Fatigue Tests In Air Research Articles

Conducting thermomechanical fatigue test in air at light water reactor relevant temperature intervals

In Light Water Reactors (LWR), many structural components are made of austenitic stainless steels (SS). These components are subject to extreme conditions, such as large temperature gradients and pressure loads during service. Hence, the fatigue and fracture behavior of austenitic SS under these conditions has evoked consistent interest over the years. Most studies dealing with this problem in the past, investigated the isothermal fatigue (IF) condition, which is not the case in the service, and less attention has been paid to thermomechanical fatigue (TMF). Moreover, the existing codes of practice and standards for TMF testing are mainly derived from the high temperature TMF tests ( T mean > 400 °C). This work presents the development of a facility to perform TMF tests under LWR relevant temperature interval in air. The realized testing parameters and tolerances are compared with the recommendations of existing codes of practice and standards from high temperature tests. The effectiveness of the testing facility was verified with series of TMF and IF tests performed on specimens made out of a commercial austenitic SS TP347 pipe material. The results revealed that the existing tolerances in standards are quite strict for the application of lower temperature ranges TMF tests. It was found that the synchronous, in-phase (IP) TMF tested specimens possess a higher lifetime than those subjected to the asynchronous, out-of-phase (OP) TMF and IF at T max in the investigated strain range for austenitic SS. Nevertheless, the fatigue lifetime of all the test conditions was similar in the engineering scale.

602 高強度アルミニウム合金の疲労特性に及ぼす重水素の影響(OS4-(1)オーガナイズドセッション《材料と組織》)

So far, 6061 aluminum alloy has been being used as a liner material of high-pressure hydrogen containers for fuel cell vehicles, while higher-strength alloy is being demanded to increase the maximum fill pressure with a minimum increase in cost and weight. In this study, aluminum alloy plates of 6061 and 7075 both T6-tempered are subjected to fatigue test in air and heavy-water-vapor atmosphere at 303K with relative humidity of 90% (HWA). The amount of deuterium in front of the crack tip is measured by means of thermal desorption spectroscopy, and the effect of hydrogen on the fatigue crack propagation is discussed based on the results obtained. It was found that crack propagation rate of aluminum alloys was little affected by external environmental hydrogen in HWA. Deuterium was detected in front of the crack tip, which means that hydrogen is introduced into the aluminum alloys from humid environment during the crack propagation.

Effect of Nitriding on Very High Cycle Fatigue Properties of Developed High Speed Steel

Evaluation of fatigue properties for new high speed steel nitrided was performed through a cantilever-type rotating bending fatigue test in air at room temperature in very high cycle fatigue (VHCF) regime. The high speed steel was made experimentally by control the chemical compositions to avoid formation of the GBF area near subsurface inclusion at crack origin, based on internal failure mechanism as ‘dispersive decohesion of spherical carbide’ model previously proposed by authors. Surface treatment was applied with both of radical nitriding and low-temperature nitriding. Fatigue strength was increased in the whole of applied stress amplitude levels by both nitridings. Fatigue failure mode was changed from a matrix cracking in surface of the untreated specimen to an internal-inclusion induced failure with GBF area of the nitrided specimens in VHCF regime, for the reason of surface layer having high hardness and high compressive residual stress. From the comparison with common high speed steel, JIS SKH51, failed by an internal- inclusion induced failure mode in VHCF regime, fatigue strength of nitrided specimens was increased about 35%. It was concluded that the GBF formation around a subsurface inclusion was restricted by decrease in distribution of small MC-carbide particles in the matrix and a large number of stress cycling were required for the formation of appropriate size for the GBF area to enable propagation as an ordinary crack.

Evaluation of Very High Cycle Fatigue Properties of High-Speed Tool Steels Developed in order to Improve the Fatigue Strength

New high speed tool steels, referred to as NF1 and NF2, were made experimentally by controlling the chemical compositions in order to improve the fatigue strength and evaluated with cantilever-type rotating bending fatigue test in air at room temperature in very high cycle regime. Larger size of MC-carbide particles were precipitated more in matrix of NF2 compared with those of NF1 and common high speed tool steel, JIS SKH51, due to the addition of more carbon and vanadium. From the experimental result of fatigue test, two kinds of the materials showed clear duplex S-N curve, composed of three or two types of failure mode, such as, a surface inclusion-induced failure mode (S-mode), a subsurface inclusion-induced failure mode without (I-mode) and with granular bright facet (GBF) area in the vicinity of inclusion (IG-mode) depending on the stress amplitude level. Fatigue strength of specimen failed by internal failure mode in the high cycle regime was the highest in NF2, followed by NF1 and JIS SKH51. As the result of discussion for the effect of microstructure of materials on subsurface crack initiation and growth behavior, size of inclusion at crack origin was the same among three kinds of the materials and crack propagation rate of NF2 was slower than those of NF1 and JIS SKH51, because the MC-carbide particles in the matrix acted as obstacle to crack growth. Also it was suggested that M6C-carbide particles distributed in the matrix did not affect for subsurface crack growth.

Effect of surface residual stress and inclusion size on fatigue failure mode of matrix HSS in very high cycle regime

Fatigue properties in high cycle and very high cycle fatigue regime of matrix high-speed tool steel (1.3C-6W-5Mo-3V-8Co) were investigated through the experimental results obtained from a cantilever-type rotary bending fatigue test in air at room temperature using hourglass shaped specimens. Two kinds of specimen were processed by grinder and cutting tool, which led to different surface compressive residual stress. Both specimens showed clear duplex S-N curve, composed of three or two types of failure mode depending on the stress amplitude, such as, a surface inclusion-induced failure mode (S-mode), a subsurface inclusion-induced failure mode without (I-mode) and with granular bright facet (GBF) area in the vicinity of inclusion (IGmode). The transition of the failure mode from S-mode to I-mode and from I-mode to IG-mode was occurred at high stress amplitude level by the effect of large compressive residual stress on the surface layer. A map of the appearance of failure modes relating the stress amplitude with the residual stress was proposed and showed a good agreement with the experimental results. Fatigue life in very high-cycle regime was almost same between the both specimens because of the existence of almost same size inclusion at crack origin. S-N curve depended on the inclusion size was provided with the estimation of crack growth rate from the S-N data.

Microstructural changes induced near crack tip during corrosion fatigue tests in austenitic‐ferritic steel

Microstructural changes occurring during fatigue tests of austenitic-ferritic duplex stainless steel (DSS) in air and in hydrogen-generating environment have been investigated. Hydrogen charging of steel samples during fatigue crack growth (FCG) tests was performed by cathodic polarization of specimens in 0.1M H(2)SO(4) aqueous solution. Microstructural investigations of specimens after FCG tests were carried out using transmission electron microscopy to reveal the density and arrangement of dislocations formed near crack tip. To determine the way of crack propagation in the microstructure, electron backscatter diffraction investigations were performed on fatigue-tested samples in both kinds of environment. To reveal hydrogen-induced phase transformations the atomic force microscopy was used. The above investigations allowed us to define the character of fatigue crack propagation and microstructural changes near the crack tip. It was found that crack propagation after fatigue tests in air is accompanied with plastic deformation; a high density of dislocations is observed at large distance from the crack. After fatigue tests performed during hydrogen charging the deformed zone containing high density of dislocations is narrow compared to that after fatigue tests in air. It means that hydrogenation leads to brittle character of fatigue crack propagation. In air, fatigue cracks propagate mostly transgranularly, whereas in hydrogen-generating environment the cracks have mixed transgranular/interfacial character.

Low cycle fatigue tests of surface modified T91 steel in 10 −6 wt% oxygen containing Pb 45Bi 55 at 550 °C

Low cycle fatigue tests in air and LBE containing 10 −6 wt% dissolved oxygen were conducted with T91 steel at 550 °C. T91 was employed in two modifications, one in the as-received state, and the other after alloying FeCrAlY into the surface by pulsed electron beam treatment (GESA process). Tests were carried out with symmetrical cycling ( R = −1) with a frequency of 0.5 Hz and a total elongation Δ ε t/2 between 0.3% and 2%. No influence from LBE on fatigue could be detected. Results in air and LBE showed similar behaviour. Additionally, no difference was observed between surface treated and none treated T91 specimens.

Subsurface crack initiation and propagation mechanism in high-strength steel in a very high cycle fatigue regime

The mechanism of subsurface crack initiation and propagation in high strength steel in a very high-cycle fatigue region was studied by a computational simulation using a fracture surface topographic analysis (FRASTA) method for specimens of a high carbon chromium bearing steel with data obtained from rotary bending fatigue testing in air. Distinctive features of the fracture surface formed in the vicinity of a non-metallic inclusion at the fracture origin inside the fish-eye zone, the GBF area, was observed in detail using a scanning probe microscope and a three-dimensional SEM for comparing the microstructures of the materials. The GBF area, in which a rich carbon distribution was detected by electron probe microanalysis, revealed a rough granular morphology compared with the area inside the fish eye. It was clearly simulated by the FRASTA method in which during the fatigue process multiple microcracks are initiated dispersively by decohesion of spherical carbide from the matrix around a non-metallic inclusion, and coalesce with each other into the GBF area. In the study, the mechanism for the formation of the GBF area was proposed as the ‘dispersive decohesion of spherical carbide’ model.

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.

Effects of Hydrogen Gas Environment on Non-Propagation Phenomena of a Type 304 Austenitic Stainless Steel

In order to investigate the hydrogen gas effect on non-propagation phenomena of a type 304 austenitic stainless steel, fatigue tests with in-situ observation using a Scanning Laser Microscope were performed in air, in 0.18MPa hydrogen gas and in 0.18MPa nitrogen gas. A nonpropagating crack was observed during the fatigue test in air. At almost the same stress level of non-propagating in air, non-propagating cracks were also observed in fatigue tests in hydrogen and in nitrogen. Stress level of the non-propagation is not sufficiently different in the three environments. However, the process up to non-propagation differs from each other, for example, the crack path and debris.

Zr基バルクアモルファス合金の疑似体液中フレッティング疲労特性

Fretting fatigue properties were studied in air and in a simulated body fluid, PBS(−), using a commercial Zr-based amorphous alloy (7.6Ni-12.3Cu-3.5Al-76.6Zr in mass%). Fretting fatigue tests were carried out under load control using a sinusoidal wave with a stress ratio of 0.1, a frequency of 20 Hz in air and 2 Hz in PBS(−) and a contact pressure of 30 MPa for fretting. There was no difference between the stress-number of cycles to failure (S-N) curves of the plain fatigue test in air and in PBS(−), and the 107-cycle fatigue strength was approximately 150 MPa for both environments. The 107-cycle fretting fatigue strength in air was one-third as high as the plain fatigue strength, while the 2×106-cycle fretting fatigue strength in PBS(−) was approximately twice as high as that in air. Although the friction coefficient between the specimen surface and the fretting pad, which affects the fretting fatigue strength, was hardly different between air and PBS(−), SEM observations showed that the actual contact area of the fretting pad on the specimen surface tested in air was smaller than that tested in PBS(−). Thus, we considered that the decrease of the fretting fatigue strength in air was caused by the higher contact pressure of the pad in air than that in PBS(−). XPS analyses also showed that the specimens tested in air and in PBS(−) had the different compositions of the oxide films on their surfaces, suggesting that the difference in the composition of the oxide films affect the fretting damage and thus the fretting fatigue strength.

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.

Corrosion Behaviour of Environmentally Friendly Treatments for Aluminium Alloys

In this paper an alternative anodising procedure, that confers a good base for painting and adhesive bonding of aluminium, is envisaged to replace the chromic acid anodising The substrate consisting of Al 2024-T3 is immersed in a cerium solution in order to create a cerium-containing film that covers the copper rich precipitates, followed by anodising in a boric acid-sodium borate/sulphuric acid solution. The results showed that this procedure confers good corrosion protection to the substrate and can potentially constitute an alternative to the Cr-VI based treatment. Introduction Painting and adhesive bonding of aluminium are essential technologies in many industrial sectors (e.g. aerospace industry) and each process depends on the generation of a strong and stable interface between an organic layer and the surface consisting of naturally formed metal oxide. Pre-treatments on the substrates, which leave the metal with an oxide structure that is stable and compatible with the organic layer, are vital to achieve the desirable levels of strength and reliable long-term performance and have been used for decades. However, they involve a range of environmentally objectionable chemicals, such as solvents and chromates, thus needing to be replaced. Several attempts have been made to replace the Cr (VI) – based treatments. In the present work the traditional chromic acid anodising was replaced by a boric acid-sodium borate/sulphuric acid bath. The alloy was pre-treated in a cerium nitrate solution, in order to form a film that covers the copper rich precipitates of the alloy, which impair good anodising. Experimental Aluminium alloy 2024-T3 coupons were used. The specimens were degreased with tricloroethylene (4 min), followed by etching in 3 g.l NaOH solution (8 min) and desmutting in 50% v/v HNO3 solution (30 sec). The anodising bath (ABS) consisted of a mixture of H2SO4 (15%) with a solution containing 0.5M H3BO3 and 0.05 M Na2B4O7.10H2O, in the proportion 70/30 (v/v). The optimised temperature and current density were respectively 40oC and 2 A.dm and the anodising time was 30 minutes. The choice of 40oC came from the fact that lower temperatures lead to higher coating thickness, which should be deleterious for fatigue resistance. The anodising was performed on as-etched surfaces and on surfaces also pre-treated by immersion in 0.01M Ce(NO3)3 solutions for 4 hours. After anodising the specimens were sealed in boiling distilled water for 30 minutes. However, before sealing the specimens were observed on a field emission scanning electron microscope (SEM), on the surface and in a section almost perpendicular to the surface. This latter observation also allowed the determination of the coating thickness. Since the fatigue is an important property of these materials, fatigue tests were carried out on anodised specimens. For comparison, fatigue resistance was also obtained on as received and chromic acid anodised specimens. The alloy was submitted to tensile-tensile fatigue tests in air and at the environmental temperature, in order to obtain S-N plots. The tests were carried out according to ASTM E 466-82 [1], using an INSTRON 8502 servo-hydraulic machine. The corrosion behaviour was studied in 3% NaCl solution by Electrochemical Impedance Spectroscopy, using a Solartron 1250 frequency response analyser connected to the cell via a Solartron 1286 electrochemical interface. Results and Discussion Figure 1a shows a SEM photograph of the transverse section of the specimen anodised in the ABS. It allows the determination of the coating thickness that is approximately 2.5 μm. Higher magnification micrograph (160 000 ×, Fig. 1b) shows in more detail the structure of this coating. Linear continuous pores perpendicular to the specimen surface, as those reported in the literature for anodised aluminium, cannot be seen. The coatings have a grain like structure with the grains separated by pores. The grains seem to have grown preferentially in one direction, revealing a lamellar shape. Coatings formed after pre-treatment in cerium solutions were also observed by SEM. The structures observed were similar to those previously reported, apart a finer structure with smaller grains. The presence of a barrier layer in contact with the metallic surface could not be detected in both cases, which indicates that it should be very thin. a) b) Figure 1. Micrographs of transverse sections of anodised Al 2024-T3 in ABS: a) 20 000×.; b)160 000 × The impedance results for 1 day and 7 days immersion in 3% NaCl solution for ABS anodising are shown in Fig. 2a. The impedance values do not change significantly with time, which reveals a good corrosion performance of the specimen. As a plateau is not usually observed at low frequencies for anodised samples that show even pitting or pronounced corrosion, practical criteria have been established to assess the corrosion behaviour of this material [2-4]. Most of them are based on the value of the impedance in the low frequency region. Thus, according to Mansfeld and Kendig [2], a damage function, D, which is defined as

A comparison of the mechanisms of fatigue-crack propagation behavior in a Zr-based bulk amorphous metal in air and an aqueous chloride solution

Bulk amorphous alloys, such as the Zr 41.2Ti 13.8Cu 12.5Ni 10Be 22.5 (at.%) alloy, have received much interest lately, particularly for their commercial application in golf club heads. This study seeks to investigate the fatigue behavior of this Zr-based amorphous metal in the presence of air and sodium chloride solution, with the specific goal of identifying mechanisms of environmentally assisted fatigue-crack growth in these environments. Results from experiments, including fatigue testing in air and sodium chloride, fatigue under potential control, and static load testing in sodium chloride, suggest that quite distinct mechanisms of fatigue-crack propagation are active in air and sodium chloride solution. Specifically, the fatigue-crack growth rates observed under static and cyclic loading in sodium chloride likely depend on an anodic process, which results in a brittle mode of failure. Conversely in air, fatigue-crack propagation is associated with alternating blunting and re-sharpening of the crack tip, as evidenced by the presence of classic fatigue striations.

Fracture, fatigue and environmentally-assisted failure of a Zr-based bulk amorphous metal

Bulk amorphous alloys, such as the Zr 41.2Ti 13.8Cu 12.5Ni 10Be 22.5 (at%) alloy, have received much interest of late. This study seeks to investigate mechanical properties of this amorphous metal, including fracture and fatigue behavior in the presence of air and sodium chloride solution. For the fully amorphous Zr 41.2Ti 13.8Cu 12.5Ni 10Be 22.5 (at%) alloy, plane-strain fracture toughness ( K Ic) values of 18 to 68 MPa√m have been measured, the significant sample-to-sample variability being associated with the presence of residual stresses following processing and a dependence on loading rate. Upon partial or complete crystallization, the alloy becomes severely embrittled, with K Ic dropping to ∼1 MPa√m. Moreover, in NaCl solution, static load testing reveals a threshold value for cracking, K ISCC, as low as ∼0.8 MPa√m. Results from fatigue testing in air and sodium chloride, as well as fatigue testing under potential control, suggest that quite distinct mechanisms of fatigue-crack propagation are active in air and NaCl solution. Specifically, in air, fatigue-crack propagation behavior is similar to that observed in polycrystalline metals and is associated with alternating blunting and re-sharpening of the crack tip, as evidenced by the presence of fatigue striations. In NaCl solution, conversely, the high crack-growth rates likely depend on an anodic process, which results in a brittle mode of failure.

408 マイクロ X 線残留応力測定を用いた表面改質材の疲労信頼性保証法の構築

The purpose of this study is to develop the fatigue reliability estimation method of surface modified materials. Rotating-bending fatigue tests in air were carried out using smooth specimens of alloy tool steel, SKH51,with two kinds of surface treatment; TiCN thin film coating by the PVD method and duplex treatment with plasma nitriding followed by TiCN coating.

106 マイクロX線残留応力測定を用いた表面改質材の疲労信頼性保証法の構築

In this study, rotating-bending fatigue test in air was conducted using the smooth specimens of alloy tool steel, SKH51, with two kinds of surface treatment; that is, TiCN coating by PVD method and duplex treatment with plasma assisted nitriding followed by TiCN coating. A "P-S-N Globe" was created by combining the experimental analysis with the mesoscopic fracture mechanics and reliability engineering. This concept represents a major contribution to the fatigue reliability design method for surface modification components.

Low-Cycle Fatigue of TiNi Shape Memory Alloy and Formulation of Fatigue Life

The low-cycle fatigue of a TiNi shape memory alloy was investigated by the rotating-bending fatigue tests in air, in water and in silicone oil. (1) The influence of corrosion fatigue in water does not appear in the region of low-cycle fatigue. (2) The temperature rise measured through an infrared thermograph during the fatigue test in air is four times as large as that measured through a thermocouple. (3) The fatigue life at an elevated temperature in air coincides with the fatigue life at the same elevated temperature in water. (4) The shape memory processing temperature does not affect the fatigue life. (5) The fatigue equation is proposed to describe the fatigue life depending on strain amplitude, temperature and frequency. The fatigue life is estimated well by the proposed equation. [S0094-4289(00)01102-6]

Fatigue Strength and Subsurface Crack Growth Properties of Plasma Assisted Duplex Surface Treated Tool Steel (SKD61).

Numerous methods of surface treatment have been developed for purpose of the increase in high cycle fatigue strength and extension of lifetime of the engineering materials in various kinds of environment. In this study, cantilever-type rotating-bending fatigue test in air was conducted using the smooth specimens of alloy tool steel, SKD 61, with three kinds of surface treatment ; that is, TiN coating by PVD method, plasma assisted ion-nitriding and duplex treatment with the ion-nitriding followed by the TiN coating. Fatigue strength of the surface treated specimens was improved as compared with untreated specimen. The highest fatigue strength was the duplex surface treated specimen, followed by the ion-nitriding one and the lowest was the TiN coating one. This tendency was obvious at the region of high stress amplitude and disappeared at low stress amplitude region. Increase of fatigue strength of specimen by surface treatment was due to transition of fatigue crack initiation site from surface to subsurface by an effect of compressive residual stresses occurred at surface layer by ion-nitriding, and barrier to the formation of surface crack nuclei beneath the TiN coating film. The difference in fatigue strength between the duplex surface treated specimen and the ion-nitriding one, of which crack initiation site was subsurface, was discussed by the effect of TiN coating film on subsurface crack growth behavior. It was found that TiN coating film acts as barriers to formation of plastic zone at the front of subsurface crack and retards crack growth twoard surface.