Evaluation of equi-biaxial fatigue of stainless steel by the pressurized disc fatigue test

The pressurized disc fatigue (PDF) test technique was employed to obtain fatigue lives of Type 316 stainless steel under equi-biaxial stress conditions. In the PDF test, a disc-type specimen was subjected to the cyclic bulge test. The biaxial fatigue lives were successfully obtained by the PDF tests, and they were longer than those obtained by the uni-axial and plate bending fatigue tests under the same equivalent strain range. Observations of crack initiation and growth behavior during the PDF test revealed that the relatively large size of the disc-type specimens had only a minor influence on the fatigue lives. Finite element analysis results showed the PDF test was valid for evaluating the fatigue lives under equi-biaxial conditions. It was concluded that the influence of equi-biaxial condition was not necessary to be considered in the design fatigue curve.     

Reprint of “Multiaxial fatigue property of Ti–6Al–4V using hollow cylinder specimen under push-pull and cyclic inner pressure loading”

This paper studies a multiaxial fatigue crack mode and a fatigue life of Ti–6Al–4V. Load controlled fatigue tests at room temperature were carried out using a hollow cylinder specimen under multiaxial loading with principal stress ratio λ equal to 0, 0.4, 0.5 and 1.0 and loading ratio R kept constant and equal to 0. λ is defined as λ = σ₂/σ₁, where σ₁ and σ₂ are maximum and intermediate/minimum principal stresses, respectively. Here, the test at λ = 0 is a uniaxial loading test and that at λ = 1.0 an equi-biaxial loading test. A testing machine employed was a newly developed multiaxial fatigue testing machine which can apply push-pull and reversed torsion loading with inner pressure into the hollow cylinder specimen. Based on the obtained results in this study, multiaxial fatigue properties are examined, where the fatigue life evaluation and the crack mode are discussed. The fatigue life is reduced with an increase of λ, due to cyclic ratcheting and crack mode in multiaxial loading. The crack mode is also affected by the surface condition resulting from cut-machining.     

Multiaxial fatigue property of Ti–6Al–4V using hollow cylinder specimen under push-pull and cyclic inner pressure loading

This paper studies a multiaxial fatigue crack mode and a fatigue life of Ti–6Al–4V. Load controlled fatigue tests at room temperature were carried out using a hollow cylinder specimen under multiaxial loading with principal stress ratio λ equal to 0, 0.4, 0.5 and 1.0 and loading ratio R kept constant and equal to 0. λ is defined as λ = σ₂/σ₁, where σ₁ and σ₂ are maximum and intermediate/minimum principal stresses, respectively. Here, the test at λ = 0 is a uniaxial loading test and that at λ = 1.0 an equi-biaxial loading test. A testing machine employed was a newly developed multiaxial fatigue testing machine which can apply push-pull and reversed torsion loading with inner pressure into the hollow cylinder specimen. Based on the obtained results in this study, multiaxial fatigue properties are examined, where the fatigue life evaluation and the crack mode are discussed. The fatigue life is reduced with an increase of λ, due to cyclic ratcheting and crack mode in multiaxial loading. The crack mode is also affected by the surface condition resulting from cut-machining.     

Low cycle fatigue behavior of Cr–Mo–V low alloy steel used for railway brake discs

The cyclic stress–strain response and the low cycle fatigue (LCF) behavior of Cr–Mo–V low alloy steel which was used for forged railway brake discs was studied. Tensile strength and LCF properties were examined over a range from room temperature (RT) to 600 °C using specimens cut from circumferential direction of a forged disk. The fully reversed strain-controlled LCF tests were conducted at a constant total strain rate with different axial strain amplitude levels. The cyclic strain–stress relationships and the strain–life relationships were obtained through the test results, and related LCF parameters of the steel were calculated. The studied steel exhibits cyclic softening behavior and behaves Masing type, especially at higher strain amplitudes. At higher than 600 °C, carbide particles aggregated and a decarburized layer developed near the specimen surface. Micro voids distribute within the depth of 50 μm from the specimen surface could coalesce with fatigue cracks. Multiple crack initiation sites were observed on the fracture surface. The oxide film that generated at 600 °C covered the fatigue striations and accelerated the crack propagation. Final fracture area with bigger and deeper dimples showed better ductility at higher temperature. The investigated LCF behavior can provide reference for brake disc life assessment and fracture mechanisms analysis.     

Environmental effect on the fatigue performance of bare and oxide coated 7075-T6 alloy

Fatigue behaviors of bare and anodic oxide coated 7075-T6 alloy have been investigated in laboratory air and 3.5%NaCI solution environment by using smooth cylindrical specimens. Presence of corrosive attack during fatigue test drastically reduced fatigue performance of the alloy. The deleterious effect was observed to be pronounced at high-cycles fatigue region, where the fatigue strength of the bare specimen was lowered by a factor of 2.9. However, the oxide coated specimens having a thickness of 23 μm showed a modest reduction in fatigue strength. Corrosion fatigue (CF) strength of the bare specimens was predominantly controlled by pitting-induced crack nucleation. Examinations on the surfaces of the corrosion-fatigued and immersed test specimens revealed that cyclic loading stimulated corrosion pit formation during CF tests. Also, corrosion behaviors of both the coated and bare specimen shave been investigated by potentiodynamic test. Despite superior corrosion resistance of coated specimens, fatigue performance was adversely affected under the combined action of corrosion attack and cyclic loading.     

The investigation of crack growth in specimens with rectangular cross-sections under out-of-phase bending and torsional loading

The paper presents the fatigue test results of rectangular cross-section specimens made of 10HNAP (S355J2G1W) steel. The specimen height to width ratio was 1.5. The tests under bending with torsion were performed for the following ratios of bending to torsional moments M_aB/M_aT = 0.47, 0.94, 1.87 and the loading frequency 26.5 Hz. Nominal stresses were chosen for the equivalent stress according to the Huber-Mises hypothesis equal to 360 MPa. The tests were performed in the high cycle fatigue regime for the stress ratio R = −1 and phase shift between bending and torsion loading equal to ϕ = 0 and 90°. Crack initiation and propagation phases were observed on the specimen surface using the optical microscope (magnification 20×) with an integrated digital camera. The test results for the fatigue crack growth rate versus the stress intensity factor range for mode I and mode III have been described with the Paris equation.     

Gigacycle fatigue initiation mechanism in Armco iron

Microstructure irreversibility plays a major role in the gigacycle fatigue crack initiation. Surface Persistent Slip Bands (PSB) formation on Copper and its alloy was well studied by Mughrabi et al. as typical fatigue crack nucleation in the very high cycle fatigue regime. In the present paper, Armco iron sheet specimens (1 mm thickness) were tested under ultrasonic frequency fatigue loading in tension–compression (R = −1). The test on the thin sheets has required a new design of specimen and new attachment of specimen. After gigacycle fatigue testing, the surface appearance was observed by optical and Scanning Electron Microscope (SEM). Below about 88 MPa stress, there is no PSBs even after fatigue cycle up to 5 × 10⁹. With a sufficient stress (above 88 MPa), PSBs in the ferrite grain was observed by optic microscope after 10⁸ cycles loading. Investigation with the SEM shows that the PSB can appear in the body-centered cubic crystal in the gigacycle fatigue regime. Because of the grain boundary, however, the local PSB did not continually progress to the grain beside even after 10⁹ cycles when the stress remained at the low level.     

Effect of high-temperature hot corrosion on the low cycle fatigue behavior of a directionally solidified nickel-base superalloy

The low cycle fatigue behavior of a directionally solidified nickel-based superalloy DZ125 was examined at 850 °C in air using bare and salt-coated specimens. Experimental results show that the salt-coated specimen showed relatively low fatigue life compared with the bare specimen, and this effect accelerated with the increased applied maximum stress. Damage of hot corrosion in fatigue life was found to be associated with the reduction of the bare area and the early crack initiation from the weaken grain boundaries of recrystallized grains.     

Research on fatigue behavior and residual stress of large-scale cruciform welding joint with groove

Fatigue fracture behavior of the 30 mm thick Q460C-Z steel cruciform welded joint with groove was investigated. The fatigue test results indicated that fatigue strength of 30 mm thick Q460C-Z steel cruciform welded joint with groove can reach fatigue level of 80 MPa (FAT80). Fatigue crack source of the failure specimen initiated from weld toe. Meanwhile, the microcrack was also found in the fusion zones of the fatigue failure specimen, which was caused by weld quality and weld metal integrity resulting from the multi-pass welds. Two-dimensional map of the longitudinal residual stress of 30 mm thick Q460C-Z steel cruciform welded joint with groove was obtained by using the contour method. The stress nephogram of Two-dimensional map indicated that longitudinal residual stress in the welding center is the largest.     

The role of oxidation damage in fatigue crack initiation of an advanced Ni-based superalloy

The effects of prior oxidation on the room temperature fatigue life of coarse-grained Ni-based superalloy, RR1000, have been investigated. High cycle fatigue tests were conducted, on both machined and pre-oxidised testpieces, at room temperature at an R ratio of 0.1. The oxidation damage was produced by pre-exposures at 700 °C for either 100 or 2000 h. Pre-oxidised testpieces tended to fail with shorter fatigue lives than those obtained from the as-machined testpieces although they were also observed to outperform the as-machined test pieces at peak stress levels around 900 MPa. The chromia scale and intergranular alumina intrusions formed during pre-oxidation are prone to crack under fatigue loading leading to early crack nucleation and an associated reduction in fatigue life. This has been confirmed to be the case both below and above a peak stress level of ∼900 MPa. The better fatigue performance of the pre-oxidised specimens around this stress level is attributed to plastic yielding of the weaker γ′ denuded zone, which effectively eases the stress concentration introduced by the cracking of the chromia scale and intergranular internal oxides. This γ′ denuded zone is also a product of pre-oxidation and develops as a result of the selective oxidation of Al and Ti. Over a limited stress range, its presence confers a beneficial effect of oxidation on fatigue life.     

Influence of spectrum loading sequences on fatigue life in a high-temperature environment

This paper discusses the fatigue life behaviour of aluminium alloy AA6061-T6 under spectrum loadings. Load sequences in spectrum loadings can have significant effects on fatigue life at room temperature and within the elevated temperature range. The main objective of this paper is to investigate the influences of load sequences effect on fatigue life at elevated temperature. Fatigue strain signal was obtained from the engine mount bracket of an automobile under normal driving conditions. Constant amplitude loading, high-to-low, and low-to-high loading sequences were then derived from the original fatigue strain signal to observe the fatigue behaviour at both room and elevated temperatures. The fatigue test was performed on AA6061-T6 specimen according to the ASTM E466 standard using a 100 kN servo-hydraulic fatigue testing machine within the temperature range of 27–250 °C. The elevated temperature range was chosen based on the maximum temperature of the engine mount bracket and the extreme temperature of the cylinder head that can be reached in service. After the test, fatigue fracture surfaces were sectioned and inspected using a high-magnification microscope. Results show that fatigue life behaviour at room temperature was significantly influenced by the load sequences in spectrum loadings. On the other hand, the effect of load sequences at a higher temperature was reduced.     

Cold expansion effects on cracked fastener holes under constant amplitude and spectrum loading in the 2024-T351 aluminum alloy

The increased number of aging aircraft in operation today requires a deeper understanding of fatigue life improvement methods. This research focused on the fatigue life benefit from cold expanded holes with preexisting cracks approximately 1.270 mm (0.050 in.) long under constant amplitude and wing spectrum loading. Holes with preexisting cracks were tested to simulate the worst case scenario of a hole with a crack the size of the detection threshold, 1.270 mm (0.050 in.), present before cold expansion that was not found by Non Destructive Inspection. Test results were compared to crack growth models generated in AFGROW. At high stress levels the AFGROW models yielded non conservative results greater than 150% of the test demonstrated fatigue life.     

Crack initiation in VHCF regime on forged titanium alloy under tensile and torsion loading modes

This paper is focused on the VHCF behavior of aeronautical titanium alloy under tensile and torsion fatigue loadings. Tensile tests were carried out with two different stress ratios: R = −1 and R = 0.1. Both surface and subsurface crack initiations were observed. In the case of subsurface crack initiation several fatigue life controlling mechanisms of crack initiation were found under fully-reversed loading conditions: initiation from (1) strong defects; (2) ‘macro-zone’ borders; (3) quasi-smooth facets and (4) smooth facets. Tests with stress ratio R = 0.1, have shown that initiation from the borders of ‘macro-zones’ becomes the dominant crack initiation mechanism in presence of positive mean stress. Like for the tensile results, surface and subsurface crack initiations were observed under ultrasonic torsion in spite of the maximum shear stress location on the specimen surface. But the real reason for the subsurface crack initiation under torsion was not found.     

Effect of the loading frequency on the compressive fatigue behavior of plain and fiber reinforced concrete

This paper presents the recent experimental results aimed at disclosing the loading frequency effect on the fatigue behavior of a plain concrete and two types of fiber-reinforced concrete, using polypropylene and steel fibers. Compressive fatigue tests were conducted on 123 cubic specimens (100 mm in edge length). Four different loading frequencies, 4 Hz, 1 Hz, 1/4 Hz and 1/16 Hz, were employed. The maximum stress applied on the specimen was 85% of its compressive strength and the stress ratio was kept constant as 0.3. The results show that the loading frequency effect on the fatigue behavior of the plain concrete is pronounced. The fatigue life (the number of cycles to failure) at lower frequencies is less than that at higher frequencies. However, the fibers do improve the fatigue behavior significantly under low loading frequencies. Such trend can be attributed to the effectiveness of the fibers in bridging cracks, and thus inhibiting the crack extension under cyclic loads.     

Crack precursor size for natural rubber inferred from relaxing and non-relaxing fatigue experiments

Fatigue life prediction for a dumbbell cylindrical natural rubber component under uniaxial tensile loading conditions was performed based on the Thomas fatigue crack growth model for relaxing (R = 0) load cycles and the Mars–Fatemi model for non-relaxing (R > 0) load cycles. By using a self-written program, we proposed a new approach to establish the relation between the power law exponent F and the R ratio in the Mars–Fatemi model. The approach is based on rubber fatigue life (S–N) data rather than crack growth rate and tearing energy (da/dN–T) data, avoiding certain difficulties often encountered using the crack growth method. The results indicate that the relation between F and R is a quadratic or cubic function over the range 0 < R < 0.3. Finally, the quantitative effect of initial crack size on fatigue life was studied. We found that the inferred mean size of crack precursors in the rubber component is around 30–40 μm under both relaxing and non-relaxing loading conditions, and the fluctuation of fatigue life is due to the inhomogeneity of crack precursor size except the factors such as unavoidable variations in testing conditions and specimen variations. The good agreement of inferred crack precursor sizes from different R ratio loading conditions is a strong indication that the Mars–Fatemi model provides a proper accounting for the effects of strain crystallization, and it confirms yet again the understanding that nucleated cracks originate from similarly sized precursors in both relaxing and non-relaxing fatigue experiments.     

The fatigue behavior of irradiated Reactor Pressure Vessel steel

Fatigue specimens of A508-3 steel were irradiated in the swimming-pool test reactor in China Institute of Atomic Energy, the fluence was 3 × 10¹⁹ n/cm² at 300 °C, then low-cycle fatigue tests were carried out at ambient temperature, with the fatigue strain range is 0.32–1.8%. The results indicate that, irradiated A508-3 specimens exhibit cyclic softening and instability behavior during the test, and the cyclic softening rate increased with strain range increased; fatigue life decreased from 1.7 × 10⁵ to about 5 × 10², as the strain range increased from 0.32% to 1.8%, the fatigue life of A508-3 steel increased after the neutron irradiation; fatigue fracture initiated at the surface of specimen, and more individual cracks formed on the specimens of higher strain range compared with the specimens of lower strain range.     

Very high cycle fatigue of nitrided 18Ni maraging steel sheet

Thin sheets of nitrided 18Ni maraging steel are tested under cyclic tension (load ratio R = 0.1) in the very high cycle fatigue (VHCF) regime. The ultrasonic fatigue testing method with a cycling frequency of about 20 kHz has been further developed for these experiments. Sheet specimens with 0.35 mm thickness are mounted on a carrier specimen, they are pre-stressed and are forced to vibrate jointly. Between 10⁷ and 10⁹ cycles, fatigue cracks are initiated exclusively at internal TiN inclusions. The areas of the crack initiating inclusions projected perpendicular to the applied tensile stress are evaluated. The square root of inclusion areas, (area_INC)^1/2 lies between 2.5 μm and 5.3 μm. Considering inclusions as cracks, their stress intensity range is between ΔK_INC = 1.3 MPa m^1/2 and 2.4 MPa m^1/2. The sizes of crack initiating inclusions influence fatigue lifetimes. This is considered in a crack propagation model and by presenting lifetimes versus the stress amplitudes multiplied by (area_INC)^1/12. A mean lifetime of 10⁹ cycles is found at a stress amplitude of 22% of the tensile strength, which is comparable to other high strength steels tested under cyclic tension.     

Fatigue strength and fracture mechanism of steel modified by super-rapid induction heating and quenching

Four kinds of surface hardened-specimens (ordinary structural steel with carbon content of 0.45% C) having hardened thicknesses of 0.7–1.8 mm were prepared using a ‘super-rapid induction heating (SRIH) system’. Rotation bending fatigue tests were performed with special focus on the effect of a hardened thickness on fatigue properties. Measurement of residual stress and observation of the fracture surface were also carried out to investigate the fracture mechanism of the specimen with a shallow hardened layer. It was found that there is not much improvement of fatigue strength at 10⁷ cycles for specimens with shallow hardened layers in spite of having a high compressive residual stress of about 1000 MPa. This is because the fatigue crack originating from inside the hardened layer leads to the final fracture of the specimen (internal fracture mode). Improvement of fatigue strength has been achieved on the specimen with thick hardened layers, such as those about 1.8 mm thick. In this case, fatigue cracks originate from inclusions located in hardened layers, which leads to final fracture (hardened-layer fracture mode).     

压力容器用钢低周疲劳表面裂纹扩展速率试验研究

压力容器的表面裂纹常常会导致低周疲劳破坏.因此,研究压力容器用钢的低周疲劳表面裂纹扩展规律,具有重要意义.本文在带有上下对称圆弧板状试样的上圆弧几何中心,加工预制了半椭圆表面裂纹,以模拟压力容器具有表面裂纹时的特点.同时,在上圆弧一侧设计加工了特定形状和尺寸的刀口,装卡COD位移规,用来控制表面裂纹前缘的应变.以10C...