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• Anodic oxidation coating's cracking causes a severe substrate stress concentration. • Sealing weakens anodic oxidation coating's cracking resistance during fatigue tests. • Prior shot peening enhances the fatigue property of anodic oxidation coated specimen. In this study, the effects of sealing and prior shot peening treatments on the fatigue behavior of the sulphuric acid anodic oxidation coated 6082-T6 aluminium alloy were investigated. Experimental results indicate that the preferential fracture of anodic oxidation coatings due to its higher brittleness and lower fracture toughness as well as the stress concentration of near-interface substrate caused by the interface cyclic dislocation accumulation could cause the fatigue failure of anodic oxidation coated specimens. Consequently, the fatigue property of anodic oxidation coated specimens notably decreases compared to as-machined specimens, with a decrease of 26.7 % in the fatigue limit corresponding to 2 × 107 fatigue cycles. Additionally, the sealing treatment introduces numerous cracks that penetrate the coating entirely, diminishing the coating's resistance to cracking under fatigue loading; this alteration changes the fatigue failure mechanism of anodic oxidation coated specimens, ultimately resulting in a decrease of 15.5 % in the fatigue limit of anodic oxidation coated specimens. Despite exacerbating the surface integrity and increasing the surface roughness of anodic oxidation coated specimens, the prior shot peening treatment increases the fatigue limit of anodic oxidation coated specimens by 82.8 % since the affecting layer beneath the interface plays a crucial role in inhibiting the further propagation of coating cracks into the substrate as well as the crack propagation in the substrate beneath the interface. [ABSTRACT FROM AUTHOR]

• Prior shot peening improves the CF limit of AO coated aluminum alloy by 50 %. • AO specimens exhibit different CF failure modes under low and high applied stress. • Substrate residual stress and microstructure affect the CF crack propagating path. • Substrate residual stress relaxes in distinct mode under low and high applied stress. Plain (anodic oxidation) and composite (prior shot peening + anodic oxidation) coatings were prepared to investigate their effects on the corrosion-fatigue (CF) property of 6082-T6 aluminium alloy in a 3.5 wt% NaCl aqueous solution. The results indicate that plain coated specimens exhibited similar CF properties to as-machined specimens at low applied stresses. However, under high applied stresses, the plain coating led to a preferential fracture and was unable to effectively prevent corrosion medium. Hence, the CF property of plain coated specimens was found to be inferior to as-machined specimens at high applied stresses. The synergetic effect of substrate residual stress and microstructure beneath the coating-substrate interface reduced the crack propagating rate in the initial stage by deflecting the crack at the grain boundary. Therefore, the CF property of composite coated specimens improved as a whole when compared with as-machined and plain coated specimens. Besides, under low applied stresses, the near-interface substrate residual stress of composite coated specimens occurred the relaxation due to the substrate crack propagation, while under high applied stresses, this was due to the synergetic effect of materials' local cyclic yield and plastic flow, as well as the substrate crack propagation. [ABSTRACT FROM AUTHOR]

• The surface properties of EA4T axle steel treated by ultrasonic surface rolling process (USRP) are investigated. • Effect of USRP on the electrochemical corrosion resistance and corrosion fatigue performance is elucidated. • Residual stress relaxation mechanism of USRP-treated EA4T axle steel in air and corrosion is revealed. • Influence of residual stress relaxation on the corrosion fatigue resistance is clarified. The fatigue properties and residual stress relaxation behavior of EA4T axle steel treated by ultrasonic surface rolling process (USRP) are investigated in air and corrosive environments. The result reveals that the surface roughness (Ra) is decreased from 1.42 to 0.17 μm after the USRP. A surface layer with ultrafine grains, work hardening, and compressive residual stress (CRS) is achieved by the USRP. Compared with the as-machined specimens, the USRP specimens exhibit slightly lower electrochemical corrosion resistance but superior fatigue performance in air and corrosive environments. The dislocation density is significantly decreased in the corrosive environment during fatigue loads, accompanied by a rapid relaxation of surface CRS. The influence of USRP on the corrosion fatigue performance is elucidated. The residual stress relaxation mechanism and its influence on fatigue resistance are also analyzed. [ABSTRACT FROM AUTHOR]

In this study, the very high cycle fatigue tests were conducted on a kind of high-strength austempered ductile iron (ADI) at 90 Hz and 20 kHz. The S – N curves decrease continuously and there is no fatigue limit in the very high cycle fatigue regime. The fatigue strength of the different frequencies essentially corresponds with each other and the specimens show similar fracture behaviors. Meanwhile, on the fracture surfaces of some specimens, the ‘granular-bright-facet’ (GBF) area is observed. The threshold stress intensity factor for internal crack growth is obtained and the fatigue limit is evaluated based on the defect size. [ABSTRACT FROM AUTHOR]

In this study, the effects of micro-shot peening and artificial defect on the fatigue properties of EA4T railway axle steel were evaluated. Un-peened (UP) and micro-shot peened (MSP) EA4T axle steel specimens were drilled to introduce artificial defects on the surface. Then the fatigue tests were conducted on a rotary bending fatigue machine (R = −1). The test results showed that the fatigue limit of the UP specimens was improved by 25% after MSP; when the equivalent defect sizes were less than 60 and 70 μm respectively, they had no negative effect on the fatigue strength of the UP and MSP drilled specimens, otherwise the fatigue strength of both specimens decreased with the increase of the defect size. The fracture surfaces were analyzed by scanning electron microscopy (SEM) observations, which indicated that crack initiation sites for the as-prepared UP and MSP specimens were on the surface, while for the drilled specimens, they were on the vicinity of hole defects. The effect of artificial defect size on the fatigue strength of the UP and MSP specimens and the critical defect size were analyzed with fracture mechanics approach, the results of which showed that the relationship between defect size and fatigue strength of the UP and MSP specimens can be well evaluated by the modified Haddad model, and the critical defect size of the UP specimens was 65 μm while that of the MSP specimens was increased to 76 μm due to the introduction of compressive residual stress. [ABSTRACT FROM AUTHOR]

Highlights • Effect of a shear-affected-zone (SAZ) on fatigue limit was investigated. • Tests on specimens with and without SAZ were conducted. • Propagation resistance of the fatigue crack has been lost by the punching process. • Fatigue crack growth mode had a possibility of transition from Mode I to Mode II. • Fatigue limit design for the punching processed plate was coarse-grained. Abstract It is well known that the fatigue limits of some precipitation-hardened steels with a punched hole do not increase, even if the tensile strengths of base-metal increases. This study suggested that the dominant reason for this strange phenomenon is a transition from the Mode I type fatigue crack propagation due to the cyclic plastic deformation to Mode II type propagation due to the damage accumulation by a quantitative analysis of influential factors based on fracture mechanics. This result can open a probability that the fatigue limit of the punched hole with shear-affected zone can be actively quantified. [ABSTRACT FROM AUTHOR]

• Vanadium addition causes an increase in both yield strength and yield ratio. • Vanadium addition reduces strain hardening ability, and weakens wear resistance. • RCF crack initiation of V-alloyed stee is delayed by increasing yield strength. • RCF crack growth size is restricted by reducing plastic deformation capacity. • Increasing yield ratio improves RCF resistance, but weakens wear resistance. This study aims to evaluate the effect of vanadium microalloying on rolling contact fatigue (RCF) and wear behavior of railway wheel steels. It involves microstructural analysis and mechanical testing, including hardness, tensile, and rolling/sliding tests on the test steels, followed by electron backscatter diffraction, residual stress, and microhardness analyses after rolling/sliding tests. The findings demonstrate that vanadium microalloying leads to the alleviation of dynamic recrystallization, strain hardening, and compressive residual stress within the outermost layer beneath the worn surface, indicating a reduction in the plastic deformation capacity during dry rolling/sliding conditions. RCF resistance of railway steel can be enhanced through vanadium microalloying, as it simultaneously delays crack initiation by increasing yield strength and restricts crack growth by reducing plastic deformation capacity. While vanadium microalloying enhances the matrix hardness of wheel steel, it concurrently diminishes its strain hardening ability, potentially compromising its wear resistance. In addition, this study found that the yield ratio, being indicative of plastic deformation capacity and strain hardening ability, can serve as a suitable parameter for characterizing the RCF and wear resistances of tested steels. It exhibits a positive correlation with RCF resistance and a negative correlation with wear resistance. [ABSTRACT FROM AUTHOR]

• The preferentially dissolved ferrite in pearlite and shedding of cementite lamellae contribute to the formation of corrosion pits. • Corrosion fatigue crack initiation and growth mechanism of U75V rail steel is clarified. • Corrosion fatigue crack initiation behavior is related to morphology ratio and local shape of corrosion pits. Understanding the corrosion fatigue mechanism of rail materials is of great importance to guide the assessment of the structural integrity of modern railway components. To this purpose, the corrosion fatigue crack (CFC) initiation and growth behaviors of heat-treated U75V rail steel are investigated under 3.5 wt% NaCl solution in terms of high-cycle fatigue, fatigue crack growth (FCG) rate, together with surface morphology and microstructure. Experimental results reveal that the U75V rail steel is susceptible to corrosion fatigue failure. Examinations on fractured samples clearly show that the electric potential difference between ferrite and cementite triggers the preferential anodic dissolution of ferrite, leading to corrosion fatigue crack initiation. The crack initiation is not only solely determined by the size of the corrosion pit, but also strongly influenced by its local shape. In comparison with the fatigue crack growth in air, the corrosion FCG tends to grow in a relatively straight path, because of hydrogen embrittlement. Additionally, the CFC behaviors indicate a tendency to transverse through soft-oriented grains and deflect upon encountering hard-oriented grains. [ABSTRACT FROM AUTHOR]

• Fatigue tests are conducted for small specimen and press-fitted axle with defect. • Press-fitted axles exhibit notably lower fatigue strength than small specimens. • Goodman relation is used to consider mean tensile stress induced by press-fitting. • The error of predicted notch fatigue strength is controlled to less than 11%. In order to evaluate the notch fatigue strength of transition groove in railway axle, a series of fatigue tests were conducted on small specimens and press-fitted axles containing surface defects. Based on the experimental and finite element (FE) analyses, a novel method considering mean stress effect was proposed to evaluate the fatigue strength of scale railway axle with surface defect. It is found that press-fitted axles exhibit significantly lower notch fatigue strength than small specimens. Furthermore, the theory of critical distance (TCD) provides a good correlation between the notch fatigue strength of small specimens and experimental results. FE analyses revealed that the press-fitting reduces the fatigue strength of the transition groove through increasing stress concentration and introducing a mean tensile stress that decreases with an increase in the applied loading. To account for the mean stress effect, the axial stress at the groove is corrected as an equivalent stress amplitude at R = -1 based on Goodman relation. Through the combination of TCD and Goodman relation, an estimated fatigue strength with an error of less than 11 % can be achieved for the scale railway axle with surface defect. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • The cold working causes obvious texture evolution in Cu-Ni-Si alloy. • The cold working has obvious effect on fatigue strength and fatigue fracture mode. • The salt atmosphere changes the fatigue failure from shear mode to tensile mode. Abstract In this study, the tensile and fatigue tests were conducted on precipitate strengthened Cu-Ni-Si alloy. The fatigue properties of specimens with cold working and without cold working in air, as well as that of the specimen with cold working in salt atmosphere were investigated. The results show that the monotonic tensile strength and the yield strength are improved obviously, while the elongation decreases by cold working. The S-N curves of all the types of specimens show a continuous decrease. The fatigue strength of specimens in air is slightly reduced by cold working, however, that of the specimen with cold working at 107 cycles is largely decreased from 190 MPa to 120 MPa in salt atmosphere. All the fatigue cracks initiate from the specimen surface by slip deformation. In air condition, the specimens with cold working show shear mode fracture with transgranular crack propagation and it is caused by the obvious texture, while that without cold working show normal mode fracture with both transgranular and intergranular crack propagation. However, in salt atmosphere condition, the specimens with cold working show normal mode fracture with totally intergranular crack propagation. [ABSTRACT FROM AUTHOR]

• Effect of artificial notches is studied in terms of fatigue crack non-propagation behaviors. • Non-propagation crack rate firstly increases and then decreases with the increased notch depth. • Gradient residual stress and microstructure are responsible for the crack non-propagation. • Crack traces of post-mortem specimen in the hardened layer and the matrix are compared. Abnormal damages in railway axles can lead to a significant hazard to running safety and reliability. To this end, a surface treatment was selected to effectively inhibit fatigue crack initiation and growth. In this study, a single-edge notch bending fatigue test campaign with artificial notches was conducted to elucidate the fatigue crack non-propagation behavior in railway S38C axles subjected to an induction hardening process. The fatigue cracking behavior in the gradient structure was revealed by optical microscopy, electron backscatter diffraction, and fractography. The microhardness distribution was measured using a Vickers tester. The obtained results show that the microhardness of the strengthening layer is nearly triple that of the matrix. Owing to the gradient microstructures and hardness, as well as compressive residual stress, the fatigue long crack propagates faster once it passes through the hardened zone (approximately 2.0 mm in the radial depth). Thereafter, local retarding (including deflection, branching, and blunting) of the long crack occurs because of the relatively coarse ferrite and pearlite in the transition region and matrix. Totally, this fatigue cracking resistance is reasonably believed to be due to the gradient microstructure and residual stress. These findings help to tailor a suitable detection strategy for maximum defects or cracks in railway axles. [ABSTRACT FROM AUTHOR]

• Surface rolling deformed severity-dependent fatigue mechanism of titanium alloy was revealed. • The microstructural evolutions of the samples with different surface rolling deformation severities during fatigue loading were revealed. • The residual stress evolutions during fatigue loading for the different surface rolling deformed samples were clarified. • A surface strengthening strategy was given to enhance the stress-controlled fatigue property of titanium alloy. This work investigates the effect of initial surface plastic deformation on the fatigue behavior of Ti-6Al-4V alloy based on the microstructure and residual stress evolution under fatigue loading. The high strain hardening ability, full play of compressive residual stress, and excellent surface integrity of samples with low plastic deformation severity resulted in relatively higher fatigue strength. Microstructural analysis revealed that the ultrasonic surface rolling process (USRP) applied to samples with low plastic deformation severity accommodated plastic strain by refining the grains, whereas the grains coarsened in the USRP samples with high plastic deformation severity for accommodating the plastic strain. [ABSTRACT FROM AUTHOR]