known to exist around the fracture origins. This type of fracture surface has been observed not only in high strength steel, but also in Ti alloy. To reveal the formative factors of the fine concavo-convex pattern, this study focuses on a unique environment around subsurface cracks. Subsurface cracks are not surrounded by atmosphere; therefore, the environment around them can be similar to vacuum. Based on this idea, fatigue crack growth tests using high strength steel (SNCM439) and Titanium alloy (Ti-6Al-4V) were conducted in high vacuum under various conditions, and fracture surfaces were investigated by SEM analyses. As a result, the fine concavo-convexo morph similar to ODA in subsurface fractures was observed on the fracture surfaces obtained in the crack growth tests. In addition, it was clarified that the unique fracture surface was not formed during crack growth process but formed by the long term repeating contacts of already initiated fracture surfaces in high vacuum. The fine concavo-convexo morph can be formed regardless of materials when the following two conditions are satisfied ; one is a vacuum environment and the other is a long term repeating contacts of fracture surfaces over about 107 cycles.

1117 高強度鋼のODA型き裂進展領域における極低速き裂進展挙動(OS11-4 材料の疲労挙動と損傷評価-超長寿命-)

Fish eye failure that is unique fatigue fracture occurs in very long life region of high strength steel. The fatigue life of Fish eye failure dominated ODA (Optically Dark Area) around the fracture origin. A granular looking area of ODA is visible close to the crack initiation, Fatigue life of Fish eye failure is almost exclusively spent propagating the fatigue clack in ODA. Therefore, to predict fatigue life of Fish-eye failure is important to know ODA of formative mechanism. In this study, fatigue and tensile test was performed in vacuum that simulated environment of subsurface failure and compared the result by the test condition. The observation of Process when ODA is formed on the precrack tip was examined in order to make clear a mechanism of ODA formation. When the tensile stress of 800MPa or less was given, the remarkable growth of the ODA crack was not seen. Crack growth rate has depended on maximum stress by range of ΔK≦4MPam. Under the fixed maximum stress of value, initiated fatigue cracks became non-propagating cracks.

The effect of inclusion geometry according to forging ratio and metal flow direction on very high-cycle fatigue properties of steel bars

The objectives of the present paper are to clarify the effect of inclusion geometry according to the forging ratio and the metal flow direction on very high-cycle fatigue properties of steel bars and to propose an evaluation method for characteristic inclusion geometries observed at fracture origin. Specimens were made of steel plates rolled to three levels of thickness from steel bars of low alloy steel. Then the specimens were cut in both parallel and perpendicular directions to the metal flow of the steel plates. Fatigue tests were conducted in a long life regime by using an ultrasonic fatigue testing machine. The fatigue limit for 10 9 cycles decreased with the increase of the thickness of steel plate, i.e. a decrease of the forging ratio. The fatigue limit of specimens cut parallel to metal flow was higher than that of specimens cut perpendicularly to metal flow. Fish-eye fractures appeared on the fracture surface of most specimens. Especially, characteristic inclusions, which are clusters of oxides, sulfides and their combination, were mainly observed at the origin of fish-eye fractures in specimens cut perpendicularly from thick plates. In this paper, these characteristic inclusions were treated as the extended elliptical inclusion/crack from the point of view of fracture mechanics and the area parameter model. More careful inspection at the fracture origin revealed that the rough surface area called the optically dark area (ODA) appeared only around the edge of the short radius on the extended elliptical inclusion where the higher stress intensity factor is generated. This result suggests that the middle part of the extended elliptical inclusion dominates the formation of ODA and the very high-cycle fatigue property. Finally, based on the fracture mechanics approach, area L/ W=4 , which is the square root of the area of the surrounded part where length/width < 4 at the middle of the extended elliptical inclusion, is proposed as the evaluation parameter for area parameter model.

Gigacycle Fatigue Properties of High-Strength Steels According to Inclusion and ODA Sizes

Gigacycle fatigue tests were conducted for several versions of JIS-SCM440 low-alloy and JIS-SUJ2 bearing steels using 20-kHz ultrasonic fatigue testing to elucidate the relationship of the inclusion size and type to fish-eye fracture properties. The total number of tested specimens was over 200. Most of the specimens revealed Al2O3 or (Cr, Fe)3C inclusion-originating types of fish-eye fractures, while TiN inclusions and the matrix also caused fish-eye fractures in some specimens. Based on these fatigue test results, 109-cycle fatigue limits were estimated according to inclusion size by resorting the obtained data points according to their inclusion sizes at the fish-eye fracture origin. The estimated fatigue limits revealed saturation when the inclusion sizes were smaller than 15 μm, while those fatigue limits depended on the inclusion sizes to the −1/6th power in the case of inclusions above 15 μm in size. The saturation of the 109-cycle fatigue limits was considered to be caused by the effects of the optically dark areas (ODAs). Moreover, the fatigue limits also depended on inclusion type. In comparing the Al2O3 and (Cr, Fe)3C inclusions, the key features causing the difference in the fatigue limits were likely to be bonding between the inclusion and the matrix, i.e., the (Cr, Fe)3C inclusions were tightly bonded to the matrix, unlike the Al2O3 inclusions, although both inclusions were of the hard type.

Study of Ultra-Long Life Fatigue of High Strength Steel with Duplex-Phase of Carbide-Free Bainite and Martensite

The ultra-long life fatigue properties of novel high strength steel with duplex-phase of carbide-free bainite and martensite (CFB/M) were studied in this article. The specimens were heat-treated by the following procedure: austenitizing at 900°C for 20 min followed by air cooling and then tempering at 340°C for 2 h. All the fatigue tests were carried out by ultrasonic fatigue testing equipment, at a cycling frequency of 20 kHz. Experimental results show that there is no conventional fatigue limit for the steel studied in this paper. From the S-N curve it could be suggested that it is more reliable to use the fatigue strength at 10 8 cycles than that at 10 7 cycles for the safe-life design. Based on the observations of fracture surface, it is obvious that the fatigue fracture surface can be classified into two types: initiating from surface and interior. A fish-eye mark and an area with a particular morphology surrounding the fracture origin termed ODA (optically dark area) by Japanese scholars are always found in the latter type, but no obvious inclusion is observed inside it. By calculating the values of ΔK at ODA and fish-eye, it is found that the values of Δk fish-eye and ΔK ODA tend to be constant respectively, independent of the stress amplitude, fatigue life and the distance of the initiation site to the edge of specimen, therefore, it is thought that the contribution of hydrogen to the formation of ODA should not be considered.

Super-Long Life Fatigue Properties of Aluminum Alloy Die-casting Related with Internal or Surface Defects

By the ultrasonic fatigue test for Al-Si-Cu die-casting, the influence of internal or surface defects on super-long life properties was investigated. The results were as follows : (1) When surface defects were the crack starting points, the S-N curve leveled off around 107 cycles. On the other hand when the defects were internal defects, the S-N curve showed continued degradation as Nf increase. (2) When the crack starting point was an internal defect, ODA (Optically Dark Area) was observed around the crack initiation point after 108 cycles. However, ODA was not observed when the compulsory fatigue was applied for no-break sample that was tested over 1010 cycles of Nf. This suggests that the incidence of fatigue crack from the defect point or its progress were not recognized. (3) By using the following formula, which compares the relationship between ΔKCD ∝ (√area) 1/3 and ΔKth∝ (√area) 1/3 based on the √area parameter model, it is possible to explain the breaking or non-breaking phenomenon caused by the casting defect in the super-long life area (over 107 cycles).

Effects of Hydrogen Charge on Fatigue Strength of Stainless Steels

The effect of hydrogen on fatigue strength of stainless steels was investigated. There were no definite differences in fatigue life of unnotched specimens between hydrogen-charged specimen (2.46.4 ppm) and uncharged specimen (2.2ppm) of SUS 304. High content of hydrogen (10100 ppm) was detected in a very thin surface layer, approximately at 100200 μm from specimen surface of SUS 304 specimens hydrogen-charged for 336672 hours. There were also no definite differences in fatigue life of unnotched specimens between hydrogen-charged specimen (1.85.8 ppm) and uncharged specimen (0.1ppm) of SUS 405. The uncharged specimens of 0.7 C13 Cr steel (0.2 ppm) failed from subsurface non-metallic inclusions in surface layer. ODAs (Optically Dark Area) were not observed due to the tensile residual stress in the surface layer although the specimens failed in the high cycle regime of 107 cycles. Increasing hydrogen content from 0.2 ppm to 2.42.7 ppm, the fatigue strength and fatigue life were markedly decreased in 0.7 C13 Cr steel.

真空疲労試験による高強度鋼のＯＤＡ形成条件の検討

A high strength steel shows fish-eye failure in very long life fatigue. In this region, the fatigue crack growth in ODA (Optically Dark Area) controls the fatigue process. However, ODA's formative mechanism is unknown. Therefore, to predict fatigue life of fish-eye failure, it is important to know ODA's formative mechanism. So, the present study examined for the influential factors on ODA in the fatigue process through uniaxial push-pull fatigue tests in vacuum as well as computer simulation. In experiment, vacuum environment is employed to simulate fish-eye type crack growth on surface of specimens becase of shutting off supply of air. The test results in vaccum showed that the fatigue life property of surface failure has corresponded to fish-eye fatigue life. And the site of inclusion which has fracture origin size determines whether surface failure or internal failure in vacuum. The results of fractographic observation of crack initiation showed that the rough area of surface fracture in vacuum was formed by mechanism of ODA type crack. Furthermore, the fatigue life of numerical simulation in vacuum showed that the fatigue life which was necessary to form ODA on specimen surface corresponded experimental life.