Fish-eye Fracture Research Articles

Retained Austenite Reduction near Fracture Surface in Repeatedly Quenched SUJ2 Steel

High-carbon high-strength JIS-SUJ2 bearing steel is an alloy having the characteristics of high wear resistance and fatigue strength as a result of quenching. When this kind of high-strength material is exposed to long-lasting low stress application loadings, fisheye fractures can often be seen. In this research, we measured the retained austenite near the fracture surface in JIS-SUJ2. After the measurement, we confirmed the decrease of the retained austenite in the vertical surface under both fisheye fracture and ductile fracture.

Comparison of the Internal Fatigue Crack Initiation and Propagation Behavior of a Quenched and Tempered Steel with and without a Thermomechanical Treatment

Previous studies have shown that a thermomechanical treatment (TMT) consisting of cyclic plastic deformation in the temperature range of dynamic strain aging can increase the fatigue limit of quenched and tempered steels by strengthening the microstructure around non-metallic inclusions. This study considers the influence of a TMT on the shape, size and position of crack-initiating inclusions as well as on the internal crack propagation behavior. For this, high cycle fatigue tests on specimens with and without TMT were performed at room temperature at a constant stress amplitude. The TMT increased the average lifetime by about 40%, while there was no effect of the TMT on the form or size of critical inclusions. Surprisingly, no correlation between inclusion size and lifetime could be found for both specimen types. There is also no correlation between inclusion depth and lifetime, which means that the crack propagation stage covers only a small portion of the overall lifetime. The average depth of critical inclusions is considerably higher for TMT specimens indicating that the strengthening effect of the TMT is more pronounced for near-surface inclusions. Fisheye fracture surfaces around the critical inclusions could be found on all tested specimens. With increasing fisheye size, a transition from a smooth to a rather rough and wavy fracture surface could be observed for both specimen types.

In-depth analysis of crack area characteristics of fisheye failures influenced by the multiaxial stress condition in the tooth root fillet of high-strength gears

Modern high-strength gears have to satisfy many requirements, such as improved tooth root bending strength. The process of shot-peening is correlated to the introduction of compressive residual stresses in the surface layer of a gear to achieve a higher tooth root bending strength. However, due to the compressive residual stresses fisheye failures can occur and can have a determining effect on the endurance of high-strength gears. By preventing such failures, it should be possible to increase further the tooth root bending strength of high-strength gears. However, this requires a deeper understanding of the crack initiation and propagation processes. Especially the unique multiaxial stress condition in the tooth root fillet of a gear could influence the crack area characteristics significantly. Though, in the literature there is no proper characterization of crack area characteristics in the tooth root fillet of gears in detail, so far. Furthermore, in previous work a model approach for the evaluation of the tooth root bending strength of gears was presented, which is based on the results of Murakami. A first comparison with experimental data showed a basic applicability of the model approach on gears. However, the derived model approach showed some room for improvement. Questions arose as to whether the approach is really fully applicable to gears, whether further modifications are needed, or whether further extension is even practical, since the fisheye fracture characteristics of gears might differ significantly from those of standard specimens. The aim of this paper is therefore to present an extensive in-depth analysis of the crack area characteristics in the event of tooth root fracture damages caused by a fisheye failure in high-strength gears. Furthermore, a case study is used to verify whether a detailed evaluation of the characteristics of non-metallic inclusions leads to more accurate results of the model approach.

On the fatigue behavior of differently deep rolled conditions of SAE 1045 in the very-high-cycle fatigue regime

Very-high-cycle fatigue (VCHF) tests were performed on differently deep rolled conditions of SAE 1045 steel. The dominant failure mode in the VHCF regime is linked to internal material defects leading to the well-known fish-eye fracture appearance. Despite lower residual stress stability, superior fatigue strength was observed for room-temperature deep rolled conditions as compared to counterparts deep rolled at elevated temperature. Crack initiation and advance is promoted by relatively higher and more stable tensile residual stresses in the core of the high-temperature deep rolled specimen, i.e., within sample volumes where cracks are initiated, eventually resulting in the inferior fatigue strength seen.

Influences of Horizontal and Vertical Build Orientations and Post-Fabrication Processes on the Fatigue Behavior of Stainless Steel 316L Produced by Selective Laser Melting.

In this paper, the influences of build orientation and post-fabrication processes, including stress-relief, machining, and shot-peening, on the fatigue behavior of stainless steel (SS) 316L manufactured using selective laser melting (SLM) are studied. It was found that horizontally-built (XY) and machined (M) test pieces, which had not been previously studied in the literature, in both stress-relieved (SR) or non-stress-relieved (NSR) conditions show superior fatigue behavior compared to vertically-built (ZX) and conventionally-manufactured SS 316L. The XY, M, and SR (XY-M-SR) test pieces displayed fatigue behavior similar to the XY-M-NSR test pieces, implying that SR does not have a considerable effect on the fatigue behavior of XY and M test pieces. ZX-M-SR test pieces, due to their considerably lower ductility, exhibited significantly larger scatter and a lower fatigue strength compared to ZX-M-NSR samples. Shot-peening (SP) displayed a positive effect on improving the fatigue behavior of the ZX-NSR test pieces due to a compressive stress of 58 MPa induced on the surface of the test pieces. Fractography of the tensile and fatigue test pieces revealed a deeper understanding of the relationships between the process parameters, microstructure, and mechanical properties for SS 316L produced by laser systems. For example, fish-eye fracture pattern or spherical stair features were not previously observed or explained for cyclically-loaded SLM-printed parts in the literature. This study provides comprehensive insight into the anisotropy of the static and fatigue properties of SLM-printed parts, as well as the pre- and post-fabrication parameters that can be employed to improve the fatigue behavior of steel alloys manufactured using laser systems.

Fatigue properties and microstructure of quasicrystalline AlFeCrTi alloy

Aluminium based material having chemical composition of Al93Fe3Ti2Cr2 (at.%) was prepared by hot extrusion of gas atomised powder. Microstructure consisting of aluminium matrix and icosahedral quasicrystalline particles was investigated by scanning and transmission electron microscopy a X-ray spectroscopy. Size, distribution and inner structure of quasicrystalline particles were determined. When quasicrystalline particles having submicron size were present in the microstructure together with fine-grained aluminium matrix, it resulted in enhancement of mechanical properties. Fatigue experiments were performed in load controlled regime with positive mean stress and fatigue life data for this quasicrystalline material were compared with similar results for commercially available aluminium alloys. Longer fatigue life together with higher fatigue strength reaching value of 350MPa was observed. It is approx. 100MPa higher than for commercially produced coarse-grained aluminium alloys. Fractographic observations revealed presence of fish eye fracture patterns and validity of weakest link fracture mechanism acting in this type of material under investigation.

Cutting and rubbing process for a Ti-6Al-4V alloys, and its effects

Aiming at an enhanced fatigue strength of Ti-6Al-4V alloys, the severe plastic deformation was given to the specimen surface at the final stage of lathe turning. The main findings of this study can be summarized as follows. A chip with unique shape for the surface treatment was developed. By using the chip, a fine-grained structure was formed on the entire surface of the Ti-6Al-4V alloy specimen during the cutting and rubbing process. The fine-grain structure with 500nm grain size was generated at about 20μm below the surface. Vickers hardness values (HV) of the specimen surface after the cutting and friction processing was reached about HV of 500 (40% higher than preprocessing). Although some studies indicated negligible increases in fatigue strength of titanium alloy specimen processed by surface treatments such as nitriding, shot-peening, the fatigue life of the cutting and friction processed specimen was much longer than that of non-surface treated specimens. Regarding the fatigue strength at 107 cycles, no enhancement due to the present surface processing was attained, because of the fish-eye fracture. The fracture was brought by the cracking of α grains located beneath the fine-grain surface layer.

G031062 高強度鋼の真空環境下における微小き裂の極低速き裂進展挙動([G031-06]材料力学部門,一般セッション : き裂疲労)

It is known that high strength steel will cause the characteristic internal fracture called Fish eye fracture in very high cycle region. From this, in order to use high strength steel more efficiently and more safely, the elucidation of the mechanism of the Fish eye fracture produced in low stress and long-life and construction of the lifetime evaluation method of Fish eye fracture are desired. In this study, we did ΔK decrease fatigue test under vacuum environment, and examined the relation between internal fracture in very high cycle region and extremely slow fatigue crack growth behavior under vacuum environment. As a result, the influence of material strength was not seen under vacuum environment. Moreover, opening stress increases with increases of maximum stress.

Fatigue life analysis of shot-peened bearing steel

Shot-peening effects on the fatigue life behavior of bearing steel (JIS SUJ2) have been investigated. Hourglass-shaped test specimens were heat-treated and then surface-treated using a shot-peening machine. Results of a rotary bending fatigue test showed that shotpeening suppressed not only much of the surface-originated fracture but also the scattering error of the probabilistic stress-life data, and improved the fatigue life by about 6 times through the load levels of the cyclic tests. Such large increase in fatigue life was driven by the following reasons: The increase of hardness in the skin caused the predicted fatigue limit stress to increase by 15%; fracture-initiating inclusions were distributed at deeper locations, experiencing low nominal stresses; and the increase in the fish-eye fracture size by an average of 180%.

Fatigue Properties of an Ultrafine-Grained Steel Processed by Warm Tempforming

Giga-cycle fatigue properties were investigated for an ultrafine-grained steel processed by warm tempforming. The fatigue properties of the tempformed (TF) steel were compared with conventional tempered martensite (QT) steel. Fatigue tests were carried out at frequencies of 20Hz and 20kHz, using servo-hydraulic and ultrasonic fatigue testing machines, respectively. Both theTF and the QT steels showed surface fracture in short life regions and fish-eye fracture in long life regions. The fish-eye fracture origins were oxide inclusions comprised of Si, Mg, Ca other than Al. Difference of fatigue strength was not notable between the TF and the QT steels, i.e. the fatigue strength of the TF steel was the same as that of the QT steel.

Effect of Hydrogen Charging on Low Cycle Fatigue Life and its Dependence on Cementite Morphology

Hydrogen deteriorates the strength of steel, and it is well known as hydrogen embrittlement. The susceptibility to this embrittlement is generally increased more with higher strength steels under the usual static loading. Meanwhile fatigue strength is more important in structural use; it is desired to clarify the effect of hydrogen on the properties under cyclic loading. Present paper deals with the effects of hydrogen entry on the strain controlled low cycle fatigue properties of low carbon steel (JIS S10C), which is the most basic kind of steels. With the information about hydrogen in steels, the extent of hydrogen damage in fatigue life and its dependence to cementite morphology were discussed. The total hydrogen content of 0.5–1.5 mass-ppm severely degrades the fatigue life of normalized S10C, which is not so strong and almost insusceptible to hydrogen embrittlement under static loading. The fracture surface is accompanied with fish-eye fracture surface. This is originated at inclusion and propagating with quasi-cleavage and vague striation. The hydrogen gas atmosphere formed around inclusion during cyclic loading is claimed to be the major reason for the degradation. Spheroidizing heat treatment makes the steel free from the degradation. The hydrogen claimed for the degradation of fatigue life is what has been trapped by pearlite or cementite/ferrite interface, because this interface is reduced by the spheroidizing treatment. It is not the hydrogen that was trapped by non-metallic inclusion before fatigue test.

OS0614 温間テンプフォーミング処理を施した超微細粒鋼のギガサイクル疲労特性(OS6-3 材質および熱処理の影響,OS-6 金属材料の超高サイクル疲労と信頼性評価)

Giga-cycle fatigue properties were investigated for an ultrafine-grained steel processed by warm tempfonning. The fatigue properties were compared with conventional tempered martensite steel. Fatigue tests were carried out at frequencies of 30Hz and 20kHz, using servo-hydraulic and ultrasonic fatigue testing machines, respectively. Both tempformed and conventional steels showed surface fracture in short life regions and fish-eye fracture in long life regions. The fatigue strength at 10^7 cycles was higher in the tempformed steel than in the conventional, while it was not in case of the 10^<10> cycles fatigue strength. The fish-eye fracture origins showed different aspects between the two types of the steel. In case of the conventional steel, Al_2O_3 inclusions caused the fish-eye fracture, while in case of the tempformed steel, Mo carbides surrounded by Mo-rich layers were observed at the origins. Sizes of the Mo carbides were smaller, while those of the Mo-rich layers were identical to the Al_2O_3 inclusions of the conventional steel. Therefore, the Mo-rich layers, as well as the carbides, would probably cause the fish-eye fracture. This means that suppression of the Mo-rich layers could lead to more improvement of the giga-cycle fatigue strength of the tempformed steel.

Size effects in gigacycle fatigue of high-strength steel under ultrasonic fatigue testing

Ultrasonic fatigue testing was conducted for high-strength steel using enlarged specimens with a straight section. The results showed good agreement with conventional servo-hydraulic fatigue test results under conditions where fish-eye fracture occurs, showing the validity of ultrasonic fatigue testing using the enlarged specimens. When the results were compared with those using conventional specimens, size effects were clearly observed: enlarged specimens showed lower fatigue strength due to appearance of larger inclusions in the fish-eye fracture origin.

Effect of hydrogen absorption on strain-induced low-cycle fatigue of low carbon steel

The effects of hydrogen have been studied on low carbon steel (JIS S10C), which contains limited amount of pearlite. Fatigue life and its strain rate dependence have been examined. The possible mechanism of hydrogen effect on those properties has been discussed. The hydrogen absorbed was 0.5–1.5 mass-ppm, according to the thermal desorption analysis (TDA). Main part of the hydrogen is dissolved in the matrix; the rest part is trapped by pearlite. This small hydrogen severely reduces the fatigue life, even in S10C steel that is considered to be less susceptible to hydrogen embrittlement. The more the hydrogen is absorbed, the less the fatigue life is. Hydrogen absorption also causes the increase in stress amplitude. Fractographic examination revealed the fracture process as the followings. The hydrogen gasifies at the interface between the non-metallic inclusion and matrix. Then the high pressure hydrogen gas atmosphere maintains hydrogen content in the matrix around inclusion. This assists the crack initiation at the interface of inclusion, and also assists the crack propagation through producing the fish-eye fracture. According to the published works on non-hydrogen charged specimens, vacancy plays an important role in both crack initiation and propagation, and determines the fatigue life. The stress amplitude is believed also influenced by the vacancy that is produced by the movement of jogged dislocation. When these vacancies are stabilized by hydrogen, fatigue life will be decreased with the increased stress amplitude. Fatigue life is decreased with slower strain rate test. Stress amplitude shows the negative dependence to the strain rate. These are claimed as the features of dynamic strain aging (DSA). Considering the above mentioned knowledge on vacancy stabilized by hydrogen, the DSA is brought about by the interaction between dislocation and vacancy-hydrogen pair.

Influence of grain size and precipitates on the fatigue lives and deformation mechanisms in the VHCF-regime

It is meanwhile established that the “classical” fatigue limit in the Wöhler-SN-plot at approximately 107 cycles is not generally valid for the design of components subjected to loading in the VHCF-regime. Fatigue failure of metallic materials might occur even at stress amplitudes below the “classical” fatigue limit in the VHCF regime. However, the principal understanding of the relevant damage mechanisms and of the influence of the microstructure on the fatigue lives in this regime is rather poor. It is also established to distinguish between single-phase and inclusion free (type I) materials and multiphase (type II) materials when discussing the VHCF-fatigue behaviour. For type II materials the fatigue cracks in the VHCF regime mainly originate at nonmetallic inclusions located in the interior, the so-called fish-eye fracture, often reported for high strength steels and other alloys. However, crack initiation in the interior without forming typical fish eye type fracture and without any clear influence of inclusions were also reported for aluminium alloys. The reasons for the formation of these so-called “featureless crack initiation sites” are unclear. On the other hand, in single phase type I materials crack initiation in the VHCF-regime start from the surface of the specimen, triggered by repeated, not fully reversible plastic deformation in the grains adjacent to the surface. Due to the not fully reversible and localized plastic deformation surface roughening takes place and micro-persistent slip bands (PSB) form. In order to identify relevant deformation- and damage mechanisms commercial pure (CP) aluminium AA1050 and the aluminium alloy AA6082 were investigated in the very high cycle fatigue (VHCF) regime up to 1×1010 cycles. The evolution of surface roughening was investigated with light, scanning electron and atomic force microscopy. It is shown, that the grain size, ranging from the micrometer to the nanometer range significantly affects the VHCF-behaviour. For CP Al with a grain size of ca. 55 μm no late failure was obtained, whereas for the same material with an ultrafine grain size of ca. 600 nm a stepwise SN-curve was found. In the case of the precipitation hardened material AA6082 it is found that the local mechanical properties of the matrix significantly governs the crack initiation site and hence the VHCF-behaviour.

1129 内部破壊を示すばね鋼の疲労特性に対する平均応力の影響(OS11-7 材料の疲労挙動と損傷評価-疲労損傷・寿命予測-)

Fatigue tests were conducted for 1800 MPa-class spring steels at various stress ratios. For comparison, similar fatigue tests were conducted for conventional steels whose tensile strength was lower than 1200 MPa. The spring steels exhibited fish-eye fractures, and the origins of these fractures were oxide, TiN and the matrix itself. In contrast, the conventional steels never exhibited fish-eye fractures. The fatigue strength of these steels decreased monotonously as the stress ratio increased, when the fatigue strength was evaluated in tenns of stress amplitude. With these results, application of conventional methods to evaluate mean stress effects, such as modified-Goodman diagram, a power law of a stress ratio and so on, were discussed in case of fish-eye fracture of high-strength steel. As the result, the difference between the spring and conventional steels was negligible. This means that the conventional methods were applicable to the case of fish-eye fracture of high-strength steel within the scope of this research.

ばね鋼SUP7のギガサイクル疲労特性に及ぼす試験片寸法の影響

Ultrasonic fatigue tests up to gigacycle regimes were conducted for two heats of SUP7 spring steels using φ7×20 mm and φ3 mm specimens whose risk volumes were 912 and 33 mm3, respectively. As the result, the φ7×20 mm specimens revealed much lower fatigue strength than the φ3 mm specimens, showing fish-eye fracture regardless of the specimen types. The fish-eye fracture origins were an Al2O3 inclusion in most cases, while in case of the φ3 mm specimens of heat C, a TiN inclusion and matrix itself also caused fish-eye fracture. The Al2O3 inclusion sizes of the φ7×20 mm specimens were larger than those of the φ3 mm specimens, which was the reason why the φ7×20 mm specimens showed lower fatigue strength. The minimum size of the Al2O3 inclusion observed at the fish-eye fracture origin was about 10 μm. This implied that when the specimen contained no Al2O3 inclusion exceeding 10 μm in the risk volume, a TiN inclusion or matrix itself could cause fish-eye fracture as in case of the φ3 mm specimens of heat C. These results meant that the effects of the specimen size were large in case of fish-eye fracture, recommending us to use large specimens, such as the φ7×20 mm specimens, in conducting gigacycle fatigue tests on high-strength steel.

高強度鋼の疲労特性に及ぼす応力比の影響

Fatigue tests were conducted for three heats of 1800 MPa-class spring steels under various R ratios. For comparison, similar fatigue tests were conducted also for conventional steels whose tensile strength was lower than 1200 MPa. The spring steels showed fish-eye fracture, eliminating conventional fatigue limits, and the fish-eye fracture origins were oxide, TiN and matrix itself, respectively. On the other hand, the conventional steels never showed fish-eye fracture and clearly showed fatigue limits. Fatigue strength of these steels was monotonously decreased according to increase of the R ratio, when the fatigue strength was evaluated with stress amplitude. However, the degradation of the fatigue strength was smaller than that expected from a modified-Goodman line, and the best fit line was obtained by drawing σW(R=−1)–σT line. When the stress ratio effect was evaluated with a function of (12R)γ, the estimated γ value was 0.5. In these results, difference was negligible between the spring and conventional steels. This meant that the stress ratio effect could be evaluated in conventional manners even in case of high-strength steel showing fish-eye fracture, regardless of the fish-eye fracture origin types.

窒化した超微細フェライト‐セメンタイト組織鋼の疲労特性に及ぼす諸因子の影響

Rotating bending fatigue tests were conducted for a series of plasma-nitrided ultrafine ferrite-cementite steels 5 types of the ultrafine ferrite–cementite steels, i.e., 15C, 45C, 15C-P, 45C-P and low-Mn, were prepared with a double-melting method to improve their cleanliness. 15C and 45C contained 0.15 and 0.45 mass% of carbon, respectively. 15C-P and 45C-P were 0.1 mass% phosphorus-added versions of them. Low-Mn was a Mn-decreased version of 45C. All of the nitrided specimens had a hardened layer of 1 mm in depth, while the amount of hardening was small in low-Mn. In case of 15C and 15C-P, the hardness beneath the hardened layer largely fell due to grain growth during the nitriding, in contrast to 45C and 45C-P in which the grain growth was successfully suppressed. In the fatigue tests, due to the cleanliness improvement, inclusions never caused fish-eye fracture even in the nitrided specimens. Although the local stress amplitudes beneath the hardened layer exceeded the fatigue limit expected from the local hardness, the nitrided specimens showed surface fracture only. The fatigue strength improvements due to the nitriding were larger in 45C and 45C-P than 15C-P, and those improvements in 15C and low-Mn were negligible. These results meant that C-increase and P-addition improved the surface fracture properties. The results of low-Mn demonstrated that the hardened layers of the nitrided specimens were strengthened by Mn nitrides.

Specimen size effects on gigacycle fatigue properties of high-strength steel under ultrasonic fatigue testing

Ultrasonic fatigue testing was conducted for high-strength steel, employing enlarged specimens with a straight section. The results showed good agreement with conventional servohydraulic fatigue test results, demonstrating the validity of ultrasonic fatigue testing using these enlarged specimens under conditions where fish-eye fractures occur. When the results were compared with those using conventional specimens, specimen size effects were clearly observed: enlarged specimens showed lower fatigue strength due to appearance of larger inclusions in the fish-eye fracture origin.