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The nucleation of microstructural fatigue cracks in AISI 4340 steel is investigated. The pre‐electropolished specimen is employed for the observation of the very early stage of fatigue crack initiation correlated with the microstructure. It is found that there are precipitate‐free zones (PFZs) around the lath and packet boundary, and the strength of PFZ is lower than that of the lath due to the lack of second‐phase strengthening. Therefore, the fatigue crack would first initiate within the PFZ. Besides, the initiated crack plane usually is the maximum shear stress plane. However, the MnS inclusions would cause stress concentration so that the lath boundary (LB) deviating from the maximum shear stress plane could also have sufficient driving force to crack. In contrast, when the LB is vertical to the cracked MnS inclusion, it would prevent the crack from propagating into the matrix because of the anisotropy of lath martensite. The effects of the stress concentration caused by the MnS inclusion on the maximum deviation angle and the S‐N curve are discussed. [ABSTRACT FROM AUTHOR]

The high‐cycle fatigue (HCF) experiments were carried out for different aging states to study the effects of the aging state and yield strength on the fatigue properties of a 6A01 Al alloy. The results show that the 6A01 Al alloy with the highest yield strength at a peak‐aging (PA) state can exhibit the highest fatigue strength in comparison with the overaged (OA) state and the underaged (UA) state. The main reason is that the increased strength of the 6A01 Al alloy at the PA state can improve the plastic deformation resistance and inhibit the fatigue crack initiation. Additionally, the UA has a similar yield strength but a higher fatigue strength compared with the OA, which shows that the aging state also has a significant influence on the fatigue properties. Finally, combined with the previous studies on the 2XXX and 7XXX Al alloys, the effects of the aging state and yield strength on the fatigue properties of Al alloys are summarized. [ABSTRACT FROM AUTHOR]

In order to obtain the optimum fatigue performance, 35CrMo steel was processed by different heat treatment procedures. The microstructure, tensile properties, fatigue properties, and fatigue cracking mechanisms were compared and analyzed. The results show that fatigue strength and yield strength slowly increase at first and then rapidly decrease with the increase of tempering temperature, and both reach the maximum values at a tempering temperature of 200 °C. The yield strength affects the ratio of crack initiation site, fatigue strength coefficient, and fatigue strength exponent to a certain extent. Based on Basquin equation and fatigue crack initiation mechanism, a fatigue strength prediction method for 35CrMo steel was established. [ABSTRACT FROM AUTHOR]

High‐cycle fatigue (HCF) properties of two Al‐Si‐Cu‐Mg‐Ni alloys with different defect sizes named as alloys A (smaller ones) and B (bigger ones) were investigated at 350°C and 425°C, respectively. The results indicate that fatigue strengths of both alloys decrease as the temperature increases. Fatigue cracks originated from pores and oxide films at both temperatures. They propagated preferentially through cracked matrix at 350°C and debonded interface and grain boundary at 425°C. Alloy A exhibits higher fatigue life and fatigue strength than alloy B at 350°C due to its smaller pore sizes. However, it has slightly worse fatigue properties than alloy B at 425°C because the fatigue crack initiation is controlled by oxide film at this temperature and is not affected by its size. This indicates that there is a transition of predominant initiation site from pores to oxide films when the temperature increases. The fatigue strength estimated through defect size is consistent with the experimental results at 350°C, while unsuitable at 425°C. [ABSTRACT FROM AUTHOR]

The very high‐cycle fatigue (VHCF) fractographies of high‐strength steel AISI 4340 with tensile strength ranging from 1285 to 2363 MPa fabricated by tempering were systematically observed and quantitatively analyzed. It is found that the fatigue crack initiation sites were gradually transferred from the specimen surface to the interior with increasing the tensile strength or decreasing the stress amplitude. Such phenomenon has become common rule in the high‐cycle fatigue (HCF) or VHCF regimes of high‐strength steels. Based on the fracture mechanics, a simplified transition mechanism for fatigue crack initiation sites is established in terms of the influence of applied stress amplitude and microstructure, which is beneficial to enhance our understanding on the variation of fatigue strength increasing the tensile strength. [ABSTRACT FROM AUTHOR]

• Microstructural recovery dominates the degradation in mechanical properties of Al wires at elevated temperatures. • Cracking at the grains with 〈0 0 1〉 orientation on the surface is the major crack initiation mode. • Fatigue crack propagation of Al wires exhibits three distinctive stages. • The proportion of plane crack propagation regions decreases with increasing temperature. • A dynamic fatigue strength prediction model of Al wires is proposed. With the increase in transmission demand, overhead conductors are always in hot service. In this work, the commonly used commercially pure Al wires and Al-Mg-Si wires were selected to carry out dynamic fatigue tests at elevated temperatures, to investigate the dynamic influences of temperature on their fatigue properties and damage mechanisms during cyclic deformation. The results show that the dynamic mechanical properties become degradated with increasing temperature due to the microstructural recovery, and such mechanical degradation is more severe for Al-Mg-Si wires. Besides, the fatigue crack initiation and propagation mechanisms were revealed using electron backscattering diffraction. Cracking at the grains with 〈0 0 1〉 orientation on surface is the major crack initiation mode. While, the crack propagation mechanism is divided into three stages, i.e., the propagation in the direction of a single slip system (Stage I), the propagation in the direction of multiple slip systems (Stage II), and the instability propagation in the direction of the maximum shear stress (Stage III). The dynamic evolution mechanism of the plane crack propagation regions with temperature was also studied, which is related to the critical stress and force for the crack instability propagation. Finally, a dynamic fatigue strength prediction model of Al wires was proposed. [ABSTRACT FROM AUTHOR]

It is of great importance to investigate the deformation and damage mechanisms of polycrystalline materials with soft crystal orientations, considering the soft grains are usually the fatigue crack initiation sites. In this study, a Ti–6Al–4V alloy fully consisted of <01 1 ‾ 0> and < 1 ‾ 2 1 ‾ 0> α (soft) grains was prepared and its uniaxial deformation behavior was investigated. Interestingly, no significant asymmetry on the tension-compression yield strength was found in this alloy with bi-grain orientations, though mechanical asymmetry was generally found in the traditional Ti alloys with strong texture. After high temperature solution treatment, the preferred bi-orientations were retained and the asymmetry degree of yield strength did not change, showing an excellent stability in texture and mechanical performance. Deformation mechanisms during tension and compression were studied and discussed. This work provides a new understanding on the deformation and damage mechanisms in the grains with soft crystal orientations, and may guide the microstructural design of Ti alloy for pursuing high mechanical and fatigue performances in the future. [ABSTRACT FROM AUTHOR]

• Low grain sizes generate homogeneous dislocation slip to delay fatigue cracking. • High grain sizes induce deformation twinning to delay crack propagation by dissipating plastic work. • Grains refine around fatigue cracks in CrCoNi medium-entropy alloy with low grain sizes. • Fine-grained specimens with certain twinning ability prolong the fatigue life most. Low-cycle fatigue behaviors of CrCoNi medium entropy alloy with different grain sizes were investigated. It is found that grain refinement improved deformation homogeneity to delay fatigue crack initiation by dispersing slip bands and fatigue damage. Yet, the increase in grain size promoted deformation twinning to retard fatigue crack growth by dissipating plastic work. With enough plastic deformation accumulated, extensive dislocation slip and deformation twinning significantly refined the grains localized near fatigue cracks. The grain refinement could trade off the cyclic softening and help to retard fatigue crack growth. These findings indicate that refined grains with certain twinning ability would extend the fatigue life of CrCoNi alloy the most and suppress cyclic softening at the same time. [ABSTRACT FROM AUTHOR]

• The failed mechanism of bearings in bench test made by rare earth modified, highly clean bearing steel was researched. • The rolling contact fatigue cracks initiation of bearing and its mechanism during bench test were explored. • The Influences of the characteristics of carbide particles on the rolling contact fatigue life of bearing is investigated. • The conclusions are served to optimize the rare earth modified, highly clean bearing steel. Machine tool bearings made of rare earth modified, highly clean bearing steel subjected to rolling contact fatigue (RCF) under the same bench test conditions were studied. The morphology and initiation position of contact fatigue cracks of bearings were characterized macroscopically and microscopically. According to the characteristics of carbide particles in the metallographic structure of the crack source areas, the corresponding relationships between the difference of particle size and roundness of carbide and the change in RCF lives of bearings were proposed, as well as the corresponding different failure mechanisms. The results show that RCF life increases with the decrease in the percentages of large-sized and irregular carbides in the microstructure. When early RCF failure occurs, cracks originate mainly at the surface, and the size of carbides affects RCF; with the increase of service life, the cracks begin to originate at the subsurface, and at this time, the roundness of carbides exerts a significant influence. [ABSTRACT FROM AUTHOR]

• A two-stage constitutive LCF crack growth model was proposed and developed for graded structures. • A hard core & soft shell design strategy was proposed to prolong the LCF life of TWIP steel. • Novel preparing methods were proposed to constructing microstructural gradual and sharp gradients in TWIP steel. • A method of quantitatively evaluating LCF crack length (evolution) from the cyclic stress response curve was proposed. It remains an intricate problem how to enhance low-cycle fatigue (LCF) performance, in the premise of guaranteeing a relatively high strength (or cyclic stress) which is vital to long-time service, for engineering components. The underlying cause of this puzzle is the lack of practical theory on cracking evolution/life prediction for LCF. In this study, a two-stage constitutive LCF crack growth model was proposed for a homogeneous solid and later developed for graded structures. Based on the model, a material design strategy emphasizing the strength gradient architecture (informally, hard core and soft shell structure) is proposed and analyzed to improve the LCF properties of engineering structural materials, e.g., steels. Experimentally, sharp and gradual strength gradients were carefully introduced into a TWIP steel by a series of well-designed process combinations regarding various (heterogeneous) deformation and annealing treatments. The influences of strength/microstructure and their spatial distributions on the LCF behaviors were systematically investigated. The excellent LCF properties of the TWIP steel with a negative strength gradient are originated from the repeated strengthening and weakening of strain gradient during cyclic loading, which suppresses the fatigue crack initiation and growth. The crack initiation/propagation model developed in this study is conducive and applicable to the LCF life prediction. The theory and process of constructing microstructure gradient, and therefore mechanical property gradient, may provide new and important implications for engineering metals on anti-LCF design. [ABSTRACT FROM AUTHOR]