Granular Bright Facet Area Research Articles

Comparison of Very High Cycle Fatigue Properties of 18CrNiMo7-6 Steel after Carburizing and Pseudo-carburizing

The very high cycle fatigue (VHCF) properties of case hardening steel 18CrNiMo7-6 after carburizing and pseudo-carburizing have been investigated by means of ultrasonic fatigue tests. Results showed that the pseudo-carburized specimens with the hardness of 442 HV did not show VHCF fracture, and fatigue fracture did not occur even after 109 cycles when the applied stress amplitude was lower than the high cycle fatigue limit of 650 MPa obtained at 107 cycles. When carburized thoroughly with the hardness of 700 HV, the experimental steel showed VHCF phenomenon, and it might fracture even at nearly 109 cycles when the stress amplitude was about 600 MPa. Observations of the fracture surfaces of the fatigue specimens showed that after carburizing, the fatigue crack initiation sites changed from martensite matrix to nonmetallic inclusions, forming granular bright facet (GBF) and fish-eye before final abrupt fracture. The stress intensity factor at GBF area, which was calculated to be 3.5 MPa m1/2, could be used as a critical value for VHCF fracture for the carburized specimens of the experimental steel. The VHCF fracture of the carburized specimens of the experimental steel could be related to the high hardness and large amount of retained austenite as a result of carburizing.

Non-inclusion induced crack initiation in multiphase high-strength steel during very high cycle fatigue

During very high cycle fatigue (VHCF) crack can initiate at non-inclusion site in the interior of Mn-Si-Cr bainite/martensite (B/M) multiphase steels. The characteristics of non-inclusion induced crack initiation in B/M multiphase steels and the relationship between the size of granular bright facet (GBF) and VHCF limit were studied through ultrasonic fatigue testing, in terms of fracture surface analysis, crack initiation analysis, and failure mechanism. The current study reveals that GBF area became smaller with the increase of VHCF limit, i.e. the steel with higher fatigue-endurance strength exhibited smaller GBF area in the case of steel characterized by non-inclusion crack initiation in the VHCF regime. The relationship between GBF area, fatigue life, and applied stress was consistent with the following equation areaGBF = 29.41LogNf − 0.2916 σa + 47.51. The study provides an understanding of non-inclusion induced crack initiation in VHCF and a guidance for steel fatigue design.

Very high cycle fatigue properties of high‐strength spring steel 60SiCrV7

AbstractThe very high cycle fatigue properties of spring steel 60SiCrV7 for automotive suspension system with different hydrogen contents were studied by using ultrasonic fatigue testing and fatigue crack growth testing. The results show that the S–N curves exhibit continuous drop of fatigue lives and no obvious horizontal line exists. Similar fracture surface features were observed for all the specimens that failed mainly from internal inclusions with surrounding granular bright facet (GBF). Fatigue strength decreases remarkably with increasing hydrogen content. The applied stress intensity factor range at the periphery of GBF ΔKGBF is approximately proportional to 1/3 power of the square of GBF area. The average values of ΔKGBF for uncharged specimens are close to crack growth threshold ΔKth, which indicates that ΔKGBF could be regarded as the threshold value governing the beginning of stable fatigue crack propagation. The increase of hydrogen content tends to reduce ΔKGBF.

Very high cycle fatigue properties of bearing steel under axial loading condition

It has been observed to appear a step-wise or duplex S– N curve under the test of rotary bending fatigue using high strength steel. This behavior was caused by the transition of fracture mode from surface-induced fracture to subsurface inclusion-induced fracture. The aim of this study is to clarify the S– N characteristic under an axial loading fatigue in the very high cycle fatigue regime. In order to investigate the mean-stress effects, fatigue tests were carried out in air at room temperature under three applied stress ratios of −1, 0 and 0.5 using a hour-glass shaped specimen of high carbon–chromium bearing steel, JIS SUJ2. From the results, three types of fracture mode were observed on the fracture surface, such as surface-induced fracture, subsurface inclusion-induced fracture without granular bright facet (GBF) area and that with GBF area around an inclusion. Fatigue lifetime for transition in the fracture mode depended on the applied stress ratio. Shape of the S– N curve was a smooth and continuous under three testing conditions in spite of the occurrence on the three types of fracture. Detail discussion for fatigue fracture behavior was made through the observation of fracture surface and from point in view of the fracture mechanics. In addition, an effect of residual stress in the specimen surface layer on the transition of fracture mode was discussed and compared with the experimental results.

Effect of Non-Metallic Inclusion Size and Residual Stresses on Gigacycle Fatigue Properties in High Strength Steel

Fatigue fracture of some high-strength steels occurs at small defect in the subsurface zone of a material at low stress amplitude level and in a high-cycle region of more than 106 cycles (gigacycle fatigue life), whereas surface fatigue crack initiation occurs at high-stress amplitude and low cycles. There is a definite stress range where the crack initiation site changes from a surface to a subsurface defect, giving a step-wise S-N curve or a duplex S-N curve. From the experimental results, fatigue fracture mode was classified into three types, such as, surface inclusion induced fracture mode, subsurface inclusion induced fracture mode without granular bright facet (GBF) area and that with the GBF, depending on stress amplitude level and stress ratio. The GBF area was observed in the vicinity of a non-metallic inclusion at the fracture origin inside the fish-eye in gigacycle fatigue regime. It was made clear from the discussion with fracture mechanics that the transition of fracture mode was affected by compressive residual stresses on the specimen surface. Fracture-mode transition diagram was proposed through the experimental and theoretical investigation. Also, from the evaluation of the fatigue life based on the estimated subsurface crack growth rate from the S-N data, effect of inclusion size on the dispersion of fatigue life was explained, and S-N curve for subsurface inclusion-induced fracture depended on the inclusion size was provided.

Internal Fatigue Failure Mechanism of High Strength Steels in Gigacycle Regime

Gigacycle fatigue behavior in high-strength steels tested under rotary bending fatigue was summarized in this paper. Characteristic of the very high cycle fatigue is to be caused the transition of fracture mode from surface-induced fracture to subsurface inclusion-induced one. In the vicinity of an inclusion at the origin of internal crack, granular-bright-facet (GBF) area was formed during extremely long fatigue cycles. It was pointed out that the formation of GBF area was an important factor for the control of the internal fatigue fracture in gigacycle regime. The GBF area revealed a very rough granular morphology compared with the area outside the GBF inside the fish-eye zone, and was related to the carbide distribution in the microstructure of the matrix. From the detailed observation of fracture surface and computer simulation by FRASTA method, the GBF area formation mechanism in a gigacycle fatigue regime was proposed as the ‘dispersive decohesion of spherical carbide model’.

Effect of Tempering Temperature on Very High Cycle Fatigue Behavior in Low-Alloy Steel, SNCM439

In order to investigate the influence of the tempering temperature on a S-N characteristic in very high cycle fatigue regime, cantilever-type rotary bending fatigue tests up to 109 cycles were performed in air at room temperature using hour-glass shaped specimens heat treated at four kinds of tempering temperature in low-alloy steel, JIS SNCM439. Specimens tempered at low temperature of 433K and 573K showed a step-wise S-N curve, and were ruptured by internal fracture mode at high cycle fatigue regime above 106cycles. On the other hand, surface fracture mode only appeared on the specimens tempered at high temperature of 773K and 893K, and the fatigue limit existed clearly. Granular bright facet (GBF) area was observed in the vicinity of a non-metallic inclusion which was origin of an internal fracture. Properties of the GBF area were verified from the experimental facts that the roughness of the GBF area was larger than that outside the GBF in a fish-eye, the size and the distribution of convex particles on the GBF area were same as those of spherical carbide particles in the matrix, and a high density of carbon was detected on the GBF area by EPMA. It was suggested that a decohesion of spherical carbide from the matrix for the formation of the GBF area was affected by the size of the carbide which became large with decreasing the tempering temperature.

Axial Loading Fatitgue Properties of Low Alloy Steel, SNCM439, in Very High Cycle Regime

In order to study the fatigue properties of low alloy steel, JIS SNCM439, under axial loading condition in very high cycle regime, fatigue test under the stress ratio of -1 was carried out in air at room temperature using the hourglass shaped specimen. From the experimental results, clear duplex S-N curve existed in this material ; that is, there were two curves composed of a surface inclusion-induced fracture mode and a subsurface inclusion-induced fracture mode with granular bright facet (GBF) area in the vicinity of inclusion. Roughness of the GBF area was about 2 times larger than the area outside the GBF in the fish-eye zone relating to the size and distribution of carbide particle in the matrix. An appearance of the dual-stage S-N behavior was related to the difference of threshold for crack initiation and crack growth rate between surface inclusion and subsurface inclusion. Stress amplitude for transition of the fracture mode was explained by considering an effect of compressive residual stress on the surface layer. From the evaluation of fatigue life based on the estimated subsurface crack growth rate from the S-Ndata, effect of inclusion size on the dispersion of fatigue life was explained, and S-N curve for subsurface inclusion-induced fracture depended on the inclusion size was provided.

Effect of Stress Ratio on Axial-Loading Fatigue Characteristics of High Carbon-Chromium Bearing Steel in Very High Cycle Fatigue Regime

Axial-reversed loading fatigue tests under three applied stress ratios of –1, 0 and 0.5 were carried out in air at room temperature using hour-glass shaped specimen of high carbon-chromium bearing steel, JIS SUJ2, to investigate fatigue fracture characteristic in very high cycle regime. A granular bright facet (GBF) was observed around subsurface inclusion in all specimens ruptured at long-life fatigue over 106cycles. Three types of fracture mode were observed from the fracture surface, such as surface-induced fracture, subsurface inclusion-induced fracture without GBF area and that with GBF area. Number of cycles to transition in the fracture mode depended on the applied stress ratio. Shape of the S–N curve obtained from the axial loading fatigue was a smooth and continuous in spite of the occurrence on three types of fracture mode, and was different from that of rotary bending fatigue. The characteristic of fatigue fracture behavior was discussed from point of view in fracture mechanics and also residual stress distribution on the specimen surface.

Subsurface Crack Initiation and Propagation Mechanism under the Super-Long Fatigue Regime for High Speed Tool Steel (JIS SKH51) by Fracture Surface Topographic Analysis

In order to study the subsurface crack initiation and propagation mechanism of high strength steel under a very high cycle fatigue regime, computational simulation with fracture surface topographic analysis (FRASTA) was carried out for subsurface fatigue crack initiated specimens of high speed tool steel (JIS SKH51) obtained from the rotating bending fatigue test in air. A remarkable area formed around the nonmetallic inclusion inside the fish-eye region on the fracture surface, which is a feature on the fracture surface in super long fatigue. This so-called GBF (granular-bright-facet) was observed in detail by a scanning probe microscope and a three-dimensional SEM. The GBF area, in which a rich carbide distribution was detected by EPMA, revealed a very rough and granular morphology in comparison with the area inside the fish-eye. It was clearly simulated by FRASTA that multiple microcracks were initiated and dispersed by the decohesion of a spherical carbide from the matrix around a nonmetallic inclusion, and converged into the GBF area during the fatigue process. After the formation of the GBF area, interior cracks grew radially and a fish-eye pattern formed on the fracture surface.

Effect of Stress Ratio on Subsurface Fatigue Fracture of High-Speed Tool Steel, SHK51

The effect of stress ratio (R=σmin/σmax) on subsurface fatigue fracture of high-speed tool steel, SKH51, was investigated using smooth specimen subjected to axial reversed loading in air at room temperature. From the experimental results, interior inclusion induced fracture occurred in each stress ratio. The number of cycles to the transition from surface fracture mode to subsurface fracture mode depended on R. A granular-bright-facet (GBF) area formed around the nonmetallic inclusion inside the fish-eye zone on the fracture surface of the specimen in long-life fatigue regime, Nf>106, at R=-1.3, -1 and 0. From the detail observation of GBF area by three-dimensional SEM, the roughness of GBF area depended on the stress intensity factor range, Δkinc, s, at the inclusion of facture origin.

Subsurface Crack Initiation and Propagation Mechanism in the Super-Long Fatigue Regime for High Speed Tool Steel, JIS SKH51, by Fracture Surface Topographic Analysis

In order to study the subsurface crack initiation and propagation mechanism of high strength steel in very high cycle fatigue regime, computational simulation with fracture surface topographic analysis (FRASTA) was carried out for subsurface fatigue crack initiated specimens of high speed tool steel, JIS SKH 51, obtained from the rotating bending fatigue test in air. A remarkable area formed around the nonmetallic inclusion inside the fish-eye region on the fracture surface, which is a feature on the fracture surface in super long fatigue and named as GBF (granular-bright-facet), was observed in detail by a scanning probe microscope and a three-dimensional SEM. The GBF area, in which a rich carbide distribution was detected by EPMA, revealed a very rough and granular morphology in comparison with the area inside the fish-eye. It was clearly simulated by FRASTA method that multiple microcracks initiated depressively by the decohesion of spherical carbide from the matrix around a nonmetallic inclusion and coalesce each other into the GBF area during fatigue process. After the formation of GBF area, interior crack grew radially and the fish-eye pattern formed on the fracture surface.

トポグラフィ破面解析による高強度鋼の内部疲労き裂発生・進展機構に関する研究

In order to study the mechanism of the subsurface crack initiation and propagation of high strength steel in very high-cycle region, computational simulation with fracture surface topographic analysis (FRASTA) method was carried out for specimens of high-carbon-chromium bearing steel, JIS SUJ2, obtained from the rotary-bending fatigue test in air. A remarkable area formed around the nonmetallic inclusion inside the fish-eye zone on the fracture surface, which is a feature on the fracture surface in superlong fatigue and named as GBF (granular-bright-facet), was observed in detail by a scanning probe microscope and a three-dimensional SEM. The GBF area, in which a rich carbide distribution was detected by EPMA, revealed a very rough and granular morphology in comparison with the area inside the fish eye. It was clearly simulated by the FRASTA method that multiple microcracks initiate dispersively by the decohesion of spherical carbide from the matrix around a nonmetallic inclusion and coalesce each other into the GBF area during fatigue process. After formation of the GBF area, interior crack grew radially and the fish-eye pattern formed on the fracture surface.